Casimir Effect, Dark Energy, Dark Matter, Negative Energy

[INRM]

The Casimir effect, is that energy released positive energy, dark energy etc?

Also, what is Dark energy, and how does it differ from regular energy?

Also has negative energy been proven to exist? I know quantum physics predicts it, but has it even been identified?

[Ziggurat]

Also has negative energy been proven to exist? I know quantum physics predicts it, but has it even been identified?

Generally speaking, "negative energy" doesn't mean much at all. In most of physics (including standard quantum mechanics), the only quantity that matters is changes in energy. That means what you define as your "zero energy" state is arbitrary, and so whether something has positive or negative energy depends upon that arbitrary choice. It's exactly analogous to being able to choice the zero longitude on a globe - differences in longitude don't depend upon that choice, but the absolute value of longitude at any particular place does, and is therefore also arbitrary.

There are contexts in which the choice of your zero for your energy scale isn't so arbitrary (such as general relativity), but that's not most of the time. And even then, negative energies just mean energies less than your zero reference (such as two masses which have approached each other, and hence have less gravitational energy than if they were infinitely far apart).

[TV's Frank]

I only have time to answer briefly right now...

I don't think the Casimir effect is a suitable explanation for dark energy. Why? As far as we can tell, the dark energy density in the universe has remained constant for several billion years. The Casimir effect energy density drops off as 1/length^4. We live in an expanding universe, so as the universe gets larger the Casimir effect would diminish rapidly.

"Dark energy" is just a name we give to the observed accelerated expansion of the universe. There could be many causes for it, or General Relativity may be wrong on this big of a scale. However, we can pretend this expansion is due to a certain form of energy. We make pretend like this because it makes life easier to describe the properties of the accelerated expansion. Dark energy has a special property: it has negative pressure (not negative energy).

I don't have enough time to dig into the third question...

Hope I could help!

[TV's Frank]

Ok, maybe I do have time to take a stab at #3...

Like Ziggarut said, there are different ways of defining "energy". In quantum mechanics, for a physical system, you have states and associated energies (called energy eigenvalues). In normal QM, all energy eigenvalues are positive-definite. In relativistic QM, at first blush it appears that you can have negative energy states. You may have heard of the "Dirac sea", which is the set of all negative energy states. However, this is just a conceptual model and not really accurate. When you go to actually solve the equations, what you thought were negative energy states just turn into anti-particles, which have positive energy eigenvalues.

I wrote this kind of half-assed, so there may be some issues I left too vague or didn't quite get right. I'm sure the legion JREFers will dissect this issue further.

[Reality Check]

The Casimir Effect (http://en.wikipedia.org/wiki/Casimir_Effect) is a quantum mechanical effect not related to "energy released positive energy, dark energy etc".

[Reality Check]

Dark energy (http://en.wikipedia.org/wiki/Dark_energy) differs from regular energy in that it is not known to interact through any of the fundamental forces other than gravity.

[Ziggurat]

In normal QM, all energy eigenvalues are positive-definite.

I don't think so. The standard convention for electron orbitals in a hydrogen atoms references the unbound electron at infinite distance from the proton as being zero energy, which makes the bound state negative.

[TV's Frank]

I don't think so. The standard convention for electron orbitals in a hydrogen atoms references the unbound electron at infinite distance from the proton as being zero energy, which makes the bound state negative.

You are absolutely right. I'll only respond with a Wikipedia link:

http://en.wikipedia.org/wiki/Negative_energy

[robinson]

Dark energy (http://en.wikipedia.org/wiki/Dark_energy) differs from regular energy in that it is not known to interact through any of the fundamental forces other than gravity.

According to popular theory, Dark Energy is the opposite of gravity. It is causing Galaxies, the entire known Universe, to speed up, rather than slow down from the effect of gravity.

[Reality Check]

According to popular theory, Dark Energy is the opposite of gravity. It is causing Galaxies, the entire known Universe, to speed up, rather than slow down from the effect of gravity.

Whose "popular theory"?

