Non-baryonic cold dark matter ("CDM"), the observational evidence

[DeiRenDopa]

I've started this thread to avoid the Quarks, [OIII], neutron stars, black holes OK; CDM not OK - Huh? thread (http://forums.randi.org/showthread.php?t=112199) getting bogged down.

Of course the nature of the observational evidence, and the analysis that takes observations and concludes 'here be (lots and lots of) CDM' is relevant to that thread! And it has already been discussed, in an ad hoc way, starting with this post by sol invictus (http://forums.randi.org/showpost.php?p=3654579&postcount=39).

However, it is a separate topic, and one that could easily generate a dozen pages of posts ... and it is not central to the question I want to address in that other thread ("How does it come about that an apparently intelligent, educated, thoughtful person can be quite OK with 'quarks' (which have not be 'seen' in any experiments), [OIII] 5007 (which has never been produced in any lab), neutron stars (ditto), and black holes (double ditto!), yet balk at the very thought of non-baryonic cold dark matter (CDM)?").

I'll take the following approach to addressing the observational evidence:

* observations concerning CDM in our Milky Way galaxy, and other galaxies

* observations concerning CDM in rich clusters of galaxies

* CDM in cosmology.

For the first two, I will look at the different kinds of observations that lead to the conclusion 'lots of CDM', with an emphasis on the different physical mechanisms at work (a.k.a. the different physics theories involved in the observations themselves and in the analyses of those observations), the leading limitations and questions on these, and whether there are any viable alternative conclusions (to 'here be (lots and lots of) CDM'). Of necessity, most of the history will be omitted; this is, in some ways, a pity, because that history is really quite fascinating - the errors, the wrong turns, the prescient early insights, the slow elimination of all alternatives, the huge effort put into corroboration, etc, etc, etc.

The last one (cosmology) needs to be treated in a different way, partly because it is most powerful when considered in terms of consistency, rather than a set of independent classes of observation.

My approach deliberately omits all particle physics inputs; there is a very strong set of cases concerning the existence of CDM that arise from particle physics, and these help support the conclusion in terms of consistency.

I will also not even attempt to cover astronomical observations other than those under these three headings.

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Key term: by "CDM" I will mean 'non-baryonic cold, dark matter'.

'Cold' refers to the average speed of this matter with respect to the CMB frame of reference; basically it just says this stuff isn't zipping round the universe nearly at the speed of light, unlike cosmic rays and neutrinos (the former is an example of 'hot matter', the latter 'hot dark matter').

'Dark' refers to transparency to all forms of electromagnetic radiation; DM and photons are like two ships in the night, they pass each other by without either noticing the other. In practical, astronomical, terms this simply means DM does not emit light (or gamma rays, or x-rays, or ... or radio), nor does it absorb it.

'non-baryonic' means the CDM is made up of stuff other than the molecules, atoms, nuclei, and electrons we are made up of (and the Sun, and cosmic rays, and neutron stars, and dust, and gas, and ...). Neutrinos are 'non-baryonic'; however, they are not 'cold'. The question of whether black holes get counted as non-baryonic or not will be covered in the cases where it is necessary to eliminate them as a possible explanation for the various observations.

OK, time to start.

[BenBurch]

Well, galaxies cannot hold together without CDM or a force that acts just like CDM. It has been a toothache in cosmology that gravitational simulations of galaxies based on the mass that we can see and the velocities we observe do not result in a galaxy that does not fly apart.

And it appears that when galaxies collide, the CDM, which does not appear to interact with baryonic matter (or itself) except gravitationally, shoots on past the parent galaxy as it experiences no drag in the interaction, but the stars and gas of the baryonic side of the galaxies DO interact mechanically and so slow. And you can detect the effects of this in analysis of these galaxies.

[fuelair]

I've started this thread to avoid the Quarks, [OIII], neutron stars, black holes OK; CDM not OK - Huh? thread (http://forums.randi.org/showthread.php?t=112199) getting bogged down.

Of course the nature of the observational evidence, and the analysis that takes observations and concludes 'here be (lots and lots of) CDM' is relevant to that thread! And it has already been discussed, in an ad hoc way, starting with this post by sol invictus (http://forums.randi.org/showpost.php?p=3654579&postcount=39).

However, it is a separate topic, and one that could easily generate a dozen pages of posts ... and it is not central to the question I want to address in that other thread ("How does it come about that an apparently intelligent, educated, thoughtful person can be quite OK with 'quarks' (which have not be 'seen' in any experiments), [OIII] 5007 (which has never been produced in any lab), neutron stars (ditto), and black holes (double ditto!), yet balk at the very thought of non-baryonic cold dark matter (CDM)?").

I'll take the following approach to addressing the observational evidence:

* observations concerning CDM in our Milky Way galaxy, and other galaxies

* observations concerning CDM in rich clusters of galaxies

* CDM in cosmology.

For the first two, I will look at the different kinds of observations that lead to the conclusion 'lots of CDM', with an emphasis on the different physical mechanisms at work (a.k.a. the different physics theories involved in the observations themselves and in the analyses of those observations), the leading limitations and questions on these, and whether there are any viable alternative conclusions (to 'here be (lots and lots of) CDM'). Of necessity, most of the history will be omitted; this is, in some ways, a pity, because that history is really quite fascinating - the errors, the wrong turns, the prescient early insights, the slow elimination of all alternatives, the huge effort put into corroboration, etc, etc, etc.

The last one (cosmology) needs to be treated in a different way, partly because it is most powerful when considered in terms of consistency, rather than a set of independent classes of observation.

My approach deliberately omits all particle physics inputs; there is a very strong set of cases concerning the existence of CDM that arise from particle physics, and these help support the conclusion in terms of consistency.

I will also not even attempt to cover astronomical observations other than those under these three headings.

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Key term: by "CDM" I will mean 'non-baryonic cold, dark matter'.

'Cold' refers to the average speed of this matter with respect to the CMB frame of reference; basically it just says this stuff isn't zipping round the universe nearly at the speed of light, unlike cosmic rays and neutrinos (the former is an example of 'hot matter', the latter 'hot dark matter').

'Dark' refers to transparency to all forms of electromagnetic radiation; DM and photons are like two ships in the night, they pass each other by without either noticing the other. In practical, astronomical, terms this simply means DM does not emit light (or gamma rays, or x-rays, or ... or radio), nor does it absorb it.

'non-baryonic' means the CDM is made up of stuff other than the molecules, atoms, nuclei, and electrons we are made up of (and the Sun, and cosmic rays, and neutron stars, and dust, and gas, and ...). Neutrinos are 'non-baryonic'; however, they are not 'cold'. The question of whether black holes get counted as non-baryonic or not will be covered in the cases where it is necessary to eliminate them as a possible explanation for the various observations.

OK, time to start.Is there a vague chance that proof or really good evidence is going to pop up somewhere here? I ask only because real astronomers/astrophysists seem to be unsure at this time and I would rather wait to read stuff until the people who have the equiptment and expertise have it really well mapped out. Conjecture can be entertaining but fact is the real thing.

[Dancing David]

The observations are facts?

Stars rotate around galaxies as though there is more matter than we can see.

[Wangler]

DRD, this is going to be another good thread, IMO. Thanks for starting, sorry to have bogged down the other.

[BenBurch]

Basically, the evidence for CDM is just as good or better than the evidence for many other things we see out in the cosmos.

[DeiRenDopa]

The main technique used to estimate the distribution of mass in spiral galaxies, as a function of radius from the centre (nucleus) is to derive a rotation curve from observations of the light (radio, etc; electromagnetic radiation - I'll use 'light' as a synonym) emitted by such galaxies.

One technique involves observing how the line of sight velocity changes with position; here (http://www.astro.cornell.edu/academics/courses/astro201/rotcurve.htm) is a concise but accurate summary of how it's done, using a 'long slit' spectrum, and emission lines in the visual waveband.

There are many other ways to obtain a spiral galaxy rotation curve, using different wavebands, different lines; integrated light from sizable chunks of the target galaxy, individual sources (e.g. HII regions, bright stars); and so on.

The results are the same: the curves either keep rising or flatten out, right out (radially) to where no more light seems to be coming from the galaxy.

Using textbook physics, these curves can be interpreted to mean that the mass 'closer in' to the centre of the galaxy (than at any radius) keeps increasing as the radius increases. In fact, no other standard physics textbook interpretation has been proposed, that is also consistent with all the relevant observations.

However, adding up all the mass in these galaxies, estimated from the light emitted (by stars, dust, and gas/plasma) or light absorbed (by dust and gas), gives totals that are just too small ... and the difference (between 'rotation curve mass' and 'stars/dust/gas mass') gets larger as the radius increases.

In addition to spiral galaxy rotation curves, the mass of galaxies can be estimated by several other techniques.

Beyond the faintest (integrated light) edges of galaxies are objects which are moving within the gravitational well of the galaxies. These objects include planetary nebulae, globular clusters, and satellite (dwarf) galaxies. Just as the rotation curves can be interpreted to estimate the total mass 'closer in' (using physics which Newton pioneered), the motions of these more distant objects can also be interpreted, using the same physics, to estimate the total mass 'closer in'. This work is much, much more challenging than rotation curves! However, it probes the mass of galaxies at considerably greater distances than rotation curves can reach, and also gives estimates of the mass of elliptical galaxies, which do not have rotation curves.

These observations can be interpreted as being consistent with the rotation curve observations - galaxies seem to have 'halos' of mass that extend way beyond their 'visible' edges. The density of this (dark) halo mass decreases with radial distance from the nucleus.

Some elliptical galaxies, typically the giants found near the centres of clusters, emit x-rays. The physics of such emission is easily understood from a different part of the standard physics textbook, and the x-ray emission can be interpreted as tracing the (radial) mass distribution in these ellipticals - basically, for the hot plasma that emits the x-rays to be 'trapped' in the giant elliptical galaxy, the galaxy must have a mass that lies between two robust limits. Again, galaxy masses estimated using this technique are consistent with those estimated from motions of globular clusters and planetary nebulae ... and again, the total mass is considerably greater than that estimated from all the light emitted or absorbed by the stars and gas/plasma (ellipticals have essentially no dust).

Some dwarf galaxies, in our Local Group, are close enough that the line of sight velocities of individual stars can be measured, and the distribution of stars within the galaxies accurately measured. Assuming these dwarf galaxies are gravitationally bound, these observations can be interpreted, using standard textbook (Newtonian) physics to give estimates of the total mass of these dwarf galaxies. The results are both astonishing and unambiguous: these galaxies contain far, far more mass than is in the stars whose light we can detect (it's much the same with regard to gas/plasma; note that these galaxies have little dust).

Somewhat in contrast to rotation curves of spiral galaxies, interpretation of the observations using the other techniques I've briefly mentioned does not have to lead to firm conclusions about mass differences ... however, as far as I know, no alternative explanations (based on standard, textbook physics) have been proposed that are also consistent with the 'lensing' observations I will cover next.

So far, the parts of the standard physics textbooks used to interpret the millions of astronomical observations have been many, but have not included General Relativity (GR).

One last technique (two actually) involves estimating mass using GR, and is completely independent of all the techniques briefly described above. It is, to me at least, truly marvelous that 'GR observations' can be interpreted to arrive at conclusions that are completely consistent with the various other observations I've briefly described ... and this consistency across different techniques using different physics is surely one of the strongest indicators that 'unseen mass' is the right interpretation.

(to be continued)

[DeiRenDopa]

Is there a vague chance that proof or really good evidence is going to pop up somewhere here? I ask only because real astronomers/astrophysists seem to be unsure at this time and I would rather wait to read stuff until the people who have the equiptment and expertise have it really well mapped out. Conjecture can be entertaining but fact is the real thing..
As I said in the other thread (see the OP), to do justice to the vast amount of observational material relevant to the existence of CDM, even just within the three regions I intend to briefly touch, would take several months (or so I estimate).

I should also have added that the audience I am aiming at is not those with BSc (or higher) degrees in physics or astronomy; for such people, there are dozens of excellent textbooks, and hundreds of landmark papers.

Finally, do not expect a nice, simple, 'try this at home' explanation; just like the observational basis for SgrA* (the nucleus of the Milky Way) being a super-massive black hole, the observational basis for CDM is a rich, intricately-connected web of millions of observations and very large parts of standard physics textbooks. Depending on what you consider 'really good evidence' to be, you might have to convince yourself of almost all of standard astronomy and astrophysics first, before you could even begin to appreciate the evidence for CDM.

[Wangler]

The main technique used to estimate the distribution of mass in spiral galaxies, as a function of radius from the centre (nucleus) is to derive a rotation curve from observations of the light (radio, etc; electromagnetic radiation - I'll use 'light' as a synonym) emitted by such galaxies.

One technique involves observing how the line of sight velocity changes with position; here (http://www.astro.cornell.edu/academics/courses/astro201/rotcurve.htm) is a concise but accurate summary of how it's done, using a 'long slit' spectrum, and emission lines in the visual waveband.

There are many other ways to obtain a spiral galaxy rotation curve, using different wavebands, different lines; integrated light from sizable chunks of the target galaxy, individual sources (e.g. HII regions, bright stars); and so on.

The results are the same: the curves either keep rising or flatten out, right out (radially) to where no more light seems to be coming from the galaxy.

Using textbook physics, these curves can be interpreted to mean that the mass 'closer in' to the centre of the galaxy (than at any radius) keeps increasing as the radius increases. In fact, no other standard physics textbook interpretation has been proposed, that is also consistent with all the relevant observations.

However, adding up all the mass in these galaxies, estimated from the light emitted (by stars, dust, and gas/plasma) or light absorbed (by dust and gas), gives totals that are just too small ... and the difference (between 'rotation curve mass' and 'stars/dust/gas mass') gets larger as the radius increases.

In addition to spiral galaxy rotation curves, the mass of galaxies can be estimated by several other techniques.

Beyond the faintest (integrated light) edges of galaxies are objects which are moving within the gravitational well of the galaxies. These objects include planetary nebulae, globular clusters, and satellite (dwarf) galaxies. Just as the rotation curves can be interpreted to estimate the total mass 'closer in' (using physics which Newton pioneered), the motions of these more distant objects can also be interpreted, using the same physics, to estimate the total mass 'closer in'. This work is much, much more challenging than rotation curves! However, it probes the mass of galaxies at considerably greater distances than rotation curves can reach, and also gives estimates of the mass of elliptical galaxies, which do not have rotation curves.

These observations can be interpreted as being consistent with the rotation curve observations - galaxies seem to have 'halos' of mass that extend way beyond their 'visible' edges. The density of this (dark) halo mass decreases with radial distance from the nucleus.

Some elliptical galaxies, typically the giants found near the centres of clusters, emit x-rays. The physics of such emission is easily understood from a different part of the standard physics textbook, and the x-ray emission can be interpreted as tracing the (radial) mass distribution in these ellipticals - basically, for the hot plasma that emits the x-rays to be 'trapped' in the giant elliptical galaxy, the galaxy must have a mass that lies between two robust limits. Again, galaxy masses estimated using this technique are consistent with those estimated from motions of globular clusters and planetary nebulae ... and again, the total mass is considerably greater than that estimated from all the light emitted or absorbed by the stars and gas/plasma (ellipticals have essentially no dust).

Some dwarf galaxies, in our Local Group, are close enough that the line of sight velocities of individual stars can be measured, and the distribution of stars within the galaxies accurately measured. Assuming these dwarf galaxies are gravitationally bound, these observations can be interpreted, using standard textbook (Newtonian) physics to give estimates of the total mass of these dwarf galaxies. The results are both astonishing and unambiguous: these galaxies contain far, far more mass than is in the stars whose light we can detect (it's much the same with regard to gas/plasma; note that these galaxies have little dust).

Somewhat in contrast to rotation curves of spiral galaxies, interpretation of the observations using the other techniques I've briefly mentioned does not have to lead to firm conclusions about mass differences ... however, as far as I know, no alternative explanations (based on standard, textbook physics) have been proposed that are also consistent with the 'lensing' observations I will cover next.

So far, the parts of the standard physics textbooks used to interpret the millions of astronomical observations have been many, but have not included General Relativity (GR).

One last technique (two actually) involves estimating mass using GR, and is completely independent of all the techniques briefly described above. It is, to me at least, truly marvelous that 'GR observations' can be interpreted to arrive at conclusions that are completely consistent with the various other observations I've briefly described ... and this consistency across different techniques using different physics is surely one of the strongest indicators that 'unseen mass' is the right interpretation.

(to be continued)

Good post, DRD.

I hope to be able, in the next couple of days, post a discussion about this line of evidence, from this layman's perspective.

I hope to get this posted before you move much further along.

[DeiRenDopa]

Good post, DRD.

I hope to be able, in the next couple of days, post a discussion about this line of evidence, from this layman's perspective.

I hope to get this posted before you move much further along.Thanks.

Note that this is only part of the observational evidence; for example, how various classes of dark baryonic matter can be ruled out - in spiral galaxies, ellipticals, and dwarf galaxies - is the subject of a later post (or several posts) ...

[Wangler]

Note that this is only part of the observational evidence; for example, how various classes of dark baryonic matter can be ruled out - in spiral galaxies, ellipticals, and dwarf galaxies - is the subject of a later post (or several posts) ...

I'm sure I'll have many opportunities to learn a great deal.

[Dancing David]

Thanks DRD, I appreciate your time and effort.

Wangler, you didn't bog the other thread, It has it's own momentum and pattern. Now is the quiet before the spam bomb.

[Zeuzzz]

The observations are facts?

Stars rotate around galaxies as though there is more matter than we can see.


That is all assuming that the only force at work is gravity, and it also assumes that gravity remains the dominant force when you scale up above the planetary/stellar level. We know that as you assend up scales other forces change their dominance, ie, EM forces usually far dominate gravity at smaller scales, so it is fully possible that as you scale up gravity starts to become secondary to other various forces and instabilities in plasma.

I see the problems with CDM and other similar cosmologies as follows:

• Like geology and unlike particle physics, cosmology is intrinsically an historical science and lacks direct experimental testing for many key hypotheses.

• Data in cosmology are limited to remotely sensed photons; there is no direct, in situ measurement as in space physics or planetary science.

• Most existing cosmology models focus on only one long-range force field (gravity) and ignore potential long-range effects of electromagnetism and plasmas.


On this last point, it is well known that electromagnetism is very effectively shielded out, which allows gravity to generally dominate over long scale lengths. Nevertheless, electromagnetism and plasmas can still be significant because:


“By definition, plasmas are an interactive mix of charged particles, neutrals, and fields that exhibits collective effects. In plasmas, charged particles are subject to long-range, collective Coulomb interactions with many distant encounters. Although the electrostatic force drops with distance (~1/r2), the combined effect of all charged particles might not decay because the interacting volume increases as r3. Magnetic field effects are often global with their connections reaching to galactic scales and beyond” (Goedbloed and Poedts, 2004) (Also see; B. Coppi, Attilio Ferrari, 2000 [full text] (http://books.google.co.uk/books?id=5Ba2-lfLiiAC&printsec=frontcover&dq=plasma+electromagnetic+forces+gravitational#PPA 1,M1))


Of course, this will be hotly contested by Big Bang advocates that require gravity being the only force at work, but is none-the-less a valid point that we have no real reason to discount at the moment.

The potential importance of electromagnetism and plasmas is indicated by the rapidly growing field of plasma astrophysics (see links and references at plasmas.org/ (http://www.plasmas.org/space-astrophys.htm)). As one example of its significance for altering conventional assumptions, (Astrophysics: A New Approach (http://www.springer.com/astronomy/book/978-3-540-22346-7?detailsPage=otherBooks&CIPageCounter=CI_MORE_BOOKS_BY_AUTHOR0), Kundt 2005) shows in detail how observed signatures of existing “black-hole” candidates can be more effectively interpreted as neutron star magnetospheres with accretion disks or neutron star binaries. Efforts to assess the potential impact of the new plasma astrophysics on cosmology issues are just beginning. Hopefully due to this we will not need to fill our universe up with CDM or other dubious metaphysical constructs, the vast and varied properties of plasma should be able to account for many of cosmologies unsolved mysteries in the near future.

[Reality Check]

That is all assuming that the only force at work is gravity, and it also assumes that gravity remains the dominant force when you scale up above the planetary/stellar level. We know that as you assend up scales other forces change their dominance, ie, EM forces usually far dominate gravity at smaller scales, so it is fully possible that as you scale up gravity starts to become secondary to other various forces and instabilities in plasma.

I see the problems with CDM and other similar cosmologies as follows:

• Like geology and unlike particle physics, cosmology is intrinsically an historical science and lacks direct experimental testing for many key hypotheses.

• Data in cosmology are limited to remotely sensed photons; there is no direct, in situ measurement as in space physics or planetary science.

• Most existing cosmology models focus on only one long-range force field (gravity) and ignore potential long-range effects of electromagnetism and plasmas.


On this last point, it is well known that electromagnetism is very effectively shielded out, which allows gravity to generally dominate over long scale lengths. Nevertheless, electromagnetism and plasmas can still be significant because:


“By definition, plasmas are an interactive mix of charged particles, neutrals, and fields that exhibits collective effects. In plasmas, charged particles are subject to long-range, collective Coulomb interactions with many distant encounters. Although the electrostatic force drops with distance (~1/r2), the combined effect of all charged particles might not decay because the interacting volume increases as r3. Magnetic field effects are often global with their connections reaching to galactic scales and beyond” (Goedbloed and Poedts, 2004) (Also see; B. Coppi, Attilio Ferrari, 2000 [full text] (http://books.google.co.uk/books?id=5Ba2-lfLiiAC&printsec=frontcover&dq=plasma+electromagnetic+forces+gravitational#PPA 1,M1))


Of course, this will be hotly contested by Big Bang advocates that require gravity being the only force at work, but is none-the-less a valid point that we have no real reason to discount at the moment.

The potential importance of electromagnetism and plasmas is indicated by the rapidly growing field of plasma astrophysics (see links and references at plasmas.org/ (http://www.plasmas.org/space-astrophys.htm)). As one example of its significance for altering conventional assumptions, (Astrophysics: A New Approach (http://www.springer.com/astronomy/book/978-3-540-22346-7?detailsPage=otherBooks&CIPageCounter=CI_MORE_BOOKS_BY_AUTHOR0), Kundt 2005) shows in detail how observed signatures of existing “black-hole” candidates can be more effectively interpreted as neutron star magnetospheres with accretion disks or neutron star binaries. Efforts to assess the potential impact of the new plasma astrophysics on cosmology issues are just beginning. Hopefully due to this we will not need to fill our universe up with CDM or other dubious metaphysical constructs, the vast and varied properties of plasma should be able to account for many of cosmologies unsolved mysteries in the near future in my opinion.

Hi Zeuzzz: Did you not read the posts on other threads that you were particpating in that refute Plasma Cosmology?

There is no evidence that electromagnetic forces are more important than gravitational forces on a cosmological scale.

In any case your posting is off topic. If you want to discuss Plasma Cosmology and its explanation for non-existance of the observed dark matter (http://chandra.harvard.edu/photo/2006/1e0657/) then start a new thread.

[sol invictus]

Now is the quiet before the spam bomb.
<nonsense>

BOOM!!

[Zeuzzz]

Hi Zeuzzz: Did you not read the posts on other threads that you were particpating in that refute Plasma Cosmology?

