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 me