Actually Dark Energy has no relationship to gravity. Gravity is a force created by the curvature of space-time by mass. Dark Energy is a form of energy that exists throughout the universe.

[robinson]

We would all love to see a source for that. Do you have any reputable, published sources for that claim?

[INRM]

TV's Frank
"Dark energy" is just a name we give to the observed accelerated expansion of the universe. There could be many causes for it, or General Relativity may be wrong on this big of a scale. However, we can pretend this expansion is due to a certain form of energy. We make pretend like this because it makes life easier to describe the properties of the accelerated expansion. Dark energy has a special property: it has negative pressure (not negative energy).

Negative pressure? Wouldn't that mean it pulls things in instead of pushes away? How would that make the universe expand?


I don't think so. The standard convention for electron orbitals in a hydrogen atoms references the unbound electron at infinite distance from the proton as being zero energy, which makes the bound state negative.

So negative energy has been proven to exist, correct?

[robinson]

Dark energy, a mysterious force that no one understands, is causing the universe to fly apart faster and faster. Only a few years ago, if you'd suggested something like that to astronomers, they would have told you to spend less time in front of the TV and more time in the "real" world.

But dark energy is real or at least, a growing number of astronomers think it is. No one, however, can truly explain it.

"Frankly, we just dont understand it," says Craig Hogan, an astronomer at the University of Washington at Seattle. "We know what its effects are," Hogan says, but as to the details of dark energy, "Were completely clueless about that. And everybodys clueless about it."
http://www.space.com/scienceastronomy/astronomy/cosmic_darknrg_020115-1.html

Scientists don’t know what dark energy is, but it seems to work opposite gravity and to speed up an ongoing expansion of the Universe.
http://www.world-science.net/othernews/070823_void.htm


The answer lies in dark energy, which became a dominant force in the Universe very recently, when the Universe was already three-quarters of the size it is today. Dark energy works opposite gravity and is speeding up the expansion of the Universe. Thanks to dark energy, CMB photons that pass through a large void just before arriving at Earth have less energy than those that pass through an area with a normal distribution of matter in the last leg of their journey.
http://www1.umn.edu/umnnews/news_details.php?release=070823_3456&page=NS

Force That Counters Gravity Existed Early in Universe
http://www.washingtonpost.com/wp-dyn/content/article/2006/11/16/AR2006111601425.html

Dark energy is a sort of anti-gravity force that pushes things apart. In fact, the universe appears to be expanding, which would indicate that dark energy is stronger than gravity. But no one really understands what either force is.
http://mklekacz.wordpress.com/2007/03/13/great-mysteries-dark-matter-dark-energy-and-gravity/

[Reality Check]

According to popular theory, Dark Energy is the opposite of gravity. It is causing Galaxies, the entire known Universe, to speed up, rather than slow down from the effect of gravity.http://www.space.com/scienceastronomy/astronomy/cosmic_darknrg_020115-1.html

http://www.world-science.net/othernews/070823_void.htm

http://www1.umn.edu/umnnews/news_details.php?release=070823_3456&page=NS

Force That Counters Gravity Existed Early in Universe
http://www.washingtonpost.com/wp-dyn/content/article/2006/11/16/AR2006111601425.html

http://mklekacz.wordpress.com/2007/03/13/great-mysteries-dark-matter-dark-energy-and-gravity/

I see that "popular theory" means reported in the popular press. In that sense "Dark Energy is the opposite to gravity" - after all that is what the articles say.
I would be pedantic and say that Dark Energy opposes the force of gravity. It is not the literal opposite to gravity (that would be anti-gravity).

You have emphasized some sentences about no one knowing what exactly Dark Energy is. Is there a point to this?

[robinson]

Whose "popular theory"?

Actually Dark Energy has no relationship to gravity.

I answered your question, "Whose 'popular theory'?", in regards to DM and gravity.

I then asked you for a source for your statement, "Actually Dark Energy has no relationship to gravity". I note that you didn't answer.


You have emphasized some sentences about no one knowing what exactly Dark Energy is. Is there a point to this?