There is no evidence that electromagnetic forces are more important than gravitational forces on a cosmological scale.

In any case your posting is off topic. If you want to discuss Plasma Cosmology and its explanation for non-existance of the observed dark matter (http://chandra.harvard.edu/photo/2006/1e0657/) then start a new thread.


:D

"There is no evidence that electromagnetic forces are more important than gravitational forces on a cosmological scale." Can you see my post? or is your mind still blocking out anything that does not adhere to your personal views? I stated why this could be the case, quite clearly and openly.

We'll just have to see about these *cough* 'refutations of plasma cosmology' in the near future when I have time to respond to them. So far i have not seen any, but will be happy to discuss them if you think they exist. I only briefly visit here each day at the mo, and will start a thread all about PC when I have more time. I didn't even mention plasma cosmology anyway in that post. You seem to have inferred this yourself from just reading some standard plasma astrophysics material, which is very telling.

I am giving my opinion (and many other astronomers) on CDM, and whether it is needed or not. Which, if you hadn't noticed, is exactly what this thread is about.

And, I am curious why you (as usual) chose to ignore the material in my previous post and just write about your personal opinion? Are all the links in my post 'woo' in your opinion? because by my standards, it looks like quite established scientific literature all published in respected science journals. If you can refute it, then please, be my guest. :)

[DeiRenDopa]

Now is the quiet before the spam bomb.How prescient!That is all assuming that the only force at work is gravity, and it also assumes that gravity remains the dominant force when you scale up above the planetary/stellar level. We know that as you assend up scales other forces change their dominance, ie, EM forces usually far dominate gravity at smaller scales, so it is fully possible that as you scale up gravity starts to become secondary to other various forces and instabilities in plasma. After all, 99% of the observable universe is in the plasma state, and this was not known when current theories were fomulated, so it is highly likely this will change many things about the way we view space.

I see the problems with CDM and other similar cosmologies as follows:

• Like geology and unlike particle physics, cosmology is intrinsically an historical science and lacks direct experimental testing for many key hypotheses.

• Data in cosmology are limited to remotely sensed photons; there is no direct, in situ measurement as in space physics or planetary science.

• Most existing cosmology models focus on only one long-range force field (gravity) and ignore potential long-range effects of electromagnetism and plasmas.


On this last point, it is well known that electromagnetism is very effectively shielded out, which allows gravity to generally dominate over long scale lengths. Nevertheless, electromagnetism and plasmas can still be significant because:


“By definition, plasmas are an interactive mix of charged particles, neutrals, and fields that exhibits collective effects. In plasmas, charged particles are subject to long-range, collective Coulomb interactions with many distant encounters. Although the electrostatic force drops with distance (~1/r2), the combined effect of all charged particles might not decay because the interacting volume increases as r3. Magnetic field effects are often global with their connections reaching to galactic scales and beyond” (Goedbloed and Poedts, 2004) (Also see; B. Coppi, Attilio Ferrari, 2000 [full text] (http://books.google.co.uk/books?id=5Ba2-lfLiiAC&printsec=frontcover&dq=plasma+electromagnetic+forces+gravitational#PPA 1,M1))


Of course, this will be hotly contested by Big Bang advocates that require gravity being the only force at work, but is none-the-less a valid point that we have no real reason to discount at the moment.

The potential importance of electromagnetism and plasmas is indicated by the rapidly growing field of plasma astrophysics (see links and references at plasmas.org/ (http://www.plasmas.org/space-astrophys.htm)). As one example of its significance for altering conventional assumptions, (Astrophysics: A New Approach (http://www.springer.com/astronomy/book/978-3-540-22346-7?detailsPage=otherBooks&CIPageCounter=CI_MORE_BOOKS_BY_AUTHOR0), Kundt 2005) shows in detail how observed signatures of existing “black-hole” candidates can be more effectively interpreted as neutron star magnetospheres with accretion disks or neutron star binaries. Efforts to assess the potential impact of the new plasma astrophysics on cosmology issues are just beginning. Hopefully due to this we will not need to fill our universe up with CDM or other dubious metaphysical constructs, the vast and varied properties of plasma should be able to account for many of cosmologies unsolved mysteries in the near future.Zeuzzz, did you read the OP?

Would you mind, terribly, if I asked you to not spam this thread with your PC woo?

In case you missed it, here are a couple of things I said, in my first post which presented substantive content relevant to the thread (per the OP; emphasis added):Using textbook physics, these [spiral galaxy rotation] curves can be interpreted to mean that the mass 'closer in' to the centre of the galaxy (than at any radius) keeps increasing as the radius increases. In fact, no other standard physics textbook interpretation has been proposed, that is also consistent with all the relevant observations.

Somewhat in contrast to rotation curves of spiral galaxies, interpretation of the observations using the other techniques I've briefly mentioned does not have to lead to firm conclusions about mass differences ... however, as far as I know, no alternative explanations (based on standard, textbook physics) have been proposed that are also consistent with the 'lensing' observations I will cover next.And a point of clarification shortly afterwards:Note that this is only part of the observational evidence; for example, how various classes of dark baryonic matter can be ruled out - in spiral galaxies, ellipticals, and dwarf galaxies - is the subject of a later post (or several posts) ....

If you would like to ask questions about specific alternative explanations of spiral galaxy rotation curves, involving only standard textbook physics, which are also consistent with all the relevant observations, please do so.

Ditto with respect to the other, non-GR, observations of galaxies (not clusters of galaxies, not cosmology).

For cosmology, why not wait until I get to that?

Oh, and I'd like to second RC's point: if you want to present more 'plasma cosmology' woo, then please either continue in the thread you dropped out of (and start by answering the many questions you so conveniently walked away from), or start a new thread.

[DeiRenDopa]

:D

We'll just have to see about these *cough* 'refutations of plasma cosmology' in the near future when I have time to respond to them. So far i have not seen any, but will be happy to discuss them if you think they exist. I only briefly visit here each day at the mo, and will start a thread all about PC when I have more time. I didn't even mention plasma cosmology anyway in that post. You seem to have inferred this yourself from just reading some standard plasma astrophysics material, which is very telling.

I am giving my opinion (and many other astronomers) on CDM, and whether it is needed or not. Which, if you hadn't noticed, is exactly what this thread is about.

And, I am curious why you (as usual) chose to ignore the material in my previous post and just write about your personal opinion? Are all the links in my post 'woo' in your opinion? because by my standards, it looks like quite established scientific literature all published in respected science journals. If you can refute it, then please, be my guest. :) Oh Zeuzzz, are you so deluded? Have you learned so little in the physics classes you have been taking?

Here's what you just wrote (excerpt):EM forces usually far dominate gravity at smaller scales, so it is fully possible that as you scale up gravity starts to become secondary to other various forces and instabilities in plasma. After all, 99% of the observable universe is in the plasma state, and this was not known when current theories were fomulated, so it is highly likely this will change many things about the way we view space.

[...]

Most existing cosmology models focus on only one long-range force field (gravity) and ignore potential long-range effects of electromagnetism and plasmas..

Though I must say your seagull appearances, to drop woo, are getting more sophisticated; it seems you have taken lessons from the IDers, and have begun a new career as a quote miner ...

[Zeuzzz]

Oh, and i forgot to add a link to the (Goedbloed and Poedts, 2004) reference where I got the quote from. Here it is; http://www.lavoisier.fr/notice/gb405993.html or here http://www.astro.uu.nl/siu/res-hg.html

[DeiRenDopa]

Though I must say your seagull appearances, to drop woo, are getting more sophisticated; it seems you have taken lessons from the IDers, and have begun a new career as a quote miner ...Oh, and i forgot to add a link to the (Goedbloed and Poedts, 2004) reference where I got the quote from. Here it is; http://www.lavoisier.fr/notice/gb405993.html or here http://www.astro.uu.nl/siu/res-hg.html...

right on cue ...

Don't you mean 'mined the quote'?

[Zeuzzz]

Oh Zeuzzz, are you so deluded? Have you learned so little in the physics classes you have been taking?

Here's what you just wrote (excerpt):.

Though I must say your seagull appearances, to drop woo, are getting more sophisticated; it seems you have taken lessons from the IDers, and have begun a new career as a quote miner ...


:D

So your not going to comment on any of the material in my post then? and what relevance this may have to whether CDM exists? Fair enough. I should have known better.

So, now using a quote from a series of peer reviewed science publications to directly back up my opinion can now be dismissed purely for the reason that I quoted it? Amazing.

[DeiRenDopa]

:D

So your not going to comment on any of the material in my post then? and what relevance this may have to whether CDM exists? Fair enough. I should have known better..

Of course I'm going to comment on it! :mad::mad:

But not until I get to the part where I cover the actual observational evidence first, as I said in the OP:I'll take the following approach to addressing the observational evidence:

* observations concerning CDM in our Milky Way galaxy, and other galaxies

* observations concerning CDM in rich clusters of galaxies

* CDM in cosmology.

For the first two, I will look at the different kinds of observations that lead to the conclusion 'lots of CDM', with an emphasis on the different physical mechanisms at work (a.k.a. the different physics theories involved in the observations themselves and in the analyses of those observations), the leading limitations and questions on these, and whether there are any viable alternative conclusions (to 'here be (lots and lots of) CDM'). Of necessity, most of the history will be omitted; this is, in some ways, a pity, because that history is really quite fascinating - the errors, the wrong turns, the prescient early insights, the slow elimination of all alternatives, the huge effort put into corroboration, etc, etc, etc.

The last one (cosmology) needs to be treated in a different way, partly because it is most powerful when considered in terms of consistency, rather than a set of independent classes of observation.

My approach deliberately omits all particle physics inputs; there is a very strong set of cases concerning the existence of CDM that arise from particle physics, and these help support the conclusion in terms of consistency.

I will also not even attempt to cover astronomical observations other than those under these three headings..
Right now, all I've done is write one substantive post, and one clarification, on the first ("observations concerning CDM in our Milky Way galaxy, and other galaxies"). There are at least two more on just that first point ... and the cosmology point (the third) comes last. Unless I missed it, there was nothing in any of your woo spam posts in this thread on either observations concerning CDM in our Milky Way galaxy, and other galaxies or observations concerning CDM in rich clusters of galaxies.

[bokonon]

However, adding up all the mass in these galaxies, estimated from the light emitted (by stars, dust, and gas/plasma) or light absorbed (by dust and gas), gives totals that are just too small ... and the difference (between 'rotation curve mass' and 'stars/dust/gas mass') gets larger as the radius increases.
I guess I'm the sort of interested layman without a BSc you are aiming to educate, so I'll try to follow along and ask the questions that occur to me.

At this point, I wonder - how do you know what quantity of matter in a distant galaxy is doing the absorbing? Hypothetically, wouldn't a roomful of swirling marble-sized black holes absorb less light than a roomful of thick black smoke, and yet be many times more massive?

[sol invictus]

At this point, I wonder - how do you know what quantity of matter in a distant galaxy is doing the absorbing? Hypothetically, wouldn't a roomful of swirling marble-sized black holes absorb less light than a roomful of thick black smoke, and yet be many times more massive?

Yes, that's right. A marble sized black hole weighs about 10^26 kg - which is a heck of a lot of smoke.

Actually, black holes of that size would probably make a pretty decent DM candidate, except that there's no reason they should be there.

[bokonon]

Yes, that's right. A marble sized black hole weighs about 10^26 kg - which is a heck of a lot of smoke.Make it no more than 7 to a room, then.

ETA: I'm assuming with that kind of mass, the event horizon would be bigger than a city block. Since you seem capable of "doing the math," would you mind providing me with a more educated range?

ETA2: I just looked, and the mass of the earth is 5.9 x 10^24 kg, so I guess we're talking 600 earths. I'll assume the event horizon is "bigger than the earth," which is precise enough for me.

Actually, black holes of that size would probably make a pretty decent DM candidate, except that there's no reason they should be there.Is there a reason for "It's not like anything we've ever encountered before, Captain!" to be there?

[sol invictus]

ETA: I'm assuming with that kind of mass, the event horizon would be bigger than a city block. Since you seem capable of "doing the math," would you mind providing me with a more educated range?

Eh? That was the mass for a marble sized black hole.

To be a little more precise, a BH with the mass of the earth has a horizon radius of .9 cm. Mind boggling, isn't it?

It might also be useful to know that the radius of a BH horizon scales linearly with the mass. So a solar mass BH will be roughly 500,000 times as large, i.e. 5km in radius.

Is there a reason for "It's not like anything we've ever encountered before, Captain!" to be there?

A good DM model includes an explanation of where the DM came from. I know of a (somewhat questionable) mechanism that might make solar mass BHs, but not one that could make marble sized BHs. But the solar mass BH as DM model is pretty solidly excluded by gravitational lensing searches. Still, it isn't totally impossible - it's just not the most likely candidate in my opinion.

[DeiRenDopa]

However, adding up all the mass in these galaxies, estimated from the light emitted (by stars, dust, and gas/plasma) or light absorbed (by dust and gas), gives totals that are just too small ... and the difference (between 'rotation curve mass' and 'stars/dust/gas mass') gets larger as the radius increases.I guess I'm the sort of interested layman without a BSc you are aiming to educate, so I'll try to follow along and ask the questions that occur to me.

At this point, I wonder - how do you know what quantity of matter in a distant galaxy is doing the absorbing? Hypothetically, wouldn't a roomful of swirling marble-sized black holes absorb less light than a roomful of thick black smoke, and yet be many times more massive?Excellent question! :)

And this is one place where a short, no-more-than-a-para-or-two summary cannot possibly do justice to the vast amount of material on the topic, not to mention the centuries of work by astronomers and physicists which lead to the conclusions so briefly summarised.

Also, it's part of the answer to the question 'so why can't the unseen mass be baryonic?' that I will address in a later post.

Without further ado then, what are galaxies made of?

Near us, in the Milky Way galaxy, we get a very up-close-and-personal view of things, and we 'see' stars (ranging in mass from ~100 sols down to ~0.1 sol), gas/plasma (with a huge range of densities and temperatures; however, it's almost entirely hydrogen (ionised, atomic, or molecular) and helium, and 'dust' (which is not pure H or pure He! more on this later). Solid stuff that's bigger than dust but smaller than a star is quite hard to detect, either by the light it emits or the light it absorbs (one of the most interesting techniques in astronomy is to observe 'at night' ... you want to know how bright the distant sky is, in x-rays or gammas, beyond the solar system? just look at the Earth, or Moon - they block the distant sky; you want to know how much dust there is in spiral galaxies? just look at one that is back-lit by a more distant galaxy; and so on).

However, we can get a handle on the total mass near us, in our part of the Milky Way galaxy, by observing how the stars (and gas, and dust) are moving, relative to us and each other. The math for this is quite clean, and quite powerful (look up 'collisionless Boltzmann equation'), but it took until the second half of the 20th century for observations to be extensive and accurate enough to apply it, in all its power, to our immediate galactic environment.

The result is this: the amount of mass, 'near us', that is not accounted for by stars, gas/plasma, and dust, is small ... possibly too small to be estimated (i.e. the error bars, or uncertainties, from the rest of the inputs are larger than the small residual). Of course, these calculations also include estimates of stars that are too faint to be directly observed, especially red and brown dwarfs. However, these estimates are robust, in the sense that the 'mass function' (proportion of stars of a given mass vs mass) is well-constrained from detailed studies of various nearby open clusters and from studies of binary stars (also by microlensing, covered later).

So where's the CDM 'near us'? It turns out that if the other results about the amount and distribution of CDM in our Milky Way galaxy are right, then there's too little 'near us' to be detected by this 'mass difference' method ... one of the many, many "consistent with all the relevant observations" I mentioned earlier.

Here's another aspect: the interstellar medium (ISM) is known to be a pretty bracing environment: cosmic rays zap through it, high energy photons flood it, and so on. Solids in the ISM are, therefore, being eroded, albeit very slowly (most parts of the ISM anyway; deep inside Bok globules for example such erosion is reduced). Now He can be only a gas in the ISM, because it must be at a temperature of at least 2.73 K (do you know why?), and any solid H will sublime quite fast; so if there were lots of eroding solids in the ISM, there should be lots of elements other than H and He in it ... but there isn't. Of course, if the solids were predominantly boulders, mountains, and dwarf planets ....

From 'near us' to galaxies in general is a journey of millions of observations by thousands of astronomers over several centuries; suffice it to say that while there certainly are galactic environments that are very different from 'near us' (especially in galactic nuclei, and also in star forming regions), the nature of most galactic environments can now be estimated fairly well, provided they are not too far away, and have been observed across the whole electromagnetic spectrum (most of it anyway), with the full range of telescopes, instruments, and techniques we now have at our disposal.

So, no swarm of mini-black holes (but more on this later).

[DeiRenDopa]

... snip ...

So, now using a quote from a series of peer reviewed science publications to directly back up my opinion can now be dismissed purely for the reason that I quoted it? Amazing.(To respond to this later edit ...)

Of course not! :mad::mad::mad::mad:

If you are prepared to be a leeeettle bit patient, and wait until I've covered the cosmology part (point number three), and if when I've done that you still have questions (or want to make a non-PC, non-woo point), I'll be more than happy to try to answer them.

One thing I get mad at you for Zeuzzz is that when I take the trouble to say, clearly, unambiguously, what I intend to do, you come along in your seagull clothes and post something that blatantly ignores my intro, and seems (to me at least) to be a gross attempt at a thread-jack.

It's bad enough that you don't bother to read the OP; it's even worse that you explicitly state you have no intention of discussing anything that you post! It must be a nice life; introduce a whole lot of material on your fave topic, get grilled on just one tiny part of it (and be shown to be serious ignorant, a very poor physics student, or perhaps just not comprehend what you read), then blithely walk away ... only to do a seagull in any other thread that takes your fancy. Don't you wonder why some people call you a troll?

[DeiRenDopa]

One more thing to add about the 'galaxies' part:

Because it's so close, we can study the halo of our Milky Way (MW) galaxy much more closely than that of almost any other galaxy; and we can also, potentially, use the motions of objects in that halo (or just beyond it) to probe the distribution of mass there.

However, to some extent, this takes me into an aspect I said I'd not cover (astronomical observations other than those under the three headings in the OP; specifically, observations of galaxy groups).

"Sky Survey Unveils Star Cluster Shredded By The Milky Way" is the cool title of a 2002 SDSS PR (http://www.sdss.org/news/releases/20020603.pal5.html). One thing that this very nice demonstration of 'tidal stripping' can be used for is to estimate the mass of the MW galaxy, by using the physics of Newton.

The two Magellanic Clouds, so familiar to those who live in the southern hemisphere (outside big cities, of course), are embedded in a stream of hydrogen gas called the Magellanic Stream. In the last years of the 20th century, this was observed, in the radio part of the spectrum, in sufficient detail to show that it too is a result of tidal stripping, of the gas in the SMC and LMC in this case.

Crunching the numbers gives a consistent result: most of the mass of the MW is in the halo.

SDSS has also started to independently confirm the MW halo mass, through the observations of thousands of stars in the halo; it's the same physics as used for planetary nebulae (etc) as probes, only with far, far more datapoints, and far greater precision. For more details, read up on SEGUE (http://segue.uchicago.edu/) (Sloan Extension for Galactic Understanding and Exploration).

[Dancing David]

That is all assuming that the only force at work is gravity, and it also assumes that gravity remains the dominant force when you scale up above the planetary/stellar level. We know that as you assend up scales other forces change their dominance, ie, EM forces usually far dominate gravity at smaller scales, so it is fully possible that as you scale up gravity starts to become secondary to other various forces and instabilities in plasma.

I see the problems with CDM and other similar cosmologies as follows:

• Like geology and unlike particle physics, cosmology is intrinsically an historical science and lacks direct experimental testing for many key hypotheses.

• Data in cosmology are limited to remotely sensed photons; there is no direct, in situ measurement as in space physics or planetary science.

• Most existing cosmology models focus on only one long-range force field (gravity) and ignore potential long-range effects of electromagnetism and plasmas.


On this last point, it is well known that electromagnetism is very effectively shielded out, which allows gravity to generally dominate over long scale lengths. Nevertheless, electromagnetism and plasmas can still be significant because:


“By definition, plasmas are an interactive mix of charged particles, neutrals, and fields that exhibits collective effects. In plasmas, charged particles are subject to long-range, collective Coulomb interactions with many distant encounters. Although the electrostatic force drops with distance (~1/r2), the combined effect of all charged particles might not decay because the interacting volume increases as r3. Magnetic field effects are often global with their connections reaching to galactic scales and beyond” (Goedbloed and Poedts, 2004) (Also see; B. Coppi, Attilio Ferrari, 2000 [full text] (http://books.google.co.uk/books?id=5Ba2-lfLiiAC&printsec=frontcover&dq=plasma+electromagnetic+forces+gravitational#PPA 1,M1))


Of course, this will be hotly contested by Big Bang advocates that require gravity being the only force at work, but is none-the-less a valid point that we have no real reason to discount at the moment.

The potential importance of electromagnetism and plasmas is indicated by the rapidly growing field of plasma astrophysics (see links and references at plasmas.org/ (http://www.plasmas.org/space-astrophys.htm)). As one example of its significance for altering conventional assumptions, (Astrophysics: A New Approach (http://www.springer.com/astronomy/book/978-3-540-22346-7?detailsPage=otherBooks&CIPageCounter=CI_MORE_BOOKS_BY_AUTHOR0), Kundt 2005) shows in detail how observed signatures of existing “black-hole” candidates can be more effectively interpreted as neutron star magnetospheres with accretion disks or neutron star binaries. Efforts to assess the potential impact of the new plasma astrophysics on cosmology issues are just beginning. Hopefully due to this we will not need to fill our universe up with CDM or other dubious metaphysical constructs, the vast and varied properties of plasma should be able to account for many of cosmologies unsolved mysteries in the near future.

Hiya Zeuzzz,

that is great but it doesn't mean much when it comes to the observed motion of all sorts of objects.

When I have asked on this board
1. What force and what size is it that makes objects move faster than they ought in gravity minus dark matter?

I have gotten the folowing answer:

Zip, nada, zilch, niente, zero, empty set.

I am still waiting on that in fact from you.

I asked you to tell me the mass and the charge of the object and then we can calculate the force and field that would be needed to acoount for the acceleration. Then we can see what data is any would support that hypothesis.

But that is the issue, there must be enough of a model to say what should be observed and then see if it is.

As in many things, I am also still waiting on the scale from a 10cm plasma to a galaxy and the Birkeland scale to what features on the sun.

:)

[Dancing David]

:D

"There is no evidence that electromagnetic forces are more important than gravitational forces on a cosmological scale." Can you see my post? or is your mind still blocking out anything that does not adhere to your personal views? I stated why this could be the case, quite clearly and openly.

We'll just have to see about these *cough* 'refutations of plasma cosmology' in the near future when I have time to respond to them. So far i have not seen any, but will be happy to discuss them if you think they exist. I only briefly visit here each day at the mo, and will start a thread all about PC when I have more time. I didn't even mention plasma cosmology anyway in that post. You seem to have inferred this yourself from just reading some standard plasma astrophysics material, which is very telling.

I am giving my opinion (and many other astronomers) on CDM, and whether it is needed or not. Which, if you hadn't noticed, is exactly what this thread is about.

And, I am curious why you (as usual) chose to ignore the material in my previous post and just write about your personal opinion? Are all the links in my post 'woo' in your opinion? because by my standards, it looks like quite established scientific literature all published in respected science journals. If you can refute it, then please, be my guest. :)


we are also still waiting on your numbers and hard data, which are still lacking.

Like how is that neutron star going to avoid gravitational collapse when it is 20,000 solar masses?

Of course neutron stars can produce similar effects but the gravitational collapse of a black hole is rather inevitable given the current model.

BTW, have you done the math on how "EM forces make for a rather rigid structure between stars", The charge, mass and distance between two stars can be approximated and then you can show us how the scale of EM forces is such that at the distance between sasy our star and Alpha Centuari compares to the gravitational force.

You are big on the castles in the sky, still no foundation for it Zeuzz.

Still waiting.

[bokonon]

Eh? That was the mass for a marble sized black hole.

To be a little more precise, a BH with the mass of the earth has a horizon radius of .9 cm. Mind boggling, isn't it?

It might also be useful to know that the radius of a BH horizon scales linearly with the mass. So a solar mass BH will be roughly 500,000 times as large, i.e. 5km in radius.Okay, either I completely misunderstood what a "marble-sized" black hole was, or I'm completely misunderstanding what you're saying now. Bear in mind that when I was in school, the controversial "earth orbits the sun, not vice versa" theory was just beginning to be discussed.

When you say "marble-sized black hole," you mean the event horizon is marble-sized, right? I was (formerly) taking marble-sized to mean the matter within the black hole was marble-sized, while its gravitational dominion extended some distance beyond marble-sized.

When you say "a black hole with the mass of the earth has a horizon radius of .9 cm," I guess you're right, my mind is boggled. That suggests to me that I could stand 9 km away and be in no danger of being drawn in, but I know if I stand 9 km away from the earth I'm going nowhere but down. Am I just confused about "event horizon?" Does the event horizon mean the largest distance at which massless photons moving at lightspeed will not escape, but portly plodding people moving at 1 kph are still fair game at much larger distances? I apologize, I don't want to derail the thread, so feel free to PM me (or tell me to go read up -- maybe I will).

[DeiRenDopa]

Use of 'gravitational lensing' to independently measure the mass of galaxies.

In one sense, Einstein was lucky: very soon after he published the general theory of relativity (GR), one of its predictions was confirmed ... the deflection of light by the mass of the Sun. Of course, GR was already on a sound observational basis, through its post-diction (explanation) of the advance of the perihelion of Mercury, which had been known for many decades. It also helped that Eddington was an enthusiastic supporter, so the observations of stars near the limb of the (eclipsed) Sun were interpreted as confirmation (today we'd call those observations marginal).

It was soon determined that a massive, compact body such as a galaxy should be able to 'lens' a more distant object, such as another galaxy; it was also quickly realised that this would happen only very rarely.

However, in 1979 (http://www.astr.ua.edu/keel/agn/q0957.html) just such an example of (strong) gravitational lensing was discovered, the distant object being a quasar.

With the angular resolution of the Hubble Space Telescope (HST), many examples of such strong lensing can be discovered; a very recent HST PR (http://www.spacetelescope.org/news/html/heic0806.html) gives some examples, and the associated papers and articles explain how many more will likely be discovered in the next few decades.

Estimating the mass of the 'lens' is relatively straight-forward, using textbook physics (GR), although in practice there are complications.

The beauty of this technique is that it is completely independent of the others I've discussed so far: the only thing that matters for GR is the total mass.

And while the number of galaxies whose masses have been estimated using this technique is, today, still small, the results are consistent with those from the other techniques.

There is another kind of gravitational (due to GR) lensing, 'weak lensing', also called 'shear'. In this case, the shape of a distant object is distorted, but only subtly. So for this technique to work, you need lots and lots of distant objects whose undistorted shapes are known. Fortunately, this is exactly what some of the recent large surveys can do, though in this case you get an averaged picture of the mass distribution of a lot of galaxies, rather than of just one.

One possible result from weak lensing observations, of galaxies, is an estimate of the shape of the mass distribution in the halos; the other techniques can give only estimates of the 'mass within distance x' - i.e. the distribution of mass assuming it is spherical (detailed studies of MW halo stars are an exception, as are some studies of some dwarf galaxies; however these are too new to say much about yet, and in any case would apply to only one, or a very few, galaxies).

Weak lensing observations of galaxies is relatively new, however results (estimates of galaxy halo mass) to date are consistent with those from techniques, and there are some early results on the shape of those halos (e.g. there is some 'flattening' and 'ellipticity'), on how they differ from one type of galaxy to another (e.g. giant elliptical galaxies in the centres of rich clusters seem to have different halos than other galaxies), and on how they depend on galaxy environment (e.g. galaxy halos in rich clusters seem to be 'truncated').

Next: so how do we know that this mass, in galaxy (halos), which is not emitting or absorbing light, is CDM?

[DeiRenDopa]

we are also still waiting on your numbers and hard data, which are still lacking.

Like how is that neutron star going to avoid gravitational collapse when it is 20,000 solar masses?

Of course neutron stars can produce similar effects but the gravitational collapse of a black hole is rather inevitable given the current model.

BTW, have you done the math on how "EM forces make for a rather rigid structure between stars", The charge, mass and distance between two stars can be approximated and then you can show us how the scale of EM forces is such that at the distance between sasy our star and Alpha Centuari compares to the gravitational force.

You are big on the castles in the sky, still no foundation for it Zeuzz.

Still waiting.DD, would you mind if I asked you to continue questioning what Zeuzzz has posted, in other threads, somewhere other than in this thread (except where it is directly relevant to this one)?

Perhaps you could start a new thread on questions that Zeuzzz has not answered, much like the one for questions which BAC has not answered?

I know, from other posts you have written, that you seem to be a fan of keeping threads on track ...

[Dancing David]

Posted by Zeuzzz

As one example of its significance for altering conventional assumptions, (Astrophysics: A New Approach, Kundt 2005) shows in detail how observed signatures of existing “black-hole” candidates can be more effectively interpreted as neutron star magnetospheres with accretion disks or neutron star binaries.

Uh huh, right Zeuzzz ,so if the lower bound on the thing at the center of our galaxy is 20,000 solar masses with an upper bound around 300,000 solar masses, how are you asuggesting it isn't a black hole.

Secondly , this is really poor form, you cite a book cover as your source?

Really weak, why not at least type in a quote?

So what keeps 20,000 solar masses from collapsing?

Oh i forgot, you don't like hard numbers, do you?

[DeiRenDopa]

Okay, either I completely misunderstood what a "marble-sized" black hole was, or I'm completely misunderstanding what you're saying now. Bear in mind that when I was in school, the controversial "earth orbits the sun, not vice versa" theory was just beginning to be discussed.

When you say "marble-sized black hole," you mean the event horizon is marble-sized, right? I was (formerly) taking marble-sized to mean the matter within the black hole was marble-sized, while its gravitational dominion extended some distance beyond marble-sized.

When you say "a black hole with the mass of the earth has a horizon radius of .9 cm," I guess you're right, my mind is boggled. That suggests to me that I could stand 9 km away and be in no danger of being drawn in, but I know if I stand 9 km away from the earth I'm going nowhere but down. Am I just confused about "event horizon?" Does the event horizon mean the largest distance at which massless photons moving at lightspeed will not escape, but portly plodding people moving at 1 kph are still fair game at much larger distances? I apologize, I don't want to derail the thread, so feel free to PM me (or tell me to go read up -- maybe I will)..
One of the great things about mass is that, so far as its gravitational effect is concerned, the actual composition of the mass is irrelevant (so long as you are sufficiently far away from it).

So, if, magically, the Earth, our dearly beloved Earth, were to be replaced by a black hole with exactly the same mass, the Moon would not notice, the comsats would continue in their orbits, going round the Earth-mass black hole in exactly 24 hours, and so on.

Ditto if it were replaced with an Earth-mass object composed of neutron star material (nuclear degenerate mass), or white dwarf star material (electron degenerate mass), or pure osmium, or pure hydrogen, or even a blob of neutrinos cooled down to some incredibly small temperature just above absolute zero.

Where the distribution of mass and its composition do matter is for things like tides - the long term orbit of the Moon, for example, will change, depending on how it exchanges angular momentum with the Earth through the tides.

There's also the question of stability ... for example, I'm not sure an Earth-mass of neutronium would be stable (spectacular explosion?), an Earth-mass of hydrogen would dissipate over a few (hundred?) million years (H2 molecules at the top of the atmosphere would be dissociated by the UV from the Sun, and a small, but not insignificant, fraction of the H would have speeds greater than escape velocity ... and that's not counting the interaction with the solar wind), and so on.

[sol invictus]

When you say "marble-sized black hole," you mean the event horizon is marble-sized, right?

Right.

I was (formerly) taking marble-sized to mean the matter within the black hole was marble-sized, while its gravitational dominion extended some distance beyond marble-sized.

The best definition of the size of a black hole is the radius of its horizon. What happens to the matter inside is anybody's guess, but it's probably not very comfortable.

When you say "a black hole with the mass of the earth has a horizon radius of .9 cm," I guess you're right, my mind is boggled. That suggests to me that I could stand 9 km away and be in no danger of being drawn in, but I know if I stand 9 km away from the earth I'm going nowhere but down.

The event horizon is the surface where not even light can escape. I'm guessing you move quite a bit more slowly than light, so stnading 9km away from an earth-mass black hole is going to be hard even though you're well away from the horizon. In fact the force on you (if you managed to stand still) would be something like 400,000 g (g being the force of gravity at the surface of the earth).

.
So, if, magically, the Earth, our dearly beloved Earth, were to be replaced by a black hole with exactly the same mass, the Moon would not notice, the comsats would continue in their orbits, going round the Earth-mass black hole in exactly 24 hours, and so on.


DRD is absolutely correct, but let me just add that 9 km from the black hole is not analogous to 9km from the surface of the earth - it's more like 9 km from the center of the earth. But once you get inside the earth, the gravitational field is altered by the fact that some of the mass is over your head. Getting close to the hole is quite different - there is no mass over your head to screen out its pull.

Actually even if you fell freely towards it (so there was no net force on you), tidal forces would still rip you in half before you got to the event horizon. Ouch.

[bokonon]

Thanks, I've learned something already.

[Zeuzzz]

DD, would you mind if I asked you to continue questioning what Zeuzzz has posted, in other threads, somewhere other than in this thread (except where it is directly relevant to this one)?

Perhaps you could start a new thread on questions that Zeuzzz has not answered, much like the one for questions which BAC has not answered?



I'll do that myself soon, I haven't actually started a thread on PC myself here yet, I have just contributed to already existing ones. And I'm not going to comment on your threads anymore, its just too much hassle dealing with the emotions I seem to stir up (i'm sure you'll be very glad to hear) :D

You can start a thread on the alleged questions I have not answered if you feel the need, but I likely wont reply in detail for another month or so.

Over and out.

[Dancing David]

DD, would you mind if I asked you to continue questioning what Zeuzzz has posted, in other threads, somewhere other than in this thread (except where it is directly relevant to this one)?

Perhaps you could start a new thread on questions that Zeuzzz has not answered, much like the one for questions which BAC has not answered?

I know, from other posts you have written, that you seem to be a fan of keeping threads on track ...

No problem, most certainly. I don't usually care, but I have done so for the same reason I am sure you do. I of course will do as you say.

[geni]

What are the effects that favor theoriesing a new type of matter rather than a new fundimental force?

[ben m]

What are the effects that favor theoriesing a new type of matter rather than a new fundimental force?

In a nutshell, various physicists have written down every New Fundamental Force Law they can think of, and none of them describe the data. Simply saying "Gravity is stronger than GMm/r^2 at large r" doesn't work. Saying "Gravity is stronger than GMm/r^2 at small velocities" doesn't work. Saying "Gravity is stronger than GMm/r^2 at small GMm/r^2" doesn't work (that's MOND; it's the one that came closest to fitting rotation curves). And so on. You're welcome to think up alternatives, but the list of tried-and-failed equations (or even forms of equations, or trends, or scaling laws) is pretty comprehensive.

Meanwhile, one new type of matter ("something cold and noninteracting") fits all of the data across the board. It didn't have to; there were a dozen major chances for the data to support or falsify this, and it's come down on "support" every time.

[geni]

In a nutshell, various physicists have written down every New Fundamental Force Law they can think of, and none of them describe the data. Simply saying "Gravity is stronger than GMm/r^2 at large r" doesn't work. Saying "Gravity is stronger than GMm/r^2 at small velocities" doesn't work. Saying "Gravity is stronger than GMm/r^2 at small GMm/r^2" doesn't work (that's MOND; it's the one that came closest to fitting rotation curves). And so on. You're welcome to think up alternatives, but the list of tried-and-failed equations (or even forms of equations, or trends, or scaling laws) is pretty comprehensive.

No that is modification of a known fundimental force. What about a new one?


Meanwhile, one new type of matter ("something cold and noninteracting") fits all of the data across the board. It didn't have to; there were a dozen major chances for the data to support or falsify this, and it's come down on "support" every time.

However it would require a rewrite of the standard model. So far things that have requied that have tended not to work out too well.

[sol invictus]

No that is modification of a known fundimental force. What about a new one?

There are extremely sensitive tests of so-called "fifth forces" in laboratory experiments on earth. It's very, very difficult to invent one that's strong enough to affect galactic rotation but isn't ruled out by those experiments.

Moreover, the bullet cluster observation pretty much kills the whole idea - that was a direct observation of the dark matter halo of two galaxy clusters.

However it would require a rewrite of the standard model. So far things that have requied that have tended not to work out too well.

Actually we know the SM is not complete, and many extensions of it (proposed for totally different reasons) contain a natural DM candidate.

[ben m]

No that is modification of a known fundimental force. What about a new one?


You're just arguing over semantics. If the real force on a star is GMm*(1/r^2 + const*v^2/r^3), is that a "modification to gravity" or the addition of a new (v^2/r^3) force? That might make a difference later when you want an underlying explanation, but it's an unimportant distinction when you're trying to calculate how things move around a galaxy---and that's all you need to do in order to compare the equation to the observations.

[Wangler]

Saying "Gravity is stronger than GMm/r^2 at small GMm/r^2" doesn't work (that's MOND; it's the one that came closest to fitting rotation curves).

I thought that not only did MOND come closest to fitting rotation curves, it showed a remarkable ability to do so, across a broad range of galactic sizes.

[ben m]

I thought that not only did MOND come closest to fitting rotation curves, it showed a remarkable ability to do so, across a broad range of galactic sizes.

That's what I meant by "came closest". It does a good job of fitting rotation curves, but a terrible job of fitting any of the other data (lensing, the Bullet Cluster, cosmology, etc.). Chris Stubbs at Harvard is analyzing whether MOND fits non-rotational components of galaxy dynamics (disk oscillations) and it sounds like the answer there is going to be another "No".

[DeiRenDopa]

In a nutshell, various physicists have written down every New Fundamental Force Law they can think of, and none of them describe the data. Simply saying "Gravity is stronger than GMm/r^2 at large r" doesn't work. Saying "Gravity is stronger than GMm/r^2 at small velocities" doesn't work. Saying "Gravity is stronger than GMm/r^2 at small GMm/r^2" doesn't work (that's MOND; it's the one that came closest to fitting rotation curves). And so on. You're welcome to think up alternatives, but the list of tried-and-failed equations (or even forms of equations, or trends, or scaling laws) is pretty comprehensive.
No that is modification of a known fundimental force. What about a new one?

Although I've yet to finish even my first point, I'll comment on this now.

A new, fundamental, force cannot be ruled out ... provided that force is not quantified, not specified, or even vaguely hinted at.

The moment you start to write it in words, even using poorly-defined terms, you begin to be able to start answering this question.

When you get to writing any such 'new force' in the form of an equation, you can test the idea.

Hundreds, perhaps thousands, of people have tried to come up with something specific and testable, in the way of a 'new force'. It's very easy to do (make up a new force) ... and in most, if not nearly all, cases it's also just as easy to show that such a new force won't work. Why? Because it does not fit the relevant observations, either directly or indirectly.

One of the wonderful things about astronomy is that there are millions, billions, or even trillions of excellent observations you can use to test new ideas like 'new fundamental forces' ... and most of this data is available to anyone, anywhere in the world, essentially for free (you need an internet connection, preferably broadband, a browser, and appropriate associated software ... and that's all).
Meanwhile, one new type of matter ("something cold and noninteracting") fits all of the data across the board. It didn't have to; there were a dozen major chances for the data to support or falsify this, and it's come down on "support" every time.
However it would require a rewrite of the standard model. So far things that have requied that have tended not to work out too well..

Consider this: barely a half century ago, no one knew about quarks, not even up or down ones, much less charmed, strange, top, or bottom ones.

No one knew that there was a tau neutrino.

And so on.

And why? Many reasons, but one is that the energies physicists had available, to do their atom and nucleus smashing, were really quite low.

Today particle accelerators can get up to ~1 TeV, that's 1012 eV, and the LHC should go a factor of 10 higher.

And all that wonderful new physics was found in an energy range that is what, ~3 orders of magnitude wide (~1 GeV to ~1 TeV).

Big deal.

The Earth is hit, every day, by millions of cosmic rays that have energies at least a million times higher! That's 6 orders of magnitude above the best we can do in labs today. The highest energy cosmic ray recorded had an energy of ~1021 eV ... a full 8 orders of magnitude above the best the LHC is likely to produce.

And the universe very likely can churn out particles with energies considerably higher still, at the rate of billions of tonnes a second ...

And anyway, as has already been noted, the Standard Model, of particle physics, is known to be incomplete - neutrinos don't have mass in it, for example, yet neutrino oscillations require that at least one (two?) do.

[BenBurch]

DeiRenDopa,

In 1975 when I suggested that the answer to the Homestake detector solar neutrino problem may lie in the fact that in the laboratory our neutrinos are very young, with the Homestake detector being calibrated based on a smaller volume of carbon-tet right next to a reactor, and the ones from the sun have travelled for minutes, it was thought preposterous. Now we know about neutrino oscillation. New things come into our particle zoo ALL the time. I am not holding my breath for the end of physics.

[DeiRenDopa]

DeiRenDopa,

In 1975 when I suggested that the answer to the Homestake detector solar neutrino problem may lie in the fact that in the laboratory our neutrinos are very young, with the Homestake detector being calibrated based on a smaller volume of carbon-tet right next to a reactor, and the ones from the sun have travelled for minutes, it was thought preposterous. Now we know about neutrino oscillation. New things come into our particle zoo ALL the time. I am not holding my breath for the end of physics.Thanks.

Just a clarification, if I may ... you're saying that in the ~8 orders of magnitude of particle energy, between LHC and UHECRs detected, there is very likely to be lots of 'new' particle physics?

And that, as with anything 'new' like this, there's absolutely no way to tell, in advance, what sorts of 'new particle physics' will be discovered?

Including (drum roll please) the possibility of a stable CDM particle (or a whole zoo of them)??

[BenBurch]

Well, you never know. The Cosmic Ray physics guys see things that they can't explain sometimes, but nobody ever sets much store by that because its just not repeatable or calibrated except in the most gross fashion. I expect to be surprised.

But, I've also been out of physics for decades now, so what do I know?

:-)

[Tubbythin]

And anyway, as has already been noted, the Standard Model, of particle physics, is known to be incomplete - neutrinos don't have mass in it, for example, yet neutrino oscillations require that at least one (two?) do.

At least two I believe, based on the combination of solar (elecron type initially) and atmospheric (muon type initially) neutrino data.

[DeiRenDopa]

Well, you never know. The Cosmic Ray physics guys see things that they can't explain sometimes, but nobody ever sets much store by that because its just not repeatable or calibrated except in the most gross fashion. I expect to be surprised.

But, I've also been out of physics for decades now, so what do I know?

:-).

The cosmic ray guys (there were no gals back then, were there?) sure did a lot for particle physics - discovery of the positron, muon (and, indirectly, the muon neutrino?), pion, kaon, and more? - back in the 1930s and 1940s.

While particle discoveries have come, since then, from accelerators, the existence of natural particle accelerators far beyond the capabilities of Earthly ones is surely a nice spur for doing more research.

But my main point was that a great deal of 'new physics' was found in the ~GeV and ~TeV range (and much 'different' physics between ~eV and ~MeV, and between ~MeV and ~GeV); surely the universe cannot be so mean as to have a complete desert of 'different' physics over the next ~8 orders of magnitude, can it?

The existence of UHECRs shows particles can be accelerated to these energies, even if we still don't know (for sure) whether those particles are protons, ... iron nuclei, ... or something mildly (or quite) exotic ...

[ben m]

And anyway, as has already been noted, the Standard Model, of particle physics, is known to be incomplete - neutrinos don't have mass in it, for example, yet neutrino oscillations require that at least one (two?) do.

This is a pretty lame argument, and I'd be happy to see it die. The Standard Model is perfectly happy having the neutrinos be massive; the original practice of writing the masses as Zero (rather than as "0 < m < experimental limit") was quite simply wrong. Rather than saying "the standard model is incomplete" over and over again, we simply fixed it.

(There are *other* reasons for the SM to be incomplete, pending an understanding of the strong-CP problem, the Higgs mechanism, etc., but neutrino masses aren't on the list.)

[sol invictus]

This is a pretty lame argument, and I'd be happy to see it die. The Standard Model is perfectly happy having the neutrinos be massive; the original practice of writing the masses as Zero (rather than as "0 < m < experimental limit") was quite simply wrong. Rather than saying "the standard model is incomplete" over and over again, we simply fixed it.

(There are *other* reasons for the SM to be incomplete, pending an understanding of the strong-CP problem, the Higgs mechanism, etc., but neutrino masses aren't on the list.)

I had in mind electro-weak symmetry breaking and the hierarchy problem.

But (as you probably know) the smallness of neutrino masses also implies new physics at a higher scale.

[DeiRenDopa]

And anyway, as has already been noted, the Standard Model, of particle physics, is known to be incomplete - neutrinos don't have mass in it, for example, yet neutrino oscillations require that at least one (two?) do.This is a pretty lame argument, and I'd be happy to see it die. The Standard Model is perfectly happy having the neutrinos be massive; the original practice of writing the masses as Zero (rather than as "0 < m < experimental limit") was quite simply wrong. Rather than saying "the standard model is incomplete" over and over again, we simply fixed it.

(There are *other* reasons for the SM to be incomplete, pending an understanding of the strong-CP problem, the Higgs mechanism, etc., but neutrino masses aren't on the list.)Hmm ...

Perhaps some clarification is needed?

There's lots of stuff out on the internet, and in popsci magazines like Scientific American, which talks about neutrino masses being zero in the Standard Model (SM). Even this Wikipedia article (http://en.wikipedia.org/wiki/Standard_model_(basic_details))* ... but it does make a distinction between "SM classic" and several ways the SM can be tweaked to accommodate them.

To what extent does tweaking the SM to accommodate non-zero neutrino masses automatically lead to something potentially testable, with the LHC for example? For sure the focus is on the Higgs mechanism far more than it is on which tweak of the SM is best for neutrino masses ...

* cited with trepidation, given all the well-known problems with this source ...

[DeiRenDopa]

There's a lot of mass in the halos of a lot of galaxies ... that's a robust conclusion from a lot of astronomical observations.

Galaxy halos are 'dark' in the sense that they do not emit light, nor do they absorb it.