Obviously. Dark Energy is a huge unknown. Because gravity doesn't seem to be working like we calculate, based on the calculated mass of Galaxies and such, popular theory is that there must be an unknown force acting to counter gravity. Which is called Dark Energy, because like Dark Matter, it doesn't seem to interact with matter except by gravity. Except in the case of DE, it is not attracting matter, but causing it to move away from other matter. (Don't look at me, that is what THEY are saying, not me)

Here we argue that a possible solution for this problem may be found in the remarkable new coincidence that the antigravity of dark energy starts to dominate over the gravity of lumpy matter at the distance where the Hubble flow emerges.
Astronomy & Astrophysics
http://www.aanda.org/index.php?option=article&access=standard&Itemid=129&url=/articles/aa/full/2001/42/aah3040/aah3040.right.html

[sol invictus]

Because gravity doesn't seem to be working like we calculate, based on the calculated mass of Galaxies and such, popular theory is that there must be an unknown force acting to counter gravity.

Actually the most direct evidence for dark energy does not depend on the mass of galaxies (or the existence of dark matter). Observations of distant supernovae show that the the expansion of the universe is accelerating. Independently of anything else, that proves that there is a repulsive force active on cosmological distance scales.

By far the simplest and most economical explanation for that observation is a positive cosmological constant - something which also happens to fit perfectly into general relativity, and which also explains all the other types of observations of DE (most of which do depend on things like the total matter density). Indeed, for most physicists the surprise is that the CC is so small, not that it isn't zero.

[Ziggurat]

So negative energy has been proven to exist, correct?

As I already pointed out, in most of physics, it's only changes in energy which matter, not absolute energies. Your reference point is therefore usually arbitrary, and if you choose a particular reference point as your zero energy, then any state with energy less than that reference point has negative energy. But there's nothing inherently different about the energy of such a negative energy state than a positive energy state, and the negative energy state wouldn't even be a negative energy state if you just chose a different reference as your zero. So why is it at all significant whether or not you can label a state as having negative energy or not? You certainly can, but so what?

[sol invictus]

As I already pointed out, in most of physics, it's only changes in energy which matter, not absolute energies. Your reference point is therefore usually arbitrary, and if you choose a particular reference point as your zero energy, then any state with energy less than that reference point has negative energy. But there's nothing inherently different about the energy of such a negative energy state than a positive energy state, and the negative energy state wouldn't even be a negative energy state if you just chose a different reference as your zero. So why is it at all significant whether or not you can label a state as having negative energy or not? You certainly can, but so what?

Perhaps what he's trying to get at is the existence of a ground state?

There is an asymmetry between positive and negative energy in the sense that good theories have a minimum energy state. As Zig says you are perfectly free to define that energy to be positive or negative - unless we consider gravity, in which case it makes a difference - but whatever you choose it to be, all other states in the theory have higher energy. So you can have arbitrarily large positive energies, but you cannot have arbitrarily negative energies (if you do, the theory is unstable).

[robinson]

Observations of distant supernovae show that the the expansion of the universe is accelerating. Independently of anything else, that proves that there is a repulsive force active on cosmological distance scales.

Accepting that the redshift of very distant objects shows they are accelerating, that everything is moving away from everything else, there must be a source of energy for this motion.

Is it our mythical friend, Dark Energy? Is it that space/time itself is expanding? Both? What exactly is the popular belief about this?

If it is DE, then we are discussing an energy source that doesn't appear, (why it is called dark), that causes matter to accelerate, but only in an expanding direction. Is that correct?

According to the scant sources I can find, the amount of repulsive energy, (we can't say anti-gravity for some reason), that is needed to accelerate every known object in the known Universe, is a very large number. Which is why when converting that energy to mass, they state it is equal to 15 times the mass that we can see, or 4 times the amount of matter and dark matter. Or something like that.

But this energy causes stuff to move away from each other, not attract, like gravity does. But not just move, but accelerate. This means Galaxies are not only moving away from each other, they are moving faster every second.

Correct?

Of all the odd things about the Universe as we observe it, that has to be right up there at the top.