So what are the observations that rule out 'baryons' as the mass in galaxy halos? That's what this post is about.

First, a definition and some clarifications.

By 'non-baryonic matter' I mean matter which is not the kind of stuff that people, rocks, inter-stellar dust, (hydrogen) gas, normal stars, white dwarf stars, and neutron stars are made of. Neutrinos are an example of non-baryonic mass. Are black holes non-baryonic matter? I will leave that question open, for now.

There is certainly baryonic matter in galaxy halos, and a lot of it ... there are stars, there is gas, and some dust ... and there are almost certainly rogue planets, small chunks of iron-nickel, boulders, mountains, and so on. The question I will examine in this post is whether any one kind of baryonic matter could comprise as much as ~10% of galaxy halo mass, or whether the combination of all kinds of baryonic matter could comprise ~80%+ of it (why not ~100%? a backward recognition of observational uncertainty).

Last clarification: the more you require consistency across a range of astronomical observations, the more clearly you can rule out a particular kind of baryonic mass. For example, if you use only the deep HST images, you can't rule out a large population of (very) old, long 'dead' low mass stars in the MW halo (they'd be too faint for the HST to detect); however, if you include all the relevant observations of all stars, to date, you would have good grounds for saying that no such stars exist, period (not to mention that any such stars would have shown up in projects like OGLE (see later)). The same applies to moving away from the cosmological principle - the more you insist that the MW is special, the easier it is to make a case that the halo of some distant spiral is comprised of lots of red and brown dwarfs, for example (no CDM needed).

In a nutshell then:

hot gas/plasma: no; x-ray emission from galaxy halos is too small, and the x-ray spectra of distant sources show no 'local' (absorption) lines

cold gas: no; no 21 cm emission (from H), the spectra of distant sources show no 'local' (absorption) lines

small clumps of cold gas: no; they'd collide to form bigger clumps, they'd form stars, and too few micro-lensing events (see below)

dust: no; distant sources are not reddened (the same principle that gives you red sunsets), nor is there any far-IR emission (which you'd expect, dust should radiate more or less as a blackbody)

faint stars (red dwarfs, brown dwarfs): no; very deep Hubble images of random halo fields show no such stars, too few micro-lensing events

faint stars (white dwarfs): no; as above, plus no evidence of the matter that should accompany them (the gas, dust, etc that would form during the process by which the stars became white dwarfs)

super-Jupiters, Jupiters, etc adrift in the halo: no; too few micro-lensing events

sand, rocks, boulders, mountains, dwarf planets: no; whatever elements they'd be made of, erosion should have enriched the halo gas to an extent it'd be clearly detected; here on Earth we are not being bombarded by such things (there'd be lots of them, if they made up even 10% of the MW halo mass, and they'd be arriving at speeds far in excess of the run-of-the-mill meteorite)

hydrogen snowballs: no; they'd sublime in quick order (all that light in space, all those cosmic rays), and the resulting clouds of H would be easily detected.

A word on micro-lensing: if you stare at enough distant stars (in the Magellanic Clouds, for example) for long enough, you can expect one to show an achromatic, symmetric brightening and fading. This is a micro-lensing event, caused by a foreground object (hopefully a MACHO - massive compact halo object) moving into the line of sight of the distant star and out of it again - another example of (GR) gravitational lensing. This technique is sensitive enough to detect compact objects, from Neptune-mass 'planets' through brown and red dwarfs to ordinary stars (and plans are afoot for facilities capable of detecting Earth- and even Mercury-mass planets) in the MW halo, or at least that part of the halo close to us. There have been several projects of this kind - OGLE, EROS, MACHO, super-MACHO, MOA - the combined results are that there are too few MACHOs to account for even a small fraction of the MW halo mass.

This has been a very brief summary; I'd be happy to expand on any aspect.

Next: possible alternative (non-textbook) physics explanations.

[DeiRenDopa]

Galaxies: possible alternative explanations of the observations relevant to CDM.

Alternative as in 'not found in standard physics textbooks'.

There is only one, that I know of, and that is MOND (MOdified Newtonian Dynamics). This is spectacularly successful at modelling galaxy rotation curves. As it does away with CDM entirely, it is also, by default, good at explaining the MW halo microlensing observations. Its supporters also claim it does a good job of explaining the (nearby) dwarf galaxy observations; I've not checked carefully enough to see how well it addresses other galaxy observations (I'll leave questions about MOND and rich clusters, and MOND and cosmology, until later).

Being a classical theory (or model), MOND cannot be the whole story, no matter how successful it might be at fitting galaxy rotation curves. Various attempts have been made to extend it, to incorporate relativity.

The MOND pages (http://www.astro.umd.edu/~ssm/mond/) is an excellent source of all things MOND, being maintained by one of its leading proponents.

If any MOND fan is reading this, I wonder if you could answer one question I have, about MOND? Why has no MOND supporter sought to try to fit the M31 and M81 galaxy rotation curves? Not only are these two the first observed, but they are the most detailed, as these two galaxies are the in the top five of large, nearby, easily observed spirals.

[Wangler]

If any MOND fan is reading this, I wonder if you could answer one question I have, about MOND? Why has no MOND supporter sought to try to fit the M31 and M81 galaxy rotation curves? Not only are these two the first observed, but they are the most detailed, as these two galaxies are the in the top five of large, nearby, easily observed spirals.

I'm sure you've seen this, but if not, maybe it will help:

http://arxiv.org/abs/astro-ph/0610618

[DeiRenDopa]

I'm sure you've seen this, but if not, maybe it will help:

http://arxiv.org/abs/astro-ph/0610618Thanks!

Well, that explains why M31 is not listed in the MOND pages! :p

A pity really, the '84-0-11' hit-rate touted there is unreliable, and unless McGaugh has been slow and/or sloppy in updating the MOND pages, it counts rather badly for his credibility. I note that Bekenstein did cite the Corbelli and Salucci paper, in his 2006 review paper (good for him).

Now for M81 ...

Oh, and to conclude: as McGaugh is not shy in pointing out, MOND has essentially no free parameters, so failure to fit the M31 rotation curve (if confirmed) is fatal to MOND, period.

[sol invictus]

As it does away with CDM entirely, it is also, by default, good at explaining the MW halo microlensing observations.

Can you really say that? Vanilla MOND isn't a complete theory and doesn't make any predictions for lensing - i.e. it might affect lensing by ordinary matter, or it might not.

One should also mention the Bullet cluster observation, which conflicts very sharply with the MOND prediction.

[DeiRenDopa]

As it does away with CDM entirely, it is also, by default, good at explaining the MW halo microlensing observations.Can you really say that? Vanilla MOND isn't a complete theory and doesn't make any predictions for lensing - i.e. it might affect lensing by ordinary matter, or it might not..
No, not really (I can't say that) ... it was an extreme summary.

More fully: being classical, MOND cannot and does not reproduce GR 'lensing', period (nor does any proponent claim that it can, should, or does).

The MW halo microlensing observations can be interpreted in terms of bounds on the mass in the MW halo in the form of MACHOs (massive compact halo objects), principally low-mass stars. This estimated (MACHO) mass is consistent with a MOND analysis of the MW rotation curve (several papers on this), though the analysis is quite tricky (basically because we are inside the MW).
.One should also mention the Bullet cluster observation, which conflicts very sharply with the MOND prediction.Indeed.

I intend to cover that when I get to point 2 (observational evidence for CDM in rich clusters of galaxies) and also address how well MOND (and MOND extensions) fares with respect to cosmology, when I get to point #3 ... patience please! :D

[robinson]

Not sure which Dark matter topic this would go in, but there is Dark Matter stuff here:
http://astro.uchicago.edu/cosmus/
http://astro.uchicago.edu/cosmus/projects/evolution/
Dark Matter simulation and movies. Interesting stuff.

[Wangler]

Just one or two counter points here:

The main technique used to estimate the distribution of mass in spiral galaxies, as a function of radius from the centre (nucleus) is to derive a rotation curve from observations of the light (radio, etc; electromagnetic radiation - I'll use 'light' as a synonym) emitted by such galaxies.

One technique involves observing how the line of sight velocity changes with position; here (http://www.astro.cornell.edu/academics/courses/astro201/rotcurve.htm) is a concise but accurate summary of how it's done, using a 'long slit' spectrum, and emission lines in the visual waveband.

There are many other ways to obtain a spiral galaxy rotation curve, using different wavebands, different lines; integrated light from sizable chunks of the target galaxy, individual sources (e.g. HII regions, bright stars); and so on.

The results are the same: the curves either keep rising or flatten out, right out (radially) to where no more light seems to be coming from the galaxy.

Using textbook physics, these curves can be interpreted to mean that the mass 'closer in' to the centre of the galaxy (than at any radius) keeps increasing as the radius increases. In fact, no other standard physics textbook interpretation has been proposed, that is also consistent with all the relevant observations.

It is my understanding that the expected rotation curves would peak at some value (say in the galactic disk, at a radius << the radius at which the surface brightness decreases to a background value), and then slowly decrease to zero.

This expected rotation curve would be generated by a massive core, and somewhat less massive spiral arms, as opposed to a strictly uniform mass distribution.

Now, the observed rotation curves behave exactly as DRD says; they do not slowly decrease, but rather they increase or remain constant.

The concordance opinion is that this is due to a large, massive halo of dark matter surrounding the galaxy.

Now, DM is assumed to interact with visible matter (VM) primarily via gravity.

However, the mass distribution of DM that would provide the observed rotation curves, in conjunction with the known VM mass in the core and disk, has mass density that either remains constant for increasing radius, or possibly increases with increasing radius.

This creates a little bit of a problem, as it would be expected that the DM halo, under gravity alone, would have a mass distribution that would have greater density at the center.

If I am not mistaken, we have to make very specific assumptions about the dynamics of the halo to avoid discrepancies with observations of radial velocity close to the galactic core.

Beyond the faintest (integrated light) edges of galaxies are objects which are moving within the gravitational well of the galaxies. These objects include planetary nebulae, globular clusters, and satellite (dwarf) galaxies.

It may be interesting to note that while the globular clusters' motion around our galaxy do apparently support a DM halo, the internal globular rotational dynamics do not.

=DeiRenDopa;3661266]..........elliptical galaxies, which do not have rotation curves.

I think you meant to say that elliptical galaxies have rotation curves, but they aren't measurable to the radial extent than rotation curves for spirals can be.

The density of this (dark) halo mass decreases with radial distance from the nucleus.

As I mentioned before, the mass density is not at it's maximum at the center of the halo, however. I find this troubling from a dynamical point of view, as it seems to require some sort of self-interacting DM.

[DeiRenDopa]

This is a very big topic Wrangler; I applaud you for taking the time to try to understand it! :)

Looking at just two things, for now:
... snip ...Beyond the faintest (integrated light) edges of galaxies are objects which are moving within the gravitational well of the galaxies. These objects include planetary nebulae, globular clusters, and satellite (dwarf) galaxies.
It may be interesting to note that while the globular clusters' motion around our galaxy do apparently support a DM halo, the internal globular rotational dynamics do not.I think you misunderstood what you read ...

The internal motions of globular clusters have nothing to do with any DM halos of the galaxies they are in orbit around.

From the motions of globular cluster stars you can conclude that the (radial) mass distribution is consistent with the estimated mass distribution due to the observed stars alone (actually you have to extrapolate to include the stars too faint to be detected). In other words, there is (apparently) no CDM in globular clusters.

This is one very interesting result! :D

However, it is beyond the scope of what I want to cover in this thread (observational evidence for CDM).
..........elliptical galaxies, which do not have rotation curves.
I think you meant to say that elliptical galaxies have rotation curves, but they aren't measurable to the radial extent than rotation curves for spirals can be.

... snip ...Again, I think you have misunderstood ...

Elliptical galaxies do not have rotation curves, because the stars (and what little gas and dust there is) do not move about the centre (of mass) with the kind of common motion found in spiral galaxies (the same is true of globular clusters and (many) dwarf galaxies).

If you could trace the orbits of a million random stars in an elliptical, you'd find they go every which way; as a whole, elliptical galaxies have essentially zero net angular momentum (unlike spirals).

What I think you are referring to is the 'velocity dispersion' - along any line of sight, the distribution of the speeds of the stars (in the elliptical).

[sol invictus]

However, the mass distribution of DM that would provide the observed rotation curves, in conjunction with the known VM mass in the core and disk, has mass density that either remains constant for increasing radius, or possibly increases with increasing radius.

That is not correct. Basic Newtonian mechanics is enough to tell you that the mass density in the halo should decrease like 1 over radius squared to provide a flat rotation curve:

$m v^2/r = G m M(r) /r^2 \rightarrow M(r) \propto r \rightarrow \rho(r) \propto 1/r^2$

[robinson]

Elliptical galaxies do not have rotation curves, because the stars (and what little gas and dust there is) do not move about the centre (of mass) with the kind of common motion found in spiral galaxies (the same is true of globular clusters and (many) dwarf galaxies).


Incorrect.

[sol invictus]

Incorrect.

No, Robinson.

DRD is correct (as usual), and you are wrong in a particularly smarmy way (as usual).

[robinson]

Most elliptical galaxies have very little rotation, but some have significant velocity gradients along their optical major axis.

See
Rotationally supported Virgo dwarf elliptical galaxies
L. van Zee , E.D. Skillman and M.P. Haynes

The Proceedings of the International Astronomical Union (2005), 1: 68-72

[robinson]

By all means, just keep saying stuff, without offering a reputable source for the information. It is amusing.

Here is another paper that might be on topic.

Galaxy Rotation Curves Without Non-Baryonic Dark Matter
http://arxiv.org/abs/astro-ph/0506370

I bring it up because it mentions the rotation of elliptical Galaxies.

[DeiRenDopa]

Most elliptical galaxies have very little rotation, but some have significant velocity gradients along their optical major axis.

See
Rotationally supported Virgo dwarf elliptical galaxies
L. van Zee , E.D. Skillman and M.P. Haynes

The Proceedings of the International Astronomical Union (2005), 1: 68-72Indeed.

It's difficult to write just a few words, as a summary, and also cover all bases, and you can find lots of examples in my posts (you don't even have to try very hard). Earlier, for example, I said "As it does away with CDM entirely, it is also, by default, good at explaining the MW halo microlensing observations"; sol called me on this (rightly), it was terse to the point of being highly misleading.

So too with the paper you cite - rotation has been observed in some elliptical galaxies.

But wait! There's more!!

Here is what I actually wrote:Elliptical galaxies do not have rotation curves, because the stars (and what little gas and dust there is) do not move about the centre (of mass) with the kind of common motion found in spiral galaxies (the same is true of globular clusters and (many) dwarf galaxies).Hmm ...

I plead guilty your (robinson) Honour; I did not, in this sentence, clearly distinguish (normal, or giant) ellipticals from dwarf ellipticals.

In my defence, however, I would like to point out that I knew that observations of some dwarf galaxies can be interpreted as rotation curves, and that I knew these some included certain dwarf ellipticals (in the Virgo cluster, no less) ... that's one reason* why I used "(many)" to qualify "dwarf galaxies".

These kinds of clarifications and caveats are many, given the complexity and richness of the observations. To give just one other example: there is a class of galaxy called 'lenticulars'. They look like spiral galaxies without (much) gas or dust, but they do, generally, have rotation curves. However, from some viewing angles it is difficult to distinguish an elliptical from a lenticular, especially if the resolution is also low, so some galaxies classified as ellipticals may, in fact, be lenticulars!

Perhaps a more fruitful approach to this might be for you to ask for clarification, rather than respond with a bald 'Incorrect'?

Or should I add, for your benefit, some boilerplate caveat before almost everything I post?

* another is that some dwarf galaxies, not dwarf ellipticals, show clear rotation curves - there is a class of dwarf galaxies called 'dwarf spirals', for example.

[Wangler]

This is a very big topic Wrangler; I applaud you for taking the time to try to understand it! :)

Well, thanks for starting the thread, and giving us a forum to discuss!

Looking at just two things, for now:
I think you misunderstood what you read ...

The internal motions of globular clusters have nothing to do with any DM halos of the galaxies they are in orbit around.

From the motions of globular cluster stars you can conclude that the (radial) mass distribution is consistent with the estimated mass distribution due to the observed stars alone (actually you have to extrapolate to include the stars too faint to be detected). In other words, there is (apparently) no CDM in globular clusters.

This is one very interesting result! :D

However, it is beyond the scope of what I want to cover in this thread (observational evidence for CDM).

Well, I think that the fact that these large associations that are called globular clusters, some of which are nearly the size of a dwarf spherical galaxies (Omega Centauri, for example), do not show evidence for dark matter in their stellar rotational profiles is a piece of observational evidence that should be considered in a critical assesement of the evidence as a whole.

Again, I think you have misunderstood ...

Elliptical galaxies do not have rotation curves, because the stars (and what little gas and dust there is) do not move about the centre (of mass) with the kind of common motion found in spiral galaxies (the same is true of globular clusters and (many) dwarf galaxies).

If you could trace the orbits of a million random stars in an elliptical, you'd find they go every which way; as a whole, elliptical galaxies have essentially zero net angular momentum (unlike spirals).

What I think you are referring to is the 'velocity dispersion' - along any line of sight, the distribution of the speeds of the stars (in the elliptical).

Indeed, I think I did misunderstand this concept. Thanks for the clarification.

Can we look at the 'velocity dispersion' and draw conclusions, or do we just rely on the other observations that you mention for ellipticals?

[DeiRenDopa]

By all means, just keep saying stuff, without offering a reputable source for the information. It is amusing..
If that's what you want, by all means simply ask for it ... do you think it is not available?

One reason I am puzzled by this comment of yours is that when I took the trouble, in other threads, to provide such "reputable source(s)", in response to your (generally good) questions, you seem to have simply ignored them.

Another, more pertinent, reason for not providing such is that, based on what you have written about your own training and abilities to read and understand physics (etc) papers, I suspect you won't get much from any such source.

Would you mind clarifying please?
.Here is another paper that might be on topic.

Galaxy Rotation Curves Without Non-Baryonic Dark Matter
http://arxiv.org/abs/astro-ph/0506370

I bring it up because it mentions the rotation of elliptical Galaxies.Indeed.

But did you actually read this paper, robinson?

Here's what it says, in the caption for Figure 4 (to take just one relevant example; I added bolding):Rotation curve for the elliptical galaxy NGC 3379. Both rotation curves are the same best fit to a parametric mass distribution (independent of luminos1ity observations) – a two parameter fit to the total galactic Mass, M, and a core radius, rc. The red points (with error bars) are samplings of the circular velocity profile constrained by orbit modeling according to Romanowsky et al. (2003, 2004). The solid black line is the rotation curve determined from MSTG, the dash-dot cyan line is the rotation curve determined from MOND.And what do Romanowsky et al. (2003, 2004) have to say, concerning the observations they made?

Here's the first part of the caption from Figure 4 of the 2003 paper (just an example; I added bolding):Line-of-sight velocity dispersion profiles for three elliptical galaxies, as a function of projected radius in units of the effective radius.Hmm ...

Perhaps you could explain for all readers, robinson, how Brownstein and Moffat were able to derive a rotation curve from Romanowsky et al. (2003)'s line-of-sight velocity dispersion profiles (or the underlying raw data)?

Or, maybe, just maybe, you did some searching (using Google?), turned up some phrases that seemed to contradict what I had written, and just posted merrily away ... without understanding the underlying physics, must less what the actual papers you found were saying?

This really doesn't become you robinson; quote mining is a tactic more suited to explicitly anti-science folk like ID proponents (or Zeuzzz) than a cynical sceptic ... at least, that's my opinion.

[DeiRenDopa]

Well, thanks for starting the thread, and giving us a forum to discuss!.
You're welcome.

I should add that if you are interested in diving deeper you might consider going to BAUT (http://www.bautforum.com), which is run by Phil Plait (well known to JREF regulars, I think) and Fraser Cain (he of Universe Today fame). There are quite a few professional astronomers there, and they have an excellent Q&A section.

If you want to get into the physics, you might consider going to PhysicsForums (http://www.physicsforums.com), in addition to taking classes. I was going to suggest this in answer to the other parts of your post (as sol has said, you misunderstood what I wrote - perhaps I wasn't clear enough? - and you misunderstand the relevant, Newtonian, physics).
.Looking at just two things, for now:
I think you misunderstood what you read ...

The internal motions of globular clusters have nothing to do with any DM halos of the galaxies they are in orbit around.

From the motions of globular cluster stars you can conclude that the (radial) mass distribution is consistent with the estimated mass distribution due to the observed stars alone (actually you have to extrapolate to include the stars too faint to be detected). In other words, there is (apparently) no CDM in globular clusters.

This is one very interesting result!

However, it is beyond the scope of what I want to cover in this thread (observational evidence for CDM).
Well, I think that the fact that these large associations that are called globular clusters, some of which are nearly the size of a dwarf spherical galaxies (Omega Centauri, for example), do not show evidence for dark matter in their stellar rotational profiles is a piece of observational evidence that should be considered in a critical assesement of the evidence as a whole.OK, I'm confused ...

Whether globular clusters show evidence for CDM or not is surely irrelevant to the observational evidence showing that galaxies, in general, do?

I mean, we don't demand that globular clusters show evidence of dust and PAH just because spiral galaxies do, do we? Ditto SMBH (super-massive black holes).

Oh, and in light of the difficulty robinson is having with understanding what I write (and you too), would you please avoid the term "stellar rotational profiles" when talking about globular clusters? Or at the very least define it in such a way to link it with 'velocity dispersion', or some other standard term?
.
Indeed, I think I did misunderstand this concept. Thanks for the clarification.

Can we look at the 'velocity dispersion' and draw conclusions, or do we just rely on the other observations that you mention for ellipticals?Yes, you can ... but it's very tricky to do ...

For starters, the parts of ellipticals (not dwarf ellipticals!) that have sufficient surface brightness for reliable velocity dispersions to be observed are not, generally, expected to be dominated by CDM (the stars etc are the dominant form of mass there). Then tracers of gravitational potential ('well') beyond the visible edge - such as planetary nebulae or satellite galaxies - are rather few, the observations difficult, and the interpretations subject to many uncertainties.

The Romanowsky et al. (2003, 2004) papers are landmark ones; the 2003 one for example has been cited by 138 others (according to ADS); their conclusions have been subject to much scrutiny (which is ongoing) ...

[Wangler]

That is not correct. Basic Newtonian mechanics is enough to tell you that the mass density in the halo should decrease like 1 over radius squared to provide a flat rotation curve:

$m v^2/r = G m M(r) /r^2 \rightarrow M(r) \propto r \rightarrow \rho(r) \propto 1/r^2$

But, wouldn't we expect a density decreasing in proportion to the square of the radius, for a halo of self-gravitating matter?

Just as you show!

But, this implies that the density ramps up quickly as r->0. That doesn't match what we see in the rotation curves, is it? Doesn't the effect of DM "peter out" as r gets smaller? Is there a simple explanation for that?

[Wangler]

OK, I'm confused ...

Whether globular clusters show evidence for CDM or not is surely irrelevant to the observational evidence showing that galaxies, in general, do?

I mean, we don't demand that globular clusters show evidence of dust and PAH just because spiral galaxies do, do we? Ditto SMBH (super-massive black holes).