Most of the Universe is made of something we can't see, being moved by something we can't see, which is extremely unlikely, yet that is the current theory.

Science is way cooler than any woo woo stuff.

[Wangler]

Indeed, for most physicists the surprise is that the CC is so small, not that it isn't zero.

Sol,

I am under the opinion that the smallness of the CC is in some ways troubling to cosmology, but I am not sure as to why.

Do you find anything fundamentally troubling about a tiny CC, or is it just surprising?

[Reality Check]

I then asked you for a source for your statement, "Actually Dark Energy has no relationship to gravity". I note that you didn't answer.
I answer here: Gravity (http://en.wikipedia.org/wiki/Gravity) is a force. Dark Enery (http://en.wikipedia.org/wiki/Dark_energy) is an energy. A force is not an energy (consult a dictionary).

[Reality Check]

Obviously. Dark Energy is a huge unknown. Because gravity doesn't seem to be working like we calculate, based on the calculated mass of Galaxies and such, popular theory is that there must be an unknown force acting to counter gravity.

This is the evidence for Dark Matter. The evidence for Dark Energy (http://en.wikipedia.org/wiki/Dark_energy#Evidence_for_dark_energy) comes from supernovae observations that suggest that the expansion of the universe is accelerating. But in a later post you seem to understand this.


Which is called Dark Energy, because like Dark Matter, it doesn't seem to interact with matter except by gravity. Except in the case of DE, it is not attracting matter, but causing it to move away from other matter. (Don't look at me, that is what THEY are saying, not me)
Astronomy & Astrophysics
http://www.aanda.org/index.php?option=article&access=standard&Itemid=129&url=/articles/aa/full/2001/42/aah3040/aah3040.right.html

Just because a few papers use "antigravity" to describe the effects of Dark Energy does not make Dark Energy an opposite to gravity.
According to Google Scholar there are 215 results for "antigravity" and "dark energy" out of a total of 16,4000 for "dark energy" only.

[Reality Check]

Accepting that the redshift of very distant objects shows they are accelerating, that everything is moving away from everything else, there must be a source of energy for this motion.

There is no source of energy for the expansion of the universe.


Is it our mythical friend, Dark Energy? Is it that space/time itself is expanding? Both? What exactly is the popular belief about this?

The scientific theory is that space/time itself is expanding.


If it is DE, then we are discussing an energy source that doesn't appear, (why it is called dark), that causes matter to accelerate, but only in an expanding direction. Is that correct?

There is no energy source for DE. DE is a hypothetical form of energy that permeates all of space. It may be the cosmological constant (http://en.wikipedia.org/wiki/Cosmological_constant), the quintessence field (http://en.wikipedia.org/wiki/Quintessence_%28physics%29)(which varies in space and time) or it may be something else (http://en.wikipedia.org/wiki/Dark_energy#Alternative_ideas).


According to the scant sources I can find, the amount of repulsive energy, (we can't say anti-gravity for some reason), that is needed to accelerate every known object in the known Universe, is a very large number. Which is why when converting that energy to mass, they state it is equal to 15 times the mass that we can see, or 4 times the amount of matter and dark matter. Or something like that.

But this energy causes stuff to move away from each other, not attract, like gravity does. But not just move, but accelerate. This means Galaxies are not only moving away from each other, they are moving faster every second.

Correct?

That is correct. The current expansion of space-time is causing galaxies to move away from each other. Dark Energy is causing the expansion of space time to accelerate and so galaxies move away from each other faster than we expect from the Big Bang alone.


Of all the odd things about the Universe as we observe it, that has to be right up there at the top.

Most of the Universe is made of something we can't see, being moved by something we can't see, which is extremely unlikely, yet that is the current theory.

Science is way cooler than any woo woo stuff.

[robinson]

There is no source of energy for the expansion of the universe.

That is the sort of claim that needs one heck of a lot of evidence to back it up.

Especially the other part, about space/time expanding. First off, space/time needs to be defined, then a description of how big it is, so we can measure how much bigger it is getting.

What units do we measure space/time in? Is this proposed expansion going on everywhere? Or just between Galaxies? I know it is difficult to explain, but gosh darn, when you say something like that, it really is up to you to explain at least why you think so. How and why it is occurring, assuming you can show it is, would also be most helpful.

Oh, I almost forgot.

When I asked you for a source of evidence for your claim, posting a link to Wikipedia pages on gravity and dark energy is dumb.

I asked for a published, reputable source for your statement.

[TV's Frank]

Sol,

I am under the opinion that the smallness of the CC is in some ways troubling to cosmology, but I am not sure as to why.

Do you find anything fundamentally troubling about a tiny CC, or is it just surprising?

If I may answer this for Sol...

In quantum field theory, the vacuum energy density (which would appear as a cosmological constant in general relativity) is formally infinite. Now, you have to play a lot of games in field theory to get around this infinity, and usually at the end you get a normal answer. In field theory, the idea of an infinite vacuum energy isn't really a big deal, since there you only care about differences in energy, not the actual values. And, yes, it is possible to subract two infinities from each other (you just have to be careful).

However, infinite energy is a problem for general relativity. So, before the discovery of dark energy, everyone just thought that there was some sort of "damping" mechanism that brought this vacuum energy down from infinity to 0. Everyone just assumed that in a few years some particle physicist would be taking a shower and go "oh!" and figure out why the cosmological constant was 0.

Then, we discovered dark energy. If dark energy is caused by the vacuum energy (and hence is a cosmological constant), then the vacuum energy is very very very very very small. Less than 10^-31 grams/cm^3, actually. So, if some sort of damping mechanism exists, then it has to bring the vacuum energy from infinity to CLOSE to 0, but not exactly. This is what's troubling. Why wouldn't a damping mechanism bring it all the way down?

Another troubling aspect is this. Suppose you say that the vacuum energy density isn't really infinity. Let's say you want to cut off the value at something very large, like the GUT scale, or the Planck scale. When you do this, the answer you get is something like 10^90 grams/cm^3. So we get that the best theoretical value disagrees with the observed value by about 120 orders of magnitude! Ouch!

[sol invictus]

Sol,

I am under the opinion that the smallness of the CC is in some ways troubling to cosmology, but I am not sure as to why.

Do you find anything fundamentally troubling about a tiny CC, or is it just surprising?

It is said by many to be the most profound and difficult problem in all of modern physics. However the source of the problem isn't cosmology - it's particle physics and quantum mechanics.

The cosmological constant is a vacuum energy. If it is non-zero, any region of empty space will have an energy proportional to its volume times the CC. In quantum theories of particle physics, the "vacuum" is not really empty - it's full of so-called virtual particles, pairs of which constantly appear and annihilate with each other. This zero-point activity imparts an energy to the vacuum.

The problem is that when you calculate that energy you get an answer that's at least 60 orders of magnitude (a quadrillion quadrillion quadrillion quadrillions) larger than the value we have measured in cosmology.

[sol invictus]

Especially the other part, about space/time expanding. First off, space/time needs to be defined, then a description of how big it is, so we can measure how much bigger it is getting.

That's given to us by Einstein's theory of general relativity. I could write the metric describing expanding space/time in cosmology for you but I don't think it would mean much to you.

What units do we measure space/time in?

The usual ones. Meters and seconds, generally.

Is this proposed expansion going on everywhere? Or just between Galaxies?

Mostly just between galaxy clusters, actually.

I know it is difficult to explain, but gosh darn, when you say something like that, it really is up to you to explain at least why you think so.

While it can certainly be confusing, the theory has been around for 90 years and is universally accepted as correct (or anyway very accurate), so it's not like RC is making some radical statement there.

Let me give you an example of how to measure the curvature of 3D space. Maybe that will help. Take three points (say three mountain tops or three spacecraft), pretty far apart. Take some surveying equipment, or some lasers, and make a perfect triangle with the points the corners. Now measure the angles at each of the three corners.