I guess your point is correct. The fact that globular clusters do not show evidence for DM in their velocity dispersion (and I mean that this indicates they have little or no DM associated with them) has no bearing on the observational evidence showing that galaxies, in general, do.

Rather, it points to a different question concerning the reasons why one association of matter (globs) behave differently than others (dwarfs).

Obviously, there are some scale differences, as well as compositional differences. But, I would expect these differences to play a minor role when talking about something like DM that is beholden to none but gravity.

Oh, and in light of the difficulty robinson is having with understanding what I write (and you too), would you please avoid the term "stellar rotational profiles" when talking about globular clusters? Or at the very least define it in such a way to link it with 'velocity dispersion', or some other standard term?

That's what you get for engaging a layman in discussions of this nature!

This layman thanks you for having this discussion, again!

Yes, of course, I will try to be better in my terminology in this regard.

I have also heard a term associated with velocity dispersion, that helps to further describe the one around a spiral (very ordered, many stars moving in the same direction, etc) and the one around an elliptical, for example.

I cannot find that terminology again; would it be a "warm velocity dispersion" for an elliptical, and a "cool velocity dispersion" for a spiral, or something like that? Do you know what I am talking about?

[sol invictus]

I'm confused. You said:


However, the mass distribution of DM that would provide the observed rotation curves, in conjunction with the known VM mass in the core and disk, has mass density that either remains constant for increasing radius, or possibly increases with increasing radius.

This creates a little bit of a problem, as it would be expected that the DM halo, under gravity alone, would have a mass distribution that would have greater density at the center.

That's incorrect, as I have just shown. To produce a flat rotation curve (a velocity which doesn't depend on radius) one needs a density that falls of like 1/r^2, not a density that grows or remains constant.

So I don't understand your next comment:

But, wouldn't we expect a density decreasing in proportion to the square of the radius, for a halo of self-gravitating matter?

Just as you show!

I'm not sure what we should expect for the density - that depends on initial conditions and interactions. Certainly I'm not aware of any simple or general argument that predicts 1/r^2 - are you? But that's what we need in order to explain the observed flat rotation curves.

[Wangler]

I'm confused. You said:



That's incorrect, as I have just shown. To produce a flat rotation curve (a velocity which doesn't depend on radius) one needs a density that falls of like 1/r^2, not a density that grows or remains constant.

So I don't understand your next comment:



I'm not sure what we should expect for the density - that depends on initial conditions and interactions. Certainly I'm not aware of any simple or general argument that predicts 1/r^2 - are you? But that's what we need in order to explain the observed flat rotation curves.

Sorry, Sol, I can see why my post confused you. I have since edited, with another comment regardign the dependence of density on radius.

I thought that a self-gravitating sphere of massive particles, when equilibrium is achieved, would have a density that is proportional to 1/r^2.

Not valid for the singularity at r=0, though.

[Wangler]

cold gas: no; no 21 cm emission (from H), the spectra of distant sources show no 'local' (absorption) lines

Hey, don't some people, who think that EM could explain the galaxy rotation curves, assume these gas clouds are subject to EM forces, because they are ionized, or charged? DIscounting any polar effects due to the charge distribution in the H atom/molecule itself.

But, isn't the 21cm emission from neutral hydrogen, so EM fields wouldn't affect this gas the same way it would affect truly ionized gas?

[sol invictus]

I thought that a self-gravitating sphere of massive particles, when equilibrium is achieved, would have a density that is proportional to 1/r^2.


Why did you think that?

As you point out, such a density profile is obviously not valid at small r. But it is equally obviously not valid at large r (because otherwise the total mass would be infinite). So at most such a behavior could be valid over an intermediate range of radii (and evidently is, or close, for DM halos).

As I said, I'm not aware of an explanation of why that should be the profile (other than the results of simulations), but perhaps there is one?

[Wangler]

Why did you think that?

As you point out, such a density profile is obviously not valid at small r. But it is equally obviously not valid at large r (because otherwise the total mass would be infinite). So at most such a behavior could be valid over an intermediate range of radii (and evidently is, or close, for DM halos).

As I said, I'm not aware of an explanation of why that should be the profile (other than the results of simulations), but perhaps there is one?

I should point out that I am assuming hydrostatic equilibrium. For the DM halo, that may or may not be the case, but it seems like a reasonable firs step.

From my point of view, the density, as a function of radius, for a DM halo is non-trivial. From where I stand, the more non-trivial the density function gets, the harder it becomes to explain the density function in terms of gravity and massive particle dynamics alone.

Also, as I begin to understand and read about this further, it appears as if the galaxy rotation curves, at small r, show v proportional to r, like you would see in a rigid rotating body. Pretty cool, lots of stuff going on in an innocent looking galaxy. No wonder they call them 'grand design' spirals....there is a lot of physics at work behind the pretty pictures!

[sol invictus]

I should point out that I am assuming hydrostatic equilibrium. For the DM halo, that may or may not be the case, but it seems like a reasonable firs step.

Two questions:

a) Why would hydrostatic equilibrium produce a 1/r^2 density profile? It doesn't do that in stars, for example.

b) How could a DM halo be in hydrostatic equil.? By definition there would have to be a pressure that balances gravitational force, but DM only interacts gravitationally (at least that's the usual assumption, and there are good reasons for it) so there cannot be any such pressure.

Could you please elaborate? I'm not trying to be a pain - I'd genuinely like to know if there's some general reason why DM should have a 1/r^2 profile.

From my point of view, the density, as a function of radius, for a DM halo is non-trivial. From where I stand, the more non-trivial the density function gets, the harder it becomes to explain the density function in terms of gravity and massive particle dynamics alone.

Rather than speculate, why not have a look at the results of numerical simulations? They assume almost nothing - they simply model DM as a non-interacting gas of massive particles with some small density inhomogeneities in the initial condition - and the results match observations quite well.

[robinson]

DRD is correct (as usual), and you are wrong in a particularly smarmy way (as usual).

I plead guilty your (robinson) Honour; I did not, in this sentence, clearly distinguish (normal, or giant) ellipticals from dwarf ellipticals.

No worries mate. We all make mistakes and such. I hoped you would have a chance to expand your statement, with a simple nudge from my humble self, but alas, Sol jumped in and made it worse.

The difficulty with blanket statements about the Universe, is those odd structures and luminous bodies that don't follow our rules.

Galaxies, those abundant glowing sources of so much energy and wonder, are notorious for not fitting our categories and predictions.

Especially (getting back on topic) when it comes to rotation, "Dark Matter" halos, acceleration, even red shifts and number of supernovas. They almost seem set on confounding our best efforts.

Did you see the latest? Another damn mystery. It seems almost every time we do a new exposure with the Hubble, we are faced with yet another object that doesn't fit the theory.

It really is a pain at times.

[sol invictus]

No worries mate. We all make mistakes and such. I hoped you would have a chance to expand your statement, with a simple nudge from my humble self, but alas, Sol jumped in and made it worse.

Ah, the smarmy wrongness is back again... how refreshing.

DRD's statement was completely correct in its context.

[robinson]

No, as he pointed out. Try to keep up.

[DeiRenDopa]

cold gas: no; no 21 cm emission (from H), the spectra of distant sources show no 'local' (absorption) linesHey, don't some people, who think that EM could explain the galaxy rotation curves, assume these gas clouds are subject to EM forces, because they are ionized, or charged? DIscounting any polar effects due to the charge distribution in the H atom/molecule itself.

But, isn't the 21cm emission from neutral hydrogen, so EM fields wouldn't affect this gas the same way it would affect truly ionized gas?Not sure I follow ...

I was, very briefly, going through a list of possible baryonic components of the MW halo (and, by extension, halos of other galaxies), and ruling them out.

Note there is certainly some cold gas, just nowhere near enough to account for the estimated mass in the halo.

Detection, or non-detection, of most things in my list relies on atomic physics (mostly), which is extremely well-understood, for the kinds of baryonic matter examined.

Gases, for example, will emit and absorb at very distinct frequencies, based on the populations in particular states (singly ionised He, for example), giving rise to line spectra, band spectra (molecules), and so on. If you don't see any such lines, then there are no such populations along the line of sight. To get absorption spectra, simply look at a bright, distant source.

Other forms of baryonic matter will emit more or less like a blackbody, which has its own, very distinctive, spectrum.

Fully ionised gas (a plasma) will emit a different kind of spectrum (look up 'bremsstrahlung').

Now if you add magnetic fields (enter caveat, for robinson, here, about the necessary strength of such), you will get another kind of spectrum (look up 'synchrotron radiation').

I wasn't aware than any PC proponent, or anyone for that matter, had proposed that large masses of baryonic matter could be completely invisible, in the environment of the MW halo (enter caveat here about the need to be consistent with all the other relevant observations, such as the fact that we are not being bombarded by hypervelocity bowling balls made of a rhenium-bismuth alloy).

But maybe that's not what you meant ...

[DeiRenDopa]

What is the expected distribution of CDM in a galaxy halo? What constraints can be put on the distribution of CDM in galaxy halos, from observations?

This is big, big topic! :D

Very quickly you will come across 'NFW halos' (after Navarro, Frenk & White), 'cored halos' (halos with constant density cores), 'stellar polytrope' halos, tidally truncated halos, isothermal halos, ... interestingly, here (http://www.physicsforums.com/archive/index.php/t-106229.html) is a page from PhysicsForums on two different types (note that the latex formatting is lost).

I'd rather leave it for now ... not only do I plan to come back to it when I address the last point (cosmology - some of the simulations sol refers to are very relevant!), but also it's not directly relevant to the observational evidence for CDM in galaxy halos.

[DeiRenDopa]

OK, I can't resist ... CONSTRAINTS ON FIELD GALAXY HALOS FROM WEAK LENSING AND SATELLITE DYNAMICS (http://nedwww.ipac.caltech.edu/level5/Sept04/Brainerd/Brainerd_contents.html), by Tereasa G. Brainerd presents an excellent review of one small part of what I've covered, and is of direct relevance to some of Wrangler's recent posts (it also, I hope, satisfies robinson, in terms of depth and provenance of source).

I'll quote from Section 5, leaving out the references and formulae (note that these provide all the depth you could ask for, if you wanted to follow up on what sol wrote ...; I've added some bolding):High-resolution CDM simulations have established the existence of a "universal" density profile for dark matter halos which results from generic dissipationless collapse [...]. This density profile fits objects that span roughly 9 orders of magnitude in mass (ranging from the masses of globular star clusters to the masses of large galaxy clusters) and applies to physical scales that are less than the "virial" radius, r200. Conventionally, r200 is defined to be the radius at which the spherically-averaged mass density reaches 200 times the critical mass density [...].

Navarro, Frenk & White [...] showed that the universal density profile for dark matter halos was fitted well by a function of the form

[Equation 11 (11)]

and halos having such a density profile are generally referred to as "NFW" halos. Here [...] is the critical density of the universe at the redshift, z, of the halo, H(z) is Hubble's parameter at that same redshift, and G is Newton's constant. The scale radius rs [...] is a characteristic radius at which the density profile agrees with the isothermal profile [...], c here is a dimensionless number known as the concentration parameter, and

[Equation 12 (12)]

is a characteristic overdensity for the halo.

Formally, the above fitting function for the radial density profiles of CDM halos converges to a steep, cuspy profile: [...]. The NFW fitting formula, however, was never intended to be extrapolated to very small radii (i.e., radii smaller than the practical resolution limits of the simulations) and much fuss has been made over whether observed galaxies actually show such cuspy inner density profiles [...]. More recent numerical work has shown that the density profiles of CDM halos do not, in fact, converge to a well-defined asymptotic inner slope [...], and it has become increasingly clear that fair and direct comparisons of simulated galaxies with observed galaxies on very small physical scales is an extremely challenging thing to do [...].

Weak lensing and satellite dynamics do not have the ability to provide any information whatsoever on the cuspiness (or lack thereof) in the central regions of galaxies. Instead, both are governed by the large-scale properties of the halos (i.e., the regime in which the NFW profile is known to be an excellent description of the density profiles of CDM halos) and, at least in principle, both have the potential to discriminate between NFW halos and simpler singular isothermal sphere halos.

[sol invictus]

No, as he pointed out. Try to keep up.

He's just extremely accommodating and quite polite. He also hasn't had to deal with you as much as I have, so he's a little more patient.

Anyway, the physics is quite well explained in DRD's posts. Anyone can read them and draw their own conclusions.

[Wangler]

But maybe that's not what you meant ...

I think it is; my point was that some folks say that the rotation curves can be explained by EM effects alone, but I think the information you present says otherwise.

I was speaking from the angle that a lot of the gas that would be expected to interact with galactic EM fields seems to be of the neutral variety.

[Wangler]

Two questions:

a) Why would hydrostatic equilibrium produce a 1/r^2 density profile? It doesn't do that in stars, for example.

I thought that one of the fundamental solutions to a self-gravitating sphere of gas or dust, for example, is one that results in a 1/r^2 density profile. I could probably present my understanding of this solutions governing equations in another post, if needed.

b) How could a DM halo be in hydrostatic equil.? By definition there would have to be a pressure that balances gravitational force, but DM only interacts gravitationally (at least that's the usual assumption, and there are good reasons for it) so there cannot be any such pressure.

Could you please elaborate? I'm not trying to be a pain - I'd genuinely like to know if there's some general reason why DM should have a 1/r^2 profile.

A pain? I am enthused that you all are willing to discuss this stuff, even when my understanding is off a bit..............

I thought that a cloud of dust or gas, as it collapses under self-gravity, will eventually see some pressure, unless we are talking about collisionless cases.

As far as a general reason why DM should have a 1/r^2 profile, isn't that what the rotation curve tells us? Didn't we cover this earlier?



Rather than speculate, why not have a look at the results of numerical simulations? They assume almost nothing - they simply model DM as a non-interacting gas of massive particles with some small density inhomogeneities in the initial condition - and the results match observations quite well.

DRD has provided some links, so I will take a look, thanks!

[Wangler]

OK, I can't resist ... CONSTRAINTS ON FIELD GALAXY HALOS FROM WEAK LENSING AND SATELLITE DYNAMICS (http://nedwww.ipac.caltech.edu/level5/Sept04/Brainerd/Brainerd_contents.html), by Tereasa G. Brainerd presents an excellent review of one small part of what I've covered, and is of direct relevance to some of Wrangler's recent posts (it also, I hope, satisfies robinson, in terms of depth and provenance of source).

I'll quote from Section 5, leaving out the references and formulae (note that these provide all the depth you could ask for, if you wanted to follow up on what sol wrote ...; I've added some bolding):

Thanks, DRD.

This looks like a good start for examining the analytical models for the DM halo mass distributions.

[DeiRenDopa]

I think it is; my point was that some folks say that the rotation curves can be explained by EM effects alone, They do?!?!? :eye-poppi

Who are these people? Where have they presented such an explanation? I mean, one that also explains (or at least is not inconsistent with) all other relevant observations ...
.but I think the information you present says otherwise..
I don't think it does.

Nothing that I presented says that spiral galaxy rotation curves cannot "be explained by EM effects alone"; in fact, I don't think I even hinted at any such explanation, much less said it could be ruled out by anything I presented.

Maybe you are referring to my earlier statement, to the effect that I wasn't aware of any explanation, other than the one I presented, based on textbook physics? or any alternative, based on physics not found in standard textbooks?

One can never say never ... someone may come up with a truly amazing explanation, based on textbook physics, that no one had even considered before, and they may do so tomorrow.

However, to the extent that the following story is indicative, I doubt it.

As you know, Fermat's Last Theorem was solved, by Andrew Wiles, over 300 years after it was written. No doubt there were hundreds of (failed) attempts during that time. Not so well known is that n = 5 and n = 7 proofs were established by the 1840s, by what we'd call, today, professional mathematicians (n = 3 and n = 4 fell much earlier to Euler and Fermat, respectively). Despite the apparent simplicity of the theorem, and despite vast numbers of attempts by amateurs, it seems no amateur even came up with an n = 7 proof, independently, much less any other more powerful proofs.

It's a powerful myth that outsiders can come up with truly revolutionary ideas in mathematics, physics, etc (in a form that is capable of being worked on; i.e. not word salad), but while there are undoubtedly many people who could come up with such ideas, it seems there are essentially no examples of any such success, at least in the last century or three.
.
I was speaking from the angle that a lot of the gas that would be expected to interact with galactic EM fields seems to be of the neutral variety.I don't understand this; would you mind saying a bit more?

[sol invictus]

I thought that one of the fundamental solutions to a self-gravitating sphere of gas or dust, for example, is one that results in a 1/r^2 density profile. I could probably present my understanding of this solutions governing equations in another post, if needed.

Yes, please do.

I thought that a cloud of dust or gas, as it collapses under self-gravity, will eventually see some pressure, unless we are talking about collisionless cases.

I don't think so. Conservation of energy and angular momentum will probably prevent the density at the center from ever getting very large, and I would expect that together with the weakness of DM interactions means the pressure is never significant. But I could be wrong.

As far as a general reason why DM should have a 1/r^2 profile, isn't that what the rotation curve tells us? Didn't we cover this earlier?

Sorry for the miscue - I meant apart from that. In other words, if we simply posit that DM exists, can we predict that it will form halos with a 1/r^2 profile?

[Wangler]

They do?!?!? :eye-poppi

Who are these people? Where have they presented such an explanation? I mean, one that also explains (or at least is not inconsistent with) all other relevant observations ...
..
I don't think it does.

Nothing that I presented says that spiral galaxy rotation curves cannot "be explained by EM effects alone"; in fact, I don't think I even hinted at any such explanation, much less said it could be ruled out by anything I presented.

Maybe you are referring to my earlier statement, to the effect that I wasn't aware of any explanation, other than the one I presented, based on textbook physics? or any alternative, based on physics not found in standard textbooks?

One can never say never ... someone may come up with a truly amazing explanation, based on textbook physics, that no one had even considered before, and they may do so tomorrow.

However, to the extent that the following story is indicative, I doubt it.

As you know, Fermat's Last Theorem was solved, by Andrew Wiles, over 300 years after it was written. No doubt there were hundreds of (failed) attempts during that time. Not so well known is that n = 5 and n = 7 proofs were established by the 1840s, by what we'd call, today, professional mathematicians (n = 3 and n = 4 fell much earlier to Euler and Fermat, respectively). Despite the apparent simplicity of the theorem, and despite vast numbers of attempts by amateurs, it seems no amateur even came up with an n = 7 proof, independently, much less any other more powerful proofs.

It's a powerful myth that outsiders can come up with truly revolutionary ideas in mathematics, physics, etc (in a form that is capable of being worked on; i.e. not word salad), but while there are undoubtedly many people who could come up with such ideas, it seems there are essentially no examples of any such success, at least in the last century or three.
.I don't understand this; would you mind saying a bit more?

I am sorry, it appears as if I am confusing both you, and Sol.

First for my confusion with you: on one of these "alternate cosmology" websites, or techncial papers, they mentioned that the galaxy rotation curves could be explained by interactions of a gas halo with a galactic EM field.

I took the following post sentences:

hot gas/plasma: no; x-ray emission from galaxy halos is too small, and the x-ray spectra of distant sources show no 'local' (absorption) lines

cold gas: no; no 21 cm emission (from H), the spectra of distant sources show no 'local' (absorption) lines

small clumps of cold gas: no; they'd collide to form bigger clumps, they'd form stars, and too few micro-lensing events (see below)

to mean that the gas is in neither abundance or form that would make that position viable.

I may have read to much in these sentances on my own account.

[Wangler]

Yes, please do.

I will do that, and post later this evening. It feels like an oral exam! :D



I don't think so. Conservation of energy and angular momentum will probably prevent the density at the center from ever getting very large, and I would expect that together with the weakness of DM interactions means the pressure is never significant. But I could be wrong.

We will dissect my later post, and see!

Sorry for the miscue - I meant apart from that. In other words, if we simply posit that DM exists, can we predict that it will form halos with a 1/r^2 profile?

Maybe, or maybe not. I was taking a simplistic approach. I know that the mass distribution for galaxies is sometimes modeled as superpositions of simpler cases: spherical for the bulge, and spheroidal for the disk.

I was just thinking of adding another simplistic distribution for the dark matter.

One of my reasons for this is because the spherical assumtion for the bulge, and the spheroidal assumption for the disk both make intuitive sense for me.

So, with the dark matter, what makes sense is a large, spherical distribution of gravitating matter. Obviously, a 1/r^2 distribution will not hold for r->0 or r->infinity, but it seems to make for a good starting point.

Note that the way that I think about it, the more complex the model for the mass distribution (for either luminous or dark matter), the less desirable from my point of view.

[Wangler]

Yes, please do.

Sol,

the solutions I have seen start like so:

We have, of course, the gravitational potential:

\bigtriangledown ^ 2\phi = 4\pi G\rho

and the equation for hydrostatic equilibrium:

\bigtriangledown P + \rho \bigtriangledown \phi = 0

These are tied together by the equation of state, which I believe relates the speed of sound in the mass distribution to the pressure:

P = \rho {{c}_{s}}^{2}

If we begin assuming that:

\rho = \rho (r)

Then I can barely understand how to get to:

\left (\frac{{{c}_{s}}^{2}}{\rho (r)} \right)\left(\frac{d \rho (r)}{d r} \right) = - \frac {4 \pi G}{{r}^{2}}\int_{r}^{0}{r}^{2} \rho (r) dr

Now, I can integrate the expression on the far right, and descriptions of this integral that I have seen in on-line versions of these calculation steps often just call this M(r).

This equation overall is too complex for me, and on-line versions of this calculation use numerical methods to solve this.

A typical numerical solution looks like the attached graph, solution1 (from reference #1).

A typical numerical solution also takes the form of:

\rho (r) = \frac {{{c}_{s}}^{2}}{2 \pi G {r}^{2}} (from reference #1)

Now, I cannot do the numerical solutions, or solve the equation further myself.

I did however, try to get "further along" by assuming the following forms for \rho (r) , and seeing how far I got:

\rho (r) = \frac {\alpha }{{r}^{2}}

and:

\rho (r) = \frac {\alpha }{r}

where \alpha is just an arbitrary constant.

I hope that this math does not make you shudder, but here goes.

First, plugging in \rho (r) = \frac {\alpha }{{r}^{2}} , we have:

\left (\frac{{{c}_{s}}^{2} {r}^{2}}{\alpha} \right)\left(\frac{-2 \alpha}{{r}^{3}} \right) = - \frac {4 \pi G}{{r}^{2}}\int_{0}^{r}\alpha dr

Combinations, and evaluation of the integral leads to:

\left (\frac{- 2{{c}_{s}}^{2}}{r} \right) = - \frac {4 \pi G \alpha} {r}

This reduces to a solution for my arbitrary constant of:

\alpha = \frac {{{c}_{s}}^{2}}{2 \pi G}

This seems to me to confirm the validity of the numerical solution.

If I try plugging in \rho (r) = \frac {\alpha }{r} , I get:

\left (\frac{{{c}_{s}}^{2}{r}}{\alpha} \right)\left(\frac{- \alpha}{{r}^{2}} \right) = - \frac {4 \pi G}{{r}^{2}}\int_{0}^{r}\alpha r dr}

Combinations, and evaluation of the integral, provides:

\left (\frac{- {{c}_{s}}^{2}}{r} \right) = - 2 \pi G \alpha

Which gives:

\alpha = \frac {{{c}_{s}}^{2}}{2 \pi G r}

So, in this case, my arbitrary constant is forcing the density to be dependent upon {r}^{-2} again!

Now, my funky checks of the density function do not prove the uniqueness of the solution that is dependent upon {r}^{-2} , but I think that it shows that for these equations, it is a viable solution.

I am sorry if my calculus is so poor, this is the best way I could present the solution, and try to work through at least some checks of the numerical solution on my own.

Hopefully I haven't done anything too outrageous, although I know I played fast and loose with my integrands and differential variables, I think.

References:

#1 http://www.mpia-hd.mpg.de/homes/dullemon/lectures/starplanet/chap_05_cloudcores.pdf

#2 Primordial Stellar Evolution, E.K.L. Upton, as presented in Stars and Stellar Systems Vol. VII: Nebulae and Interstellar Matter, U of Chicago Press, 1968

[sol invictus]

Thanks, Wangler. I'll think about this and respond later.

[DeiRenDopa]

They do?!?!?

Who are these people? Where have they presented such an explanation? I mean, one that also explains (or at least is not inconsistent with) all other relevant observations ...
..
I don't think it does.

Nothing that I presented says that spiral galaxy rotation curves cannot "be explained by EM effects alone"; in fact, I don't think I even hinted at any such explanation, much less said it could be ruled out by anything I presented.

Maybe you are referring to my earlier statement, to the effect that I wasn't aware of any explanation, other than the one I presented, based on textbook physics? or any alternative, based on physics not found in standard textbooks?
... snip ...
I am sorry, it appears as if I am confusing both you, and Sol.

First for my confusion with you: on one of these "alternate cosmology" websites, or techncial papers, they mentioned that the galaxy rotation curves could be explained by interactions of a gas halo with a galactic EM field.

I took the following post sentences:
hot gas/plasma: no; x-ray emission from galaxy halos is too small, and the x-ray spectra of distant sources show no 'local' (absorption) lines

cold gas: no; no 21 cm emission (from H), the spectra of distant sources show no 'local' (absorption) lines

small clumps of cold gas: no; they'd collide to form bigger clumps, they'd form stars, and too few micro-lensing events (see below)
to mean that the gas is in neither abundance or form that would make that position viable.

I may have read to much in these sentances on my own account.OK, thanks; I think I can see where the confusion is coming from!

Assuming it's Peratt's 'pair of cosmic/giant Birkeland currents can create spiral forms and rotation curves' that you are referring to (if not, please say more) ...

As far as I can tell, Peratt's idea is consistent with all the interpretations of observations of the MW halo, in terms of its baryonic mass content.

But that's not saying very much, because (a far from complete list):

* Peratt didn't publish anything about the expected baryonic content of the MW halo (or that of any spiral galaxy, or any galaxy at all), as far as I know (so {silence} is consistent with anything!)

* only charged objects are included in Peratt's model (i.e. the motions of uncharged objects, in the gravitational field of all masses, is not considered), so observations of the motion of uncharged objects have no bearing on his model

* the simulation codes he used were not, as far as I know, published (so, among other things, no one can independently check his stated conclusions, nor run them again, using different parameters)

* his model is strongly inconsistent with a wide range of (other) astronomical observations; for example, stars could not possibly hold enough charge for their motions to be dominated by the electrical and magnetic fields Peratt requires (either as inputs in the model, or as observed; the ISM magnetic fields, for example).

In short, this is not, and cannot be, an alternative explanation for the observed spiral galaxy rotation curves, based on textbook physics ... because it is not consistent with other, highly relevant, astronomical observations.

No doubt there are hundreds, possibly millions, of such alternatives, some seemingly based on good physics (like Peratt's model), some from the furthest regions of woo-land, and most (no doubt) in between; I see no point at all in even considering them: if they are inconsistent with well-established astronomical observations, why waste time on them?

[Wangler]

Thanks, Wangler. I'll think about this and respond later.


Thanks Sol.

As always, your opinion will be valued.

One thing that I forgot to re-iterate:

1) My current understanding is that it is very likely that the DM halo cannot be represented by a simplistic density-radius relationship, as shown here. Just like the luminous matter density function is not simplistic.

2) The more complex the density-radius relationship, the more burdens are placed on our understanding of the DM halo. Our luminous matter functions are complex, true, but in my opinion they have as their basis many observations of the behaviour of this luminous, baryonic matter which we at least think we understand well.

Basically, if we posit a "low density core" spherical density-radius relationship (for example), we need to have a very good reason to do so, that can be explained by the expected behaviour of the DM material. If we just curve-fit to fit the data, then such a solution ends up being, well, MOND-ish.

[DeiRenDopa]

Thanks Sol.

As always, your opinion will be valued.

One thing that I forgot to re-iterate:

1) My current understanding is that it is very likely that the DM halo cannot be represented by a simplistic density-radius relationship, as shown here. Just like the luminous matter density function is not simplistic.

2) The more complex the density-radius relationship, the more burdens are placed on our understanding of the DM halo. Our luminous matter functions are complex, true, but in my opinion they have as their basis many observations of the behaviour of this luminous, baryonic matter which we at least think we understand well.

Basically, if we posit a "low density core" spherical density-radius relationship (for example), we need to have a very good reason to do so, that can be explained by the expected behaviour of the DM material. If we just curve-fit to fit the data, then such a solution ends up being, well, MOND-ish.Cool, very cool! :D

I don't want to be a party pooper, but I'd hoped that this thread would be about the observational evidence for CDM, not what those observations might (or might not) say about particular kinds of CDM (or how well - or otherwise - certain models of what the CDM might be match the observations) ...

I mean, the observations are the observations, and the interpretation is that there is a lot - and awful lot - of non-baryonic matter in the halos of galaxies like the MW (and many other galaxies, and several other classes of galaxy).

The self-interactions of that CDM, as well as non-gravitational, non-electromagnetic interactions with baryonic matter, is a fascinating topic ... but one that's beyond the scope of this thread ...

[Wangler]

Cool, very cool! :D

I don't want to be a party pooper, but I'd hoped that this thread would be about the observational evidence for CDM, not what those observations might (or might not) say about particular kinds of CDM (or how well - or otherwise - certain models of what the CDM might be match the observations) ...

I mean, the observations are the observations, and the interpretation is that there is a lot - and awful lot - of non-baryonic matter in the halos of galaxies like the MW (and many other galaxies, and several other classes of galaxy).

The self-interactions of that CDM, as well as non-gravitational, non-electromagnetic interactions with baryonic matter, is a fascinating topic ... but one that's beyond the scope of this thread ...

Sorry, DRD. It is at times a herculean effort to keep these threads on subject. Sorry for hijacking repeatedly.

[Wangler]

Thanks, Wangler. I'll think about this and respond later.

Sol,

I am sure you are aware of this, but I want to state the fact anyhow, just so my presentation of this subject is more complete:

For the numerical solution that I provided, it should be obvious that it requires an initial value, and I think that they typcially limit the density at the core:

{\rho}_{r} (0) = finite

Sorry, DRD, off subject again...trying to sneak it in, if I can!

;)

[DeiRenDopa]

Sorry, DRD. It is at times a herculean effort to keep these threads on subject. Sorry for hijacking repeatedly.Just to be crystal clear ...

I'm as interested as anyone, maybe more so, in what the nature of the CDM that the observations I have, and will, discuss here point to.

And it is a fascinating subject ... and lots of (astronomy) papers have been written on how various observations can constrain the nature of CDM (or not), and what research programmes might (or might not) do, in terms of testing various hypotheses about the nature of CDM, etc, etc, etc.

However, I started this thread with the express intention of covering the observational evidence for the existence of CDM, in three domains (galaxies, rich clusters of galaxies, and cosmology).

If there are questions on the observations, or the interpretations of those observations (that there is a lot of CDM), I'd like to address them ... first.

If the observations leave any reader unconvinced (of the existence of CDM, not its nature (beyond being non-baryonic)), I'd be most interested to hear from you, and why you are not yet convinced.

[Dancing David]

Thanks to all, I am enjoying the read, some and most of the math is whizzing past but it seems to make sense to my muddled mind.

[DeiRenDopa]

Could the mass in the MW halo - and the halos of other galaxies - be largely black holes?

This is a question I left open earlier, and is the last thing I'll look before going on to point two (observational evidence for CDM in rich clusters of galaxies), unless there are further questions or issues to do with the observational evidence for CDM in galaxies.

At one level, the black hole (BH) population of the MW halo can be constrained, observationally, in much the same way as that of (other) MACHOs can - by the number and type of microlensing events, and by the number of hyper-velocity BHs observed zipping through the solar system (and, maybe, colliding with a planet, moon, asteroid, comet, ...).

However, these constraints are considerably weaker than for the corresponding classes of baryonic matter, because ... well, for several reasons.

First, while there is much uncertainty over many details, the size/mass distribution of various classes of baryonic matter - from stars to brown dwarfs to planets to ... to dust - is reasonably well constrained: given an observed abundance of, say, brown dwarfs, or dust (especially AND dust!), the amount of mass per mass interval (say a log factor of 0.3) can be fairly reliably estimated. This can be done because dust, ... brown dwarfs have been observed, and the formation processes of the various kinds of objects reasonably well described and quantified.

Not so for BHs - for starters, no BHs with masses much below 1 sol have been observed, period; for seconds, no verified processes for the formation of such BHs have made their way to textbooks yet. And that is thus of no help in constraining the mass of such BHs in the MW halo, their mass distribution, or their locations and motions.

Second, while there are some limits that can be put on BH abundance, from the lack of any accretion disks (more later), these are very wide limits.

Finally, while it is expected that BHs will evaporate, in a burst of Hawking radiation, and while no such radiation has been observed (yet), from anywhere in the sky, it may turn out that BHs do not behave this way at all (perhaps 'Hawking radiation' is an extrapolation, of twenty, too far). So, for example, there could a large population of primordial BHs ... and we wouldn't have seen any evidence of it yet.

Black holes and accretion disks: using the (misleading) popular metaphor of BHs 'sucking in' external mass, a BH travelling at some non-zero velocity relative to the medium it moves through (the MW halo, say) will 'suck in' the stray gas and dust (and occasional bit of sand, rock, and even rogue planet or star) it encounters. However, because the BH is extremely small, the gravitational acceleration near its event horizon is ginormous, so matter which is 'sucked in' will be moving at relativistic speeds when it disappears from the observable universe. Further, most such matter will not fall directly in, but will first go into orbit about the BH (conservation of angular momentum) ... and will form an accretion disk. The physics of this system (BH+baryonic matter accretion disk) is still poorly understood, but one thing is for sure: to a distant observer, it will be extremely luminous!

So, except for populations of very, very small mass BHs, if the MW halo has had BH populations for billions of years, there should be at least some luminous accretion disks visible today ... yet none have been seen so far. Ergo, the numbers of such BHs can be constrained.

[DeiRenDopa]

Observational evidence for CDM in rich clusters of galaxies.

It was the Coma cluster, the closest rich cluster, in the 1930s, and Zwicky, that got the CDM ball rolling ... though 'non-baryonic' hadn't been invented yet.

The technique he used is straight out of the physics textbook, under the section devoted to Newtonian gravity, and employed mathematical tools Newton invented.

Basically, Zwicky found that the range of line of sight velocities of the galaxies in the Coma cluster he took spectra of implied an amount of mass in that cluster far, far greater than that implied by counting the galaxies and estimating the mass of stars and gas and dust in them (based on the amount of light detected by his instruments - photographic plates). For Wrangler: google on 'virial theorem'.

Fast forward ~7 decades.

General Relativity-based gravitational lensing, both 'strong' and 'weak', provides an independent method of estimating the total mass of rich clusters; quite a few such clusters have now been investigated with this technique, and the results are the same as those from investigations using the virial theorem.

Another technique for estimating the mass in rich clusters is the same as one used for estimating the mass of certain giant elliptical galaxies, and it involves studying x-ray emission. This technique is quite powerful for rich clusters, so it's worth spending some time on it ... which I'll do in a later post. The bottom line is the same: the estimated mass in rich clusters is the same (within the relevant observational and analytical uncertainties) as that derived from the two other techniques mentioned.

For studying CDM in rich clusters, there's a new technique available, based on a section of the physics textbook we've not opened yet - if you want to read ahead, look up "the Sunyaev-Zel'dovich effect" (a.k.a. the SZ effect) and "Compton scattering" and "inverse Compton effect".

To end this post, a comment on where the baryonic mass in rich clusters is found: not in the galaxies of the clusters, but in the medium between the galaxies, the inter-galactic medium (IGM) or intra-cluster medium. Some of this IGM is dust, some is stars (and planets and rocks and ...) stripped from cluster galaxies during collisions and mergers, some is stars (and planets and rocks and ...) shot out of cluster galaxies in various violent, rare events, some undoubtedly is cold gas, .... but almost all of it is very hot, highly ionised gas (plasma). Rich clusters are outstanding sources of diffuse x-ray emission.

[DeiRenDopa]

Thread's gone quiet - no one has any questions? Is everyone waiting breathlessly for the next exciting episode?

Or have I just bored you all to somnolence?

[Dancing David]

I'm following and learning, again phsyics is never simple, it is not one observation that leads to a conclusion.

[Acleron]

Awed not bored :)

And definitely waiting for more.

[DeiRenDopa]

The Sunyaev-Zel'dovich effect (SZE).

Or how to count baryons.

X-ray observations of rich clusters of galaxies invariably show them to be strong, diffuse sources (predominantly; there are, of course, point sources (such as AGNs) and some small diffuse sources (usually the giant elliptical at the centre)). These x-rays come from a very hot plasma which pervades each cluster; among other things, this means there are rather a lot of hot electrons throughout the cluster.

In the Compton effect (http://en.wikipedia.org/wiki/Compton_scattering), x-rays collide with ('scatter off') electrons, with the electrons gaining energy and the x-rays losing it.

In the inverse Compton effect, low energy photons collide with high energy electrons (i.e. hot electrons, or electrons in particle beams) and the photons gain energy (and the electrons lose it).

When we look at a cluster of galaxies, we see it backlit by the CMB, which is comprised of low energy photons. Put low energy photons together with hot electrons, and lots of inverse Compton scattering happens - and to us the CMB will look hotter towards a cluster than it does elsewhere. Further, as the line of sight through the plasma through the centre of the cluster is longer than that elsewhere, the CMB is 'hottest' at the centre. The effect was predicted by Sunyaev and Zel'dovich, and subsequently observed.

This webpage (http://astro.uchicago.edu/sza/primer.html) gives a brief, relatively non-technical description; this 1999 review (http://nedwww.ipac.caltech.edu/level5/Birkinshaw/Birk_contents.html) gives a more complete overview (the physics is right, but the observations are now way more in number and quality).

Crudely, observations of the SZE can be interpreted as estimates of the total mass of the rich cluster contained in the hot plasma, because the SZE is essentially an exercise in counting (hot) electrons. On their own, such observations do not constrain the baryonic mass much, but combined with x-ray observations - which constrain the temperature and (to some extent) density profile of a cluster - some robust constraints on the total baryonic mass of the cluster can be obtained.

It turns out that the galaxies in rich clusters are only minor (baryonic) constituents - take all the galaxies out (and all the IGM stars too), and a rich cluster would still contain pretty much the same number of baryons (humbling, isn't it?).

What about CDM? How does the SZE constitute observational evidence that rich clusters are mostly CDM (mass-wise)?

Indirectly ... observations of the SZE are independent of gravitational lensing and x-ray observations, so provide independent confirmation that rich clusters are not, mainly, just hot plasmas. They also, in combination with detailed x-ray observations, help to estimate the density profile of CDM across such clusters, because the SZE has a dependence on the electron density and electron temperature (of the plasma) that is quite different from the corresponding dependence of the x-ray emission.

Next: a more detailed look at rich clusters' x-ray emission.

[Wangler]

That was weird; I had just begun a reply with quotes about your post, as I was certain you meant "hot" in the places you said "cold".....

When the quote came up for my response, it was of your edited version, which must have been posted at the same instant.

Needless to say, I was a mite confused there for a few moments!

[DeiRenDopa]

That was weird; I had just begun a reply with quotes about your post, as I was certain you meant "hot" in the places you said "cold".....

When the quote came up for my response, it was of your edited version, which must have been posted at the same instant.

Needless to say, I was a mite confused there for a few moments!Yes, I had a bit of a disconnect between brain and fingers*, and edited the post accordingly ... you must have jumped in in the middle.

*not as bad as another of mine, some time ago now ... where I left out a vital word ("not")! :p

[DeiRenDopa]

Use of 'gravitational lensing' to independently measure the mass of galaxies.

In one sense, Einstein was lucky: very soon after he published the general theory of relativity (GR), one of its predictions was confirmed ... the deflection of light by the mass of the Sun. Of course, GR was already on a sound observational basis, through its post-diction (explanation) of the advance of the perihelion of Mercury, which had been known for many decades. It also helped that Eddington was an enthusiastic supporter, so the observations of stars near the limb of the (eclipsed) Sun were interpreted as confirmation (today we'd call those observations marginal).

It was soon determined that a massive, compact body such as a galaxy should be able to 'lens' a more distant object, such as another galaxy; it was also quickly realised that this would happen only very rarely.

However, in 1979 (http://www.astr.ua.edu/keel/agn/q0957.html) just such an example of (strong) gravitational lensing was discovered, the distant object being a quasar.

With the angular resolution of the Hubble Space Telescope (HST), many examples of such strong lensing can be discovered; a very recent HST PR (http://www.spacetelescope.org/news/html/heic0806.html) gives some examples, and the associated papers and articles explain how many more will likely be discovered in the next few decades.

Estimating the mass of the 'lens' is relatively straight-forward, using textbook physics (GR), although in practice there are complications.

The beauty of this technique is that it is completely independent of the others I've discussed so far: the only thing that matters for GR is the total mass.

And while the number of galaxies whose masses have been estimated using this technique is, today, still small, the results are consistent with those from the other techniques.

There is another kind of gravitational (due to GR) lensing, 'weak lensing', also called 'shear'. In this case, the shape of a distant object is distorted, but only subtly. So for this technique to work, you need lots and lots of distant objects whose undistorted shapes are known. Fortunately, this is exactly what some of the recent large surveys can do, though in this case you get an averaged picture of the mass distribution of a lot of galaxies, rather than of just one.

One possible result from weak lensing observations, of galaxies, is an estimate of the shape of the mass distribution in the halos; the other techniques can give only estimates of the 'mass within distance x' - i.e. the distribution of mass assuming it is spherical (detailed studies of MW halo stars are an exception, as are some studies of some dwarf galaxies; however these are too new to say much about yet, and in any case would apply to only one, or a very few, galaxies).

Weak lensing observations of galaxies is relatively new, however results (estimates of galaxy halo mass) to date are consistent with those from techniques, and there are some early results on the shape of those halos (e.g. there is some 'flattening' and 'ellipticity'), on how they differ from one type of galaxy to another (e.g. giant elliptical galaxies in the centres of rich clusters seem to have different halos than other galaxies), and on how they depend on galaxy environment (e.g. galaxy halos in rich clusters seem to be 'truncated').

Next: so how do we know that this mass, in galaxy (halos), which is not emitting or absorbing light, is CDM?Update ...

One of the great things about surveys like SDSS is that they not only turn up many interesting things, but also that the incidence of such 'interesting things' can be used, in pretty straight-forward ways, to put bounds on all kinds of other, even more interesting things ... and be used to test various hypotheses, models, etc.

One project spun off from the SDSS survey is SLACS - Sloan Lens ACS Survey (http://adsabs.harvard.edu/abs/2006ApJ...638..703B) (extract from abstract; emphasis added):The Sloan Lens ACS (SLACS) Survey is an efficient Hubble Space Telescope (HST) Snapshot imaging survey for new galaxy-scale strong gravitational lenses. The targeted lens candidates are selected spectroscopically from the Sloan Digital Sky Survey (SDSS) database of galaxy spectra for having multiple nebular emission lines at a redshift significantly higher than that of the SDSS target galaxy. The SLACS survey is optimized to detect bright early-type lens galaxies with faint lensed sources in order to increase the sample of known gravitational lenses suitable for detailed lensing, photometric, and dynamical modeling..

To date, there have been 7 papers in this series, which have already collectively garnered >200 citations.

For the purposes of this thread, perhaps the most relevant of the seven are IV ("The Sloan Lens ACS Survey. IV: the mass density profile of early-type galaxies out to 100 effective radii (http://arxiv.org/abs/astro-ph/0701589)") and VII ("The Sloan Lens ACS Survey. VII. Elliptical Galaxy Scaling Laws from Direct Observational Mass Measurements (http://arxiv.org/abs/0805.1932)").

Here are the abstracts to these two papers, in order (and yes, if you're not au fait with the jargon astronomers use, much of these may seem like gibberish; also, much of the formatting is lost):We present a weak gravitational lensing analysis of 22 early-type (strong) lens galaxies, based on deep Hubble Space Telescope images obtained as part of the Sloan Lens ACS Survey. Using advanced techniques to control systematic uncertainties related to the variable point spread function and charge transfer efficiency of the Advanced Camera for Surveys (ACS), we show that weak lensing signal is detected out to the edge of the Wide Field Camera (. 300 h−1 kpc at the mean lens redshift z = 0.2). We analyze blank control fields from the COSMOS survey in the same manner, inferring that the residual systematic uncertainty in the tangential shear is less than 0.3%. A joint strong and weak lensing analysis shows that the average total mass density profile is consistent with isothermal (i.e. ρ ∝ r−2) over two decades in radius (3-300 h−1 kpc, approximately 1-100 effective radii). This finding extends by over an order of magnitude in radius previous results, based on strong lensing and/or stellar dynamics, that luminous and dark component “conspire” to form an isothermal mass distribution. In order to disentangle the contributions of luminous and dark matter, we fit a two-component mass model (de Vaucouleurs + Navarro Frenk & White) to the weak and strong lensing constraints. It provides a good fit to the data with only two free parameters; i) the average stellar mass-to-light ratio M∗/LV = 4.48±0.46 hM⊙/L⊙ (at z = 0.2), in agreement with that expected for an old stellar population; ii) the average virial mass-to-light ratio Mvir/LV = 246+101 −87 hM⊙/L⊙. Taking into account the scatter in the mass-luminosity relation, this latter result is in good agreement with semi-analytical models of massive galaxies formation. The dark matter fraction inside the sphere of radius the effective radius is found to be 27±4%. Our results are consistent with galaxy-galaxy lensing studies of early-type galaxies that are not strong lenses, in the region of overlap (30-300 h−1 kpc).
Thus, within the uncertainties, our results are representative of early-type galaxies in general.We use a sample of 53 massive early-type strong gravitational lens galaxies with well-measured redshifts (ranging from z = 0.06 to 0.36) and stellar velocity dispersions (between 175 and 400 kms−1) from the Sloan Lens ACS (SLACS) Survey to derive numerous empirical scaling relations. The ratio between central stellar velocity dispersion and isothermal lens-model velocity dispersion is nearly unity within errors. The SLACS lenses define a fundamental plane (FP) that is consistent with the FP of the general population of early-type galaxies. We measure the relationship between strong-lensing mass Mlens within one-half effective radius (Re/2) and the dimensional mass variableMdim ≡ G−12 e2(Re/2) to be log10[Mlens/1011M⊙] = (1.03 ± 0.04) log10[Mdim/1011M⊙] + (0.54 ± 0.02) (where e2 is the projected stellar velocity dispersion within Re/2). The near-unity slope indicates that the mass dynamical structure of massive elliptical galaxies is independent of mass, and that the “tilt” of the SLACS FP is due entirely to variation in total (luminous plus dark) mass-to-light ratio with mass. Our results imply that dynamical masses serve as a good proxies for true masses in massive elliptical galaxies. Regarding the SLACS lenses as a homologous population, we find that the average enclosed 2D mass profile goes as log10[M(<R)/Mdim] = (1.10±0.09) log10[R/Re]+(0.85±0.03), consistent with an isothermal (flat rotation curve) model when de-projected into 3D. This measurement is inconsistent with the slope of the average projected aperture luminosity profile at a confidence level greater than 99.9%, implying a minimum dark-matter fraction of fDM = 0.38 ± 0.07 within one effective radius.
We also present an analysis of the angular mass structure of the lens galaxies, which further supports the need for dark matter inside one effective radius..

Of the many take-aways, perhaps just two (for now) stand out:

* the lensing observations are consistent with observations using other techniques, concerning the distribution of, and amount of, CDM (in these galaxies)

* the galaxies analysed seem to be a representative sample of the population of galaxies of this type (this is very, very important!)

[DeiRenDopa]

Hot off the (arXiv) press!

"The Milky Way's Circular Velocity Curve to 60 kpc and an Estimate of the Dark Matter Halo Mass from Kinematics of ~2500 SDSS Blue Horizontal Branch Stars"

whew, what a mouthful :p Here (http://arxiv.org/abs/0801.1232) is a link to the preprint abstract; here (http://www.sdss.org/news/releases/20080527.mwmass.html) is the SDSS PR.

Lots of interesting stuff in this paper, including a quick summary of previous estimates of this mass, the difference between two measures ("circular velocity curve" and "escape velocity curve"), role of models, and something quite neat about 'error bars' (a.k.a. uncertainties).

The net is: a much tighter constraint on the mass within 60 kpc of the MW nucleus (~1 trillion sols), consistent with earlier work (but towards the lower bounds of earlier estimates) ... still lots and lots of CDM ....

[Skwinty]

Originally Posted by Skwinty http://forums.randi.org/helloworld2/buttons/viewpost.gif (http://forums.randi.org/showthread.php?p=3840414#post3840414)
Hi DRD
Very interesting, I have saved the thread for further digestion.
Still seems to me though the evidence relates to the effects rather than dark matter itself. Sure, the inferences about dark matter are there.
I suppose that when dark matter is produced in the lab, which hopefully will be soon then clarity will prevail in the dark recesses of my knowledge base. I dont dispute the existence of CDM just expressing some doubt as to the definitive headlines that one sees in publications.
Also as Carl Sagan said, extraordinary claims require extraordinary evidence. Thanks for the fine effort at explaining all the finer points of CDM.

(bold added)

First, why not ask about this in that thread itself? After all, this one is supposed to be about Plasma Cosmology and whether it is woo or not (do you have something to add to that question, BTW)?

Second, doesn't "the evidence relates to the effects rather than dark matter itself" cover just about all of astronomy (beyond the solar system)? For example, what are spectroscopic binaries? eclipsing binaries? neutron stars (NS)? white dwarf (WD) stars? the [OIII] 500.7 nm emission line? the 21-cm HI line?

In which lab has even the [OIII] 500.7 nm line been observed (much less a 10g chunk of WD or NS matter)?

And it doesn't have to be forms of mass ... how about 10^9 T magnetic fields? or particle accelerators that can produce 10^20 eV protons?

[Skwinty]

Hi DRD
Sorry about misposting my opinion.
I was reading the PC thread when this issue arose and I asked about it.
I have nothing to say about PC as I know nothing about it.
Not that I know much about CDM either, merely asking and stating my opinion.

My point of view stems from headlines that state for example

"The Quest for Dark Matter, Astrophysicists know where it is, but not what it is"

"Dark Matter Detection or Delusion"

"Evidence that Dark Matter has been observed"

You seem to be a bit miffed at my opinion even though I admit the shallowness of my knowledge and state that I have no dispute with the evidence you present which in my opinion only points to the where dark matter is and not what it is.

[DeiRenDopa]

Hi DRD
Sorry about misposting my opinion.
I was reading the PC thread when this issue arose and I asked about it.
I have nothing to say about PC as I know nothing about it.
Not that I know much about CDM either, merely asking and stating my opinion.

My point of view stems from headlines that state for example

"The Quest for Dark Matter, Astrophysicists know where it is, but not what it is"

"Dark Matter Detection or Delusion"

"Evidence that Dark Matter has been observed"

You seem to be a bit miffed at my opinion even though I admit the shallowness of my knowledge and state that I have no dispute with the evidence you present which in my opinion only points to the where dark matter is and not what it is."Science by headlines" (or press releases) isn't really science, is it?

And to the extent that the people who write the headlines (or PRs) get their science wrong, we shouldn't necessarily conclude the science on which the headlines (or PRs) is (however tenuously) based is itself wrong, should we?

When you've had a chance to read more of this thread (and the links in it), you may appreciate that "only points to the where dark matter is and not what it is" may be a little, shall we say, too sweeping.

For example:

* INTEGRAL has observed what we currently interpret as diffuse gamma emission from the MW bulge and disk, and this can set some (rather weak) constraints on the nature of CDM (certain SUSY particles would give a diffuse gamma background, for example)

* the shape and profile of the CDM halos of galaxies, as inferred from various observations (discussed briefly in this thread) likewise sets some constraints on the nature of CDM particles

* the Millennium simulations (and others), combined with astronomical observations, also set some (weak) constraints on the nature of CDM.

And, most fundamentally (?), CDM is, indeed, "cold" ... surely describing the attributes of something constitutes at least part of the answer to any "what" question, doesn't it? Oh, at CDM is also "non-baryonic" ...

[Skwinty]

"Science by headlines" (or press releases) isn't really science, is it?

And to the extent that the people who write the headlines (or PRs) get their science wrong, we shouldn't necessarily conclude the science on which the headlines (or PRs) is (however tenuously) based is itself wrong, should we?

I agree on the science by headlines not being science. Remember though, I am not writing those headlines and I never intimated that the science was wrong, only that the headlines seem a bit hyped. Hence the Carl Sagan quote.

"When you've had a chance to read more of this thread (and the links in it), you may appreciate that "only points to the where dark matter is and not what it is" may be a little, shall we say, too sweeping

I will most certainly read the thread and links more deeply, thats why I saved the entire thread as I stated

"And, most fundamentally (?), CDM is, indeed, "cold" ... surely describing the attributes of something constitutes at least part of the answer to any "what" question, doesn't it? Oh, at CDM is also "non-baryonic" ...

Here I interprete you saying that CDM is cold ie no IR emission and non baryonic ie not normal matter. So you have stated what it is not rather than what it is.

What I dont understand perhaps are the implications that observations of WD, NS etc are comparable to observation of dark matter.
In other words, I can understand observations of "normal" stellar objects to be direct but observation of CDM in laymans understanding to be indirect ie observation of the effects.

when you observe an eclipsing binary, you obseve a direct consequence of one object eclipsing another. You can see both objects.

When you observe gravitational lensing you can only see one object, and can only infer things about the unseen object.
Now, as I have stated many times, I am not a scientist, just an interested observer, so I accept gross deficiencies in my understanding and am hoping that participation in these discussions will widen my scientific understanding rather than trying to elevate confrontation and troll behaviour.

[Reality Check]

...snip...
When you observe gravitational lensing you can only see one object, and can only infer things about the unseen object.
Now, as I have stated many times, I am not a scientist, just an interested observer, so I accept gross deficiencies in my understanding and am hoping that participation in these discussions will widen my scientific understanding rather than trying to elevate confrontation and troll behaviour.
A small point: The gravitational lensing used in the Bullet Cluster observation is micro-gravitational lensing where the distortion of the shapes of background objects (galaxies) is used to determine the distribution of mass within an area.

[DeiRenDopa]

I agree on the science by headlines not being science. Remember though, I am not writing those headlines and I never intimated that the science was wrong, only that the headlines seem a bit hyped. Hence the Carl Sagan quote.



I will most certainly read the thread and links more deeply, thats why I saved the entire thread as I stated



Here I interprete you saying that CDM is cold ie no IR emission and non baryonic ie not normal matter. So you have stated what it is not rather than what it is.

What I dont understand perhaps are the implications that observations of WD, NS etc are comparable to observation of dark matter.
In other words, I can understand observations of "normal" stellar objects to be direct but observation of CDM in laymans understanding to be indirect ie observation of the effects.

when you observe an eclipsing binary, you obseve a direct consequence of one object eclipsing another. You can see both objects.

When you observe gravitational lensing you can only see one object, and can only infer things about the unseen object.
Now, as I have stated many times, I am not a scientist, just an interested observer, so I accept gross deficiencies in my understanding and am hoping that participation in these discussions will widen my scientific understanding rather than trying to elevate confrontation and troll behaviour.Hmmm ...

When you observe some of the beautiful planetary nebulae, what do you see? If you take a spectrum of one, you'll see a strong emission line at ~500.7 nm, and your textbook will tell you it's a 'forbidden' transition of a doubly ionised oxygen atom ... but no one has ever seen this line in any lab spectrum! So why are astronomers (etc) so confident that this nice green line is, in fact, a [OIII] transition (the square brackets mean a forbidden transition)?

Take something much more mundane, acceleration due to gravity.

As far as I know, the strongest acceleration due to gravity directly tested is that here on the surface of the Earth ... of course, there are centrifuges which can create 'g forces' as high as a million gs, but whether they give the same effect as gravity depends on what sort of philosophy of science you follow. If you accept all the results of all the tests that GR (General Relativity) has been subject to to date (and which it has passed, with flying colours), then you'll have no intellectual problems with accepting that atoms behave on the surface of high density star (with an acceleration due to gravity of a million gs) the same as they behave in an ultracentrifuge in an Earthly lab.

But then if you accept GR for the behaviour of atoms (etc) under a million gs, why not accept CDM from gravitational lensing? Sure the logic chain is longer (you have to estimate all the baryonic matter that is not 'visible' and subtract it from the estimate of the total mass causing the lensing), but the philosophical principle is the same, right?

Oh, and "cold" in CDM doesn't refer to any blackbody temperature ... matter of this kind does not 'feel' the electromagnetic force, so does not emit photons period. Instead, "cold" refers to the average speed of the CDM particles, and means 'non-relativistic' (or something similar) ...

[DeiRenDopa]

[...]

when you observe an eclipsing binary, you obseve a direct consequence of one object eclipsing another. You can see both objects.Are you sure?

I thought you observe only the light curve of a single point source ('star'), and from that light curve you infer that there are, in fact, two stars in a mutual orbit whose plane has a certain geometric relationship to your line of sight ...



When you observe gravitational lensing you can only see one object, and can only infer things about the unseen object.

[...](bold added)

Indeed ...

... just like an eclipsing binary (or a spectroscopic binary, or ...)! :D

[Herzblut]

If we begin assuming that:

http://www.randi.org/latexrender/latex.php?%20%5Crho%20=%20%5Crho%20%28r%29

Then I can barely understand how to get to:

http://www.randi.org/latexrender/latex.php?%5Cleft%20%28%5Cfrac%7B%7B%7Bc%7D_%7Bs%7 D%7D%5E%7B2%7D%7D%7B%5Crho%20%28r%29%7D%20%5Cright %29%5Cleft%28%5Cfrac%7Bd%20%5Crho%20%28r%29%7D%7Bd %20r%7D%20%5Cright%29%20=%20-%20%5Cfrac%20%7B4%20%5Cpi%20G%7D%7B%7Br%7D%5E%7B2% 7D%7D%5Cint_%7Br%7D%5E%7B0%7D%7Br%7D%5E%7B2%7D%20% 5Crho%20%28r%29%20dr

Not that I know what this is all about, but..

For a radially symmetric grav. field, simply use the Laplace operator in spherical coordinates in the Poisson (field) equation, ignoring the two angular gradients. By differentiating the equation of state you can then, in the field equation, eliminate the gravitational field in favor of mass density. By integration you get to the result straight away.

[Skwinty]

Hi DRD

When observing eclipsing binaries, you can see both objects prior to eclipsing.
During the eclipse the light curves can be used to infer all sorts of things.

As for the forbidden transitions. When the lab here on Earth can perform spectroscopy on the same volumes of gas and conditions that occur in the universe, then your argument about forbidden transitions may not hold water.

Why are you implying that I do not accept CDM. I have stated numerous time that I do not have a problem with the issue or your observations of the effects of dark matter. My issue is with the claim that Dark matter has been observed.
My argument is that the effects have been observed. When science can state that CDM consists of neutrolinos, axions or any other blend of exotic particles and this has been corroborated by at least three different types of measurement, then you can state that CDM has been observed. Until then, only the effects have been observed and science can make many inferrances about this, but its based on the observation of effect.

[DeiRenDopa]

Hi DRDHi Skwinty


When observing eclipsing binaries, you can see both objects prior to eclipsing.
During the eclipse the light curves can be used to infer all sorts of things.It's actually important to get this right, in the sense of us agreeing on a description ...

What's observed: a point source ('star') on the sky - one point source (not two). The source's brightness is observed over time, and the data displayed as a 'light curve'.

AFAIK (as far as I know), there are few, if any, EBs that have been directly resolved as separate point sources - do you know of any?

From the variation in the point source's brightness over time, we infer that the point source is actually an EB.

Why is it important that we reach agreement on this description? There are several reasons, but one has to do with what is 'direct' and what is 'indirect' ("laymans understanding to be indirect ie observation of the effects"); another with the role of (physics) theory in 'observation'.



As for the forbidden transitions. When the lab here on Earth can perform spectroscopy on the same volumes of gas and conditions that occur in the universe, then your argument about forbidden transitions may not hold water.Yes and no ...

An astronomer shows you the spectrum of a star. She says that certain lines in that spectrum are due to H (say). You do your checking and discover all kinds of consistencies (lab spectrum of H under certain temp and density conditions; the astronomer's telescope, spectrograph, etc; analysis routines and procedures; ...). What - in as much detail as possible - leads you to conclude that there is H somewhere along the line of sight to the star?

From another thread I infer that you have some interest in philosophy; it may be helpful to get over some of the more (to me) unhelpful aspects (of philosophy) as they apply to astronomy, and get on to a clear understanding of what cosmological principle(s) you are comfortable with. For example, perhaps some variant of 'physics is the same everywhere in the universe'.

Back to forbidden transitions ... if the conditions for a particular transition are far, far beyond what can be created in an Earthly lab (we can't make a vacuum hard enough, for long enough, for example), whence comes the certainty that a particular line in a spectrum means the existence of a metastable excited state of a certain ion (or atom or molecule) 'out there'? It's an extrapolation, of at least one kind - I'm keen to know what kinds of extrapolation are acceptable to you, and how you parse extrapolations (in astronomy) into 'direct observation' and 'observation of the effects'.



Why are you implying that I do not accept CDM. I have stated numerous time that I do not have a problem with the issue or your observations of the effects of dark matter. My issue is with the claim that Dark matter has been observed.
My argument is that the effects have been observed. When science can state that CDM consists of neutrolinos, axions or any other blend of exotic particles and this has been corroborated by at least three different types of measurement, then you can state that CDM has been observed. Until then, only the effects have been observed and science can make many inferrances about this, but its based on the observation of effect.
Let's explore this in more detail, later ...

For now, can you confirm that techniques like gravitational lensing can be used to infer something about the distribution of mass between the source object and us ('the lens')?

[Skwinty]

Hi DRD

Inferring something about dark matter from gravitational lensing is perfectly acceptable. I dont have any problem with that at all.

Sirius is an example of a visual binary, given the correct "seeing", telescope and hartmann mask. Algol is another and there a lots more. I agree, that there are lots of eclipsing binaries that are not visual and "indirect" methods are required to infer anything about them.

ETA: My interest in philosophy extends only to what Einstein believed about philosophy. I am not a philosopher and I believe that the laws of physics are the same throughout the observable universe. As for the unobservable universe, I cannot be certain, although there should be no reason to believe that it would be any different. But never say never!

[DeiRenDopa]

Hi DRD

Inferring something about dark matter from gravitational lensing is perfectly acceptable. I dont have any problem with that at all.Thanks, that's good to know.

Now if the lurkers who feel otherwise would make themselves known please (and say why, unlike Skwinty, you do have a problem)?



Sirius is an example of a visual binary, given the correct "seeing", telescope and hartmann mask. Algol is another and there a lots more. I agree, that there are lots of eclipsing binaries that are not visual and "indirect" methods are required to infer anything about them.Indeed.

They are 'visual binaries', and all EBs could be resolved as visual binaries (in principle); however, as I said, I know of none.

Many visual binaries are discovered when resolution increases, such as with HIPPARCOS, or the Hubble Space Telescope, or when a ground-based telescope is fitted with the latest in adaptive optics.

A spectroscopic binary is a star (point source) whose spectrum has two sets of lines that move relative to each other in a particular way, corresponding to changes in the line of sight velocity of the two stars in the binary*. In principle all spectroscopic binaries could be resolved as visual binaries (do you know why?), but few, if any, are.

Are all EBs also spectroscopic binaries (if only in principle)?

Are all spectroscopic binaries also EBs (if only in principle)?


Stay tuned, there's more! :D (and this is all very relevant to cold, non-baryonic dark matter and whether it has been 'observed' or not ... trust me).


ETA: My interest in philosophy extends only to what Einstein believed about philosophy. I am not a philosopher and I believe that the laws of physics are the same throughout the observable universe. As for the unobservable universe, I cannot be certain, although there should be no reason to believe that it would be any different. But never say never!Thanks, that's good to know.

Now if the lurkers who feel otherwise would make themselves known please (and say why, unlike Skwinty, you think physics may be different in the unobserved universe)?

Note that I changed "unobservable" to "unobserved"; care to guess why?

* any inferences here?

[robinson]

My issue is with the claim that Dark matter has been observed.
My argument is that the effects have been observed. When science can state that CDM consists of neutrolinos, axions or any other blend of exotic particles and this has been corroborated by at least three different types of measurement, then you can state that CDM has been observed. Until then, only the effects have been observed and science can make many inferrances about this, but its based on the observation of effect.

Good points. I'm reminded of how planets were first "known to exist" because of gravitational anomalies on other planets orbits. Mercury's orbit led to theorizing there was an unknown planet effecting it. Which was wrong.

But then, Pluto was theorized and turned out to be there. In both cases, something was "observed", and an invisible body theorized to exist. But in both cases, before actual observation of a planet, it would be a stretch to say it was observed.

Proper language would be "evidence for invisible matter that does not act like matter that we know of has been observed". Or something like that.

[robinson]

Which, to be quite fair, is also the sort of evidence we have for many invisible things. Like magnetism.

[DeiRenDopa]

Good points. I'm reminded of how planets were first "known to exist" because of gravitational anomalies on other planets orbits. Mercury's orbit led to theorizing there was an unknown planet effecting it. Which was wrong.

But then, Pluto was theorized and turned out to be there. In both cases, something was "observed", and an invisible body theorized to exist. But in both cases, before actual observation of a planet, it would be a stretch to say it was observed.I think you're thinking of Neptune; Pluto was not 'there'!



Proper language would be "evidence for invisible matter that does not act like matter that we know of has been observed". Or something like that.Stay tuned! :D

In the meantime, would you care to give us your thoughts on planetary nebulae (PNe)? At least the ones with strong [OIII] lines in their spectra.

Why do I ask? Because the star(s) which are inferred to exist at the heart of such PNe are often not directly observed, as in no point source is seen near the geometric centre of such PNe. Where such stars are directly observed, it turns out they are invariable white dwarf (WD) stars. As I think you now know very well robinson, no one has ever created even a few grams of WD star stuff in any earthly lab.

So, for the PNe for which a central WD star is observed, should we say ""evidence for invisible matter that does not act like matter that we know of has been observed""?

And what about the PNe for which no central star is directly observed? To what extent can we say that such stars very likely exist, in such PNe, based on little more than the detection of [OIII] lines from those PNe?

[Skwinty]

Hi DRD

Here is an example of two eclipsing binaries that are visual :

V975 Cen and NSV 15737. These are two bright eclipsing binaries solved with the aid of visual observation. I am sure that there must be more.

As to: "Are all EBs also spectroscopic binaries (if only in principle)?

Are all spectroscopic binaries also EBs (if only in principle)?" it would , I suppose depend on where you are observing from and what resolution you are using.

As far as :
"Note that I changed "unobservable" to "unobserved"; care to guess why?" That would relate to our current capabilities. This of course is subject to change as time and technology progresses.


Originally Posted by Belz... http://forums.randi.org/helloworld2/buttons/viewpost.gif (http://forums.randi.org/showthread.php?p=3842852#post3842852)
Indirect observation is as good as direct observation. Otherwise science wouldn't exist

There is no argument with this concept, however there is a difference between direct and indirect in astronomy.

Binary stars can either be distinguished optically (visual binaries) or by indirect techniques, such as spectroscopy.

Another example would be that you are only indirectly aware of me through this forum and cannot claim to have directly observed me.

At the end of the day, I think that its the interpretation of words that cause the problem rather than the interpretation of scientific fact.

[DeiRenDopa]

Hi DRD

Here is an example of two eclipsing binaries that are visual :

V975 Cen and NSV 15737. These are two bright eclipsing binaries solved with the aid of visual observation. I am sure that there must be more.Nice find! :)


However, you might like to read the relevant papers a little more carefully ... for example, I can't find anything on V975 Cen being a visual binary ("visual eclipse" doesn't mean what you think it might), and the NSV 15737 system is not a simple binary ...

I don't doubt that there might be some EBs that are also visual binaries, but they are a tiny, tiny minority of all EBs.



As to: "Are all EBs also spectroscopic binaries (if only in principle)?

Are all spectroscopic binaries also EBs (if only in principle)?" it would , I suppose depend on where you are observing from and what resolution you are using.Indeed.

And unless and until you can go anywhere and observe with any resolution ...

From here in the solar system, what is the answer to each question?


As far as :
"Note that I changed "unobservable" to "unobserved"; care to guess why?" That would relate to our current capabilities. This of course is subject to change as time and technology progresses.
Close ...

... where this mini-thread is headed (well, one place) is that "observe" is so intricately and intimately entwined with physics that "unobservable" takes you out of science (unless, perhaps, you are a philosopher), and this part of the JREF forum would not be the right place to discuss non-science ...


Originally Posted by Belz... http://forums.randi.org/helloworld2/buttons/viewpost.gif (http://forums.randi.org/showthread.php?p=3842852#post3842852)
Indirect observation is as good as direct observation. Otherwise science wouldn't exist

There is no argument with this concept, however there is a difference between direct and indirect in astronomy.

Binary stars can either be distinguished optically (visual binaries) or by indirect techniques, such as spectroscopy.You can't see in IR, or microwaves, or radio, or ... You also can't see in the x-ray waveband (etc).

That makes all astronomy - other than visual astronomy - "indirect" ... or does it?

And even for visual detection, with something like HIPPARCOS I'd be curious to know if you think you could make a case that new "HIP" binaries were discovered "directly" ... (HINT: you might like to read up on that mission, carefully, before you answer ...)



Another example would be that you are only indirectly aware of me through this forum and cannot claim to have directly observed me.

At the end of the day, I think that its the interpretation of words that cause the problem rather than the interpretation of scientific fact.Yep ...

... but that's only part of the problem ... if you have a chance, you might like to read some of the posts by "robinson" (among others) in this part of the JREF forum. If you do, I expect that you will conclude there's a great deal more to it than "the interpretation of scientific fact".

[Skwinty]

You can't see in IR, or microwaves, or radio, or ... You also can't see in the x-ray waveband (etc).

That makes all astronomy - other than visual astronomy - "indirect" ... or does it?


Using a dlsr camera with ir filter removed allows you to see the ir.
With the correct equipment, microwaves, radio x-ray or gamma rays can be directly detected though.

So once again its the damn words causing the problem.;)

[Skwinty]

I can't find anything on V975 Cen being a visual binary ("visual eclipse" doesn't mean what you think it might)

Please elucidate. The paper states the eclipse was visually observed which implies a visual binary.

I don't doubt that there might be some EBs that are also visual binaries, but they are a tiny, tiny minority of all EBs.

Agreed, but this relates to "discovered" EB's. We cant say we have discovered them all as yet, so the possibility exists that there are more.


And unless and until you can go anywhere and observe with any resolution

This limitation is purely financial though.

[DeiRenDopa]

Using a dlsr camera with ir filter removed allows you to see the ir."the damn words" again?

AFAIK, the human eye is not sensitive to IR ... at least, not in terms of being able to form images on the retina that the rods and cones react to by doing their nerve firing thing ...


With the correct equipment, microwaves, radio x-ray or gamma rays can be directly detected though.

So once again its the damn words causing the problem.;)Are you sure?

Don't you "detect" x-rays (say) via an intricate piece of equipment, a marvel of highly refined materials and electronics? And if your detector is aboard a spacecraft, as it must be for astronomical observations, when you finally get to see something on a computer monitor, hasn't the signal gone through a most marvelous chain of wonderous equipment, gadgets, etc? Hardly "direct detection", is it?

And what about IACTs (http://en.wikipedia.org/wiki/Atmospheric_Cherenkov_telescope) (Imaging Air/Atmosphere Cherenkov Telescopes, like H.E.S.S. (http://www.mpi-hd.mpg.de/hfm/HESS/HESS.html))? Whatever it is that they "observe", surely you can't say they "directly detect" TeV gammas, can you?

[robinson]

Semantics. The last refuge when losing the argument

:deadhorse.

[Hellbound]

Semantics. The last refuge when losing the argument

:deadhorse.

You should well know.

Much like claiming that "directly detecting" only means "detecting by EM radiation".

[Skwinty]

AFAIK, the human eye is not sensitive to IR ... at least, not in terms of being able to form images on the retina that the rods and cones react to by doing their nerve firing thing .

Agreed, but when you look at the picture produced you can see the ir input.



Don't you "detect" x-rays (say) via an intricate piece of equipment, a marvel of highly refined materials and electronics? And if your detector is aboard a spacecraft, as it must be for astronomical observations, when you finally get to see something on a computer monitor, hasn't the signal gone through a most marvelous chain of wonderous equipment, gadgets, etc? Hardly "direct detection", is it?

Using that argument implies then, that nothing is directly observed, as the eye, brain and mind is also a chain of intricate and wonderous equipment.
If you consider the signal, from source to primary detector then that could be called direct, ie there is no intermediary interference or effect between source and detector.


And what about IACTs (http://en.wikipedia.org/wiki/Atmospheric_Cherenkov_telescope) (Imaging Air/Atmosphere Cherenkov Telescopes, like H.E.S.S. (http://www.mpi-hd.mpg.de/hfm/HESS/HESS.html))? Whatever it is that they "observe", surely you can't say they "directly detect" TeV gammas, can you?

Perhaps the HESS is a bad example as the gamma rays do not penetrate the atmosphere and reach the earth 's surface as they interact with the atmosphere and produce cerenkov radiation. The link between the gamma ray and cerenkov radiation is inferred, so the direct observation is the cerenkov radiation and the gamma ray observation is indirect. And its a good solid inferrence.No problem there.

[DeiRenDopa]

Agreed, but when you look at the picture produced you can see the ir input. (bold added)

But your acceptance of it as being an IR picture (or whatever) relies upon your acceptance of the science behind the detection mechanisms/devices and whatever goes on between detection and picture ... and that's my point (well, one of them).



Using that argument implies then, that nothing is directly observed, as the eye, brain and mind is also a chain of intricate and wonderous equipment.
If you consider the signal, from source to primary detector then that could be called direct, ie there is no intermediary interference or effect between source and detector.Indeed.

That's one of the conclusions I am hoping you will draw ... that however you define "direct", the boundary is essentially arbitrary (you say "direct" and I say "indirect", or something like this).

Alternatively, "direct" can be limited to what you can see with your eyes (i.e. the visual waveband only), whether augmented by dumb telescopes or not (i.e. mirrors and lenses).



Perhaps the HESS is a bad example as the gamma rays do not penetrate the atmosphere and reach the earth 's surface as they interact with the atmosphere and produce cerenkov radiation. The link between the gamma ray and cerenkov radiation is inferred, so the direct observation is the cerenkov radiation and the gamma ray observation is indirect. And its a good solid inferrence.No problem there.So ...

... we can have "direct detection" (the boundaries of which are somewhat arbitrary), "good solid inference/indirect detection", and {something else}.

And while you seem cool with IACTs, there are several high-post-count folk who most surely would not be ... if only because no one has ever created Cherenkov radiation with TeV gammas in the lab, much less fired them at the atmosphere from above ...

So, would like to try to draw up a set of criteria for how you feel the terms "direct detection" (or "direct observation") and "good solid inference/indirect detection/observation" and {any other categories} should be used?

In particular, how - objectively - does one decide when a detection/observation is indirect but "a good solid inferrence"?

[DeiRenDopa]

You should well know.

Much like claiming that "directly detecting" only means "detecting by EM radiation".I suspect - in robinson's case - that it's much, much worse than this ...

For example, do IACTs "detect by EM radiation"? What about the non-detection of EM radiation (as in, for example, an absorption line)?

Myself, I suspect one deep issue is a very confused, inchoate understanding of the nature of science (and, perhaps, a deep wish that it were other than what it is) ...

[robinson]

Observing large footprints that don't match any know animal is direct observation of Bigfoot.

[Hellbound]

Observing large footprints that don't match any know animal is direct observation of Bigfoot.

Thank you for comfirning your lack of understanding.

Either that, or creating a strawman.

You talk about semantic arguments, but that's the majority of what you've been doing.

Heres a clue for you:
"evidence for invisible matter that does not act like matter that we know of has been observed"

That's basically what the words "dark matter" mean. EVERY PROPERTY put forth for dark matter comes from the observations, and the limitations on those observations, and the implications of those limitations in regards tot he laws of physics.

Instead of saying "evidence for invisible matter that does not act like matter that we know of has been observed" everyt ime, which would make scientists sound like idiots besides being a mouthful, that's been shortened to "dark matter". You, however, seem to have some objection to the word (since, from your statements, it's clear you don't object to the meaning..assuming you understand it). It's not like we had this word called dark matter that had this whole, well-defined set of attributes associated with it that we simply slapped on the first unseen gravitational lensing effect we found.

[DeiRenDopa]

Which, to be quite fair, is also the sort of evidence we have for many invisible things. Like magnetism.I'm curious ...

This very recent arXiv preprint (http://arxiv.org/abs/0807.1112) uses both "infer" and "large detectable magnetic field" in the abstract (see below; I added bold).

How do you think the authors should have written their abstract robinson?

SDSS J142625.71+575218.3: The First Pulsating White Dwarf with a Large Detectable Magnetic Field

P. Dufour, G. Fontaine, J. Liebert, K. Williams, D. K. Lai

We report the discovery of a strong magnetic field in the unique pulsating carbon-atmosphere white dwarf SDSS J142625.71+575218.3. From spectra gathered at the MMT and Keck telescopes, we infer a surface field of B_s ~1.2 MG, based on obvious Zeeman components seen in several carbon lines. We also detect the presence of a Zeeman-splitted He I 4471 line, which is an indicator of the presence of a non-negligible amount of helium in the atmosphere of this Hot DQ star. This is important for understanding its pulsations, as nonadabatic theory reveals that some helium must be present in the envelope mixture for pulsation modes to be excited in the range of effective temperature where the target star is found. Out of nearly 200 pulsating white dwarfs known today, this is the first example of a star with a large detectable magnetic field. We suggest that SDSS J142625.71+575218.3 is the white dwarf equivalent of a roAp star.

[Skwinty]

Hi DRD

In a nutshell, why I am not happy with the "Dark matter has been directly observed" statement although I accept the inferred conclusions from the observational data.

DAMA in 2000 stated the dark matter had been directly observed using a sodium iodide detector.
This statement only raised eyebrows in the scientific community.
DAMA in 2008 restated this claim with a better data and a bigger detector. The scientific community accepted the results as real effect but not convinced that it indicated dark matter.

There was very little or no corroborating evidence from CERN, Fermilab or COUPP with respect to any WIMP detection. Maybe LHC willl produce a WIMP.

No corroborating evidedence from CDMS, XENON10 or 100.
No corroborating evidence from ADMX for axions or ICE-CUBE for neutrinos.

GLAST will give good data in the near future as will all the other experiments I mentioned above.

So, in my opinion, the dark matter issue has not been sufficiently resolved for us to celebrate.

My main problem is the headline hype and not the inferred data from direct or indirect observations which indicate the presence of dark matter regardless of whether dark matter is indeed matter or energy.

[robinson]

Thank you for comfirning[sic] your lack of understanding.

...

That's basically what the words "dark matter" mean. EVERY PROPERTY put forth for dark matter comes from the observations, and the limitations on those observations, and the implications of those limitations in regards tot he[sic] laws of physics.

Did you just make that up? DM was theorized to explain Gravity rotation which didn't match Kepler's equations for the planets orbits. It is assumed Galaxy rotation has to follow Kepler's laws, which they don't, so there had to be some missing matter to explain it. A LOT of missing matter.

It is amusing to consider that while the third law was the one that got the head scratching started, the first one seems to be ignored.

1. The orbit of every planet is an ellipse with the sun at one of the foci.

2. A line joining a planet and the sun sweeps out equal areas during equal intervals of time as the planet travels along its orbit.

3. The squares of the orbital periods of planets are directly proportional to the cubes of the semi-major axes of their orbits.

Assuming a plasma dominated system of hundreds of billions of stars, as well as many times more plasma and hydrogen/helium, would act like a solar system, in terms of rotation and orbits, seems kind of silly now.



Instead of saying "evidence for invisible matter that does not act like matter that we know of has been observed" everyt ime[sic], which would make scientists sound like idiots besides being a mouthful, that's been shortened to "dark matter". You, however, seem to have some objection to the word (since, from your statements, it's clear you don't object to the meaning..assuming you understand it). It's not like we had this word called dark matter that had this whole, well-defined set of attributes associated with it that we simply slapped on the first unseen gravitational lensing effect we found.

"Non-baryonic cold dark matter", from the thread title, is a made up term. It is a placeholder until we figure out what is really going on. There is no such thing as "Dark Matter", (just like there is no such thing as "Baryonic hot light matter").

At some point it will be called neutralinos, axions, axinos or Majorons, or something. The problem is wikiality hasn't decided what to call it. much less what it is. Once consensus is reached, all will be well.

[Hellbound]

robinson:

That's exactly my point. Dark matter is a place holder, or more accurately a term to include a grouping of matter that's different from regular matter.

Saying it doesn't exist because we don't know all the elements that make it up is as non-sensical as saying beetles don't exist because we haven't found and classified every beetle yet.

It's a semantic argument, rather than an argument based on anything of substance.

But by all means, continue to play word games.

[robinson]

robinson:

Saying it doesn't exist because we don't know all the elements that make it up is as non-sensical as saying beetles don't exist because we haven't found and classified every beetle yet.


Nobody said that, except you, just now.

[Hellbound]

Nobody said that, except you, just now.

Well, then, let me state it better.

Claiming we haven't directly detected dark matter is like claiming we haven't directly detected beetles because we haven't seen every variety.

We have directly detected dark mater, we just don't have all the answers as to exactly what it is yet.

[DeiRenDopa]

[...]

(just like there is no such thing as "Baryonic hot light matter").

[...]Oh?

What would you say cosmic rays are then?

After all, they:

* are composed of protons (and other atomic nuclei) ... "baryonic"

* travel at relativistic speeds .... "hot"

* 'feel' the electromagnetic force ... "light"

* and are a form of "matter"!

May I ask robinson, to what extent are you actually (really, truly) interested in learning how modern physics (including astrophysics) works?

[DeiRenDopa]

Hi DRD

In a nutshell, why I am not happy with the "Dark matter has been directly observed" statement although I accept the inferred conclusions from the observational data.

DAMA in 2000 stated the dark matter had been directly observed using a sodium iodide detector.
This statement only raised eyebrows in the scientific community.
DAMA in 2008 restated this claim with a better data and a bigger detector. The scientific community accepted the results as real effect but not convinced that it indicated dark matter.

There was very little or no corroborating evidence from CERN, Fermilab or COUPP with respect to any WIMP detection. Maybe LHC willl produce a WIMP.

No corroborating evidedence from CDMS, XENON10 or 100.
No corroborating evidence from ADMX for axions or ICE-CUBE for neutrinos.

GLAST will give good data in the near future as will all the other experiments I mentioned above.

So, in my opinion, the dark matter issue has not been sufficiently resolved for us to celebrate.OK ...

... may I ask a hypothetical then? What sorts of circumstances do you feel would constitute "sufficiently resolved", so that you could celebrate?

And during the ~40 years it took to resolve the "solar neutrino issue" (if I may call it that), how did you feel? Were you aware of the issue? of its resolution? of the details of that resolution?

What other issues - in astrophysics, astronomy, or cosmology do you feel are also insufficiently resolved?



My main problem is the headline hype and not the inferred data from direct or indirect observations which indicate the presence of dark matter regardless of whether dark matter is indeed matter or energy.Hype can be quite annoying, can't it?

Do you find the hype about DM particularly more annoying than that about some other aspect(s) of astrophysics?

[Skwinty]

Hi DRD


... may I ask a hypothetical then? What sorts of circumstances do you feel would constitute "sufficiently resolved", so that you could celebrate?

Firstly, my satisfaction in this matter is irrelevant.
For the scientific community at large I would say that its time to pop corks when:
1.WIMPS are directly detected. See end of post for a examples of direct and indirect detection.
2.When local and relic densities of WIMPS have been directly measured.
3. When WIMP masses have been directly detected.
Then we can say that at least some part of dark matter has been directly observed.


And during the ~40 years it took to resolve the "solar neutrino issue" (if I may call it that), how did you feel? Were you aware of the issue? of its resolution? of the details of that resolution?

40 years ago I had no interest in Astronomy or Science.
But, in the last 10 years I have. I do recall that the resolution of solar neutrinos involved neutrino oscillation and a change to the standard model of particle physics. I dont recall hyped headlines though, perhaps there were, I just dont remember.


What other issues - in astrophysics, astronomy, or cosmology do you feel are also insufficiently resolved?

Goodness, there are so many unresolved issues. A bit naive to expect that there would be none.


Hype can be quite annoying, can't it?

Sure, hype is bound to lead to disappointment.


Do you find the hype about DM particularly more annoying than that about some other aspect(s) of astrophysics?

Not particularly.

Examples of Large Surveys:
Sloan 1 & 2, Deep 2, Gravitational lensing(MegaCam, Panstar, LSST)

Example of Direct detection/searches:
Experiments such as CDMS2, EDelweiss 2,Cresst 2, Liquid Xenon, Argon/Neon, Pressurised Gas, COUPP, Picasso, Simple.

Direct detection involves elastic scattering.
WIMP interacting with detector.

Examples of Indirect detection/searches:
GLAST
IceCube Antares/Nemo/Nestor

Indirect detection involves annihilation.
WIMP to Gamma rays/positrons/antiprotons.high energy neutrinos.

Pretty much what I said here in an earlier post

"Perhaps the HESS is a bad example as the gamma rays do not penetrate the atmosphere and reach the earth 's surface as they interact with the atmosphere and produce cerenkov radiation. The link between the gamma ray and cerenkov radiation is inferred, so the direct observation is the cerenkov radiation and the gamma ray observation is indirect. And its a good solid inferrence.No problem there. "

[Acleron]

Hi DRD{snip}

So what hypothesis do you have to explain the non-Newtonian if not non-Einsteinian orbiting of visually detectable matter in galaxies?

[Skwinty]

So what hypothesis do you have to explain the non-Newtonian if not non-Einsteinian orbiting of visually detectable matter in galaxies?


Dark Matter

[robinson]

Well, then, let me state it better.

Claiming we haven't directly detected dark matter is like claiming we haven't directly detected beetles because we haven't seen every variety.


No, it would be like claiming there is a new kind of life form that doesn't fit into any known category, because something is eating the leaves of a tree, and we don't know what it is, but we are sure it isn't any known animal.

And the tree is so far away we can only see it with a telescope, and we don't see any other animals eating the tree, so it has to be a new kind of life form, one that has never been seen, and doesn't act like any known animal, and there are more of these animals than all the other species combined.

[DeiRenDopa]

No, it would be like claiming there is a new kind of life form that doesn't fit into any known category, because something is eating the leaves of a tree, and we don't know what it is, but we are sure it isn't any known animal.

[...]After all this time, you still don't get it robinson?!? :eye-poppi

What part of "matter" don't you understand?

It walks like (cold) mass, talks like (cold) mass, fights like (cold) mass ... sure must be a new species of sith then ...

[Skwinty]

After all this time, you still don't get it robinson?!? :eye-poppi

What part of "matter" don't you understand?

It walks like (cold) mass, talks like (cold) mass, fights like (cold) mass ... sure must be a new species of sith then ...

Robinson might well ask of you, "What dont you get about the"allegory".
Check the meaning of the word and "The allegory of the cave"
Now I understand why people, who are highly educated in "science" cant agree with "philosophy" or the more figurative trains of thought.
Not everything has to be black and white all the time.
There is actually a lesson or moral in robinsons story here which is flying right over your heads.

Just a thought here.

[DeiRenDopa]

Robinson might well ask of you, "What dont you get about the"allegory".
Check the meaning of the word and "The allegory of the cave"
Now I understand why people, who are highly educated in "science" cant agree with "philosophy" or the more figurative trains of thought.
Not everything has to be black and white all the time.
There is actually a lesson or moral in robinsons story here which is flying right over your heads.

Just a thought here.If you say so.

However, this part of the JREF forum is entitled "Science, Mathematics, Medicine, and Technology". I can't see where "philosophy" fits in, for this section. In fact, the forum has a whole section called "Religion and Philosophy", so why do you think it appropriate to have discussions of philosophy here (and not in the section explicitly devoted to it)?

But thanks, I think you have put your finger on one possible reason for miscommunication ('talking past each other') - some people post here with a default perspective other than that of contemporary science.

[Acleron]

Robinson might well ask of you, "What dont you get about the"allegory".
Check the meaning of the word and "The allegory of the cave"
Now I understand why people, who are highly educated in "science" cant agree with "philosophy" or the more figurative trains of thought.
Not everything has to be black and white all the time.There is actually a lesson or moral in robinsons story here which is flying right over your heads.

Just a thought here.

My bolding.

First of all, being highly educated doesn't mean anything at all. A highly educated scientist can be as stupid as a highly educated anything else.

Secondly, in science, it has to be black and white. If you cannot clearly describe what you are talking about it is not science. If you don't see that point, you have no idea about how scientists think or work.

Thirdly, if you think that Robinson's use of English to describe or interpret deeper parts of scientific thought is of any use at all then you must be part of the modern philosophers movement. Unfortunately, the universe does not operate according to responses and reflexes learned when the maximum speed and mass approximate to that attained by a human being.

I could ask my question again ie What does philosophy have to offer to our understanding of the universe? But why should I wait for an answer?

[Acleron]

If But thanks, I think you have put your finger on one possible reason for miscommunication ('talking past each other') - some people post here with a default perspective other than that of contemporary science.

Precisely

[Skwinty]

First of all, being highly educated doesn't mean anything at all. A highly educated scientist can be as stupid as a highly educated anything else..

Agreed, stupidity has no limitations.

Secondly, in science, it has to be black and white. If you cannot clearly describe what you are talking about it is not science. If you don't see that point, you have no idea about how scientists think or work.

Agreed, but with some reservation. For example, the brain is divided into two hemispheres and I believe that logical thinking uses one hemisphere predominately. Lateral thinking predominately uses the othe hemisphere.
My point is that surely a balance is required. You dont have to be in the lab all the time. Same for philosophers, they dont have to be in the mind all the time.

Thirdly, if you think that Robinson's use of English to describe or interpret deeper parts of scientific thought is of any use at all then you must be part of the modern philosophers movement. Unfortunately, the universe does not operate according to responses and reflexes learned when the maximum speed and mass approximate to that attained by a human being..

I'm not part of any movement, just that I try to maintain a balance and overall understanding. Call me a fence sitter. Also , your objection to philosophy being discussed even as an aside is puzzling. Does philosophy only have something in common with religion? I thought philosophy was the seed of science. I have no interest in religion. My interest is in the science with a smattering of philosophy.

I could ask my question again ie What does philosophy have to offer to our understanding of the universe? But why should I wait for an answer?

It is not so much that philosophy can offer specific answers to the facts of the universe, but rather to apply some understanding to the non scientist as to why etc.

Coming back to what is discussed in this thread, I thought I made myself clear about dark matter and direct and indirect observations. You then come along and say

Originally Posted by Acleron http://forums.randi.org/helloworld2/buttons/viewpost.gif (http://forums.randi.org/showthread.php?p=3847621#post3847621)
So what hypothesis do you have to explain the non-Newtonian if not non-Einsteinian orbiting of visually detectable matter in galaxies?


Lets get back onto this topic instead of bashing non scientists.

[Skwinty]

I apologise for this shameless bump, but I want to hear more on this topic.:wackychatter:

[sol invictus]

I apologise for this shameless bump, but I want to hear more on this topic.

In previous threads I've tried to lay out my (philosophical, if you want) view of this sort of issue.

Scientific theories need to pass several tests to be worth considering.


They must be specific and quantitative, so that they are capable of making precise predictions.

They must be internally self-consistent.

They must be consistent with all existing experimental data.


Given two such theories, how can we decide between them? I think the best answer is to use a combination of goodness of fit and number of parameters. Usually theories contain some continuous parameters (the density and temperature of dark matter, for example). The more parameters you add, the easier it is to fit the data (add another type of dark matter with a different temperature and different density and find the best fit to data for those four parameters - it will always be at least as good, and usually better, than the two parameter model).

Theories with lots of parameters are less predictive, less elegant, and should be strongly disfavored. So generally one wants to strongly disfavor theories with lots of parameters, and mildly favor theories that fit the data well. There are mathematical formulas for this, although they're a bit ad hoc.

On the topic of this thread - PC passes none of the three tests above and therefore cannot be compared to a scientific theory like CDM. Lambda-CDM on the other hand is extremely successful by this kind of measure, much more so than any alternative I'm aware of.