If they add up to 180 degrees, space is flat in that plane. If they add up to more than 180, it's positively curved; less than 180, negatively curved. The difference between 180 and the angle you measure is the curvature times the area of the triangle, so the curvature has units of 1/meters^2.

[Reality Check]

That is the sort of claim that needs one heck of a lot of evidence to back it up.

Especially the other part, about space/time expanding. First off, space/time needs to be defined, then a description of how big it is, so we can measure how much bigger it is getting.

Space/time is space + time: refer to any GR paper - I suggest that you start with the 1905 paper by an Albert Einstein.


What units do we measure space/time in? Is this proposed expansion going on everywhere? Or just between Galaxies? I know it is difficult to explain, but gosh darn, when you say something like that, it really is up to you to explain at least why you think so. How and why it is occurring, assuming you can show it is, would also be most helpful.

Distance and time so metres and seconds (SI units).
Space/time is everything so everywhere is expanding.
Read any physics textbook (or Wikipedia - and yes I know that it is unreliable but it is a good place to start).


Oh, I almost forgot.

When I asked you for a source of evidence for your claim, posting a link to Wikipedia pages on gravity and dark energy is dumb.

I asked for a published, reputable source for your statement.

I guess you are too lazy to go to the papers cited in the Wikipedia articles so I will add a couple for you here:
Dark Energy: P. J. E. Peebles and Bharat Ratra (2003). "The cosmological constant and dark energy (http://www.arxiv.org/abs/astro-ph/0207347)
Gravity: Feynman, R. P.; Morinigo, F. B., Wagner, W. G., & Hatfield, B. (1995). Feynman lectures on gravitation. Addison-Wesley. ISBN 0201627345.

Of course there are many other papers: Try arXiv (http://arxiv.org/).

[sol invictus]

Space/time is everything so everywhere is expanding.


That part is not correct.

Space doesn't expand uniformly, because the equation for it depends on the local energy density. Within structures smaller than galaxy clusters (which are millions of lightyears across) it isn't expanding at all - it's actually collapsing, or doing something more complicated. But averaged over the observable universe - on the scale of which galaxy clusters are quite small - it is expanding.

The classic example is a balloon with pennies glued to it being inflated. Only the rubber (space) between the pennies (galaxies or clusters) expands.

[robinson]

I hope it is obvious why sometimes it seems confusing when people just say stuff, without using a source, a reputable source, to back up the claim.

Is the Universe (space/time) expanding everywhere?


Mostly just between galaxy clusters, actually.


Space/time is everything so everywhere is expanding.

(edit) I see Sol did speak to that. Thanks.

Back to the original claim I asked about.

Actually Dark Energy has no relationship to gravity. Gravity is a force created by the curvature of space-time by mass. Dark Energy is a form of energy that exists throughout the universe.

Again, can you quote a source for those statements? Especially the DE part.

When I hear statements that make no sense, I like to know who came up with them.

There is no source of energy for the expansion of the universe.

I find it hard to imagine I am the only one who looks at that and asks, "wtf?". How can anyone even say that?


The scientific theory is that space/time itself is expanding.

Now we are back to the question. Is this a Universal expansion? Or just between clusters of Galaxies?


The current expansion of space-time is causing galaxies to move away from each other. Dark Energy is causing the expansion of space time to accelerate and so galaxies move away from each other faster than we expect from the Big Bang alone.

Again, what is the source of the energy causing acceleration? And what is space/time?

If time is expanding, does that mean a second gets larger, or smaller? Or in other words, does time expanding mean it gets longer? Or shorter? And how does that effect the speed of light? Particle/wave functions? Matter? Gravity?

[robinson]

For those who may be confused, acceleration requires energy, which is why I ask about the source of the energy.

If the answer is space/time is expanding, then what is the energy source for that?

If no answer is available, that is understandable.

[sol invictus]

For those who may be confused, acceleration requires energy, which is why I ask about the source of the energy.

The acceleration is due to dark energy, by definition of "dark energy".

To give a slightly more useful answer, Einstein's equations tell us that certain forms of energy lead to accelerated expansion. The simplest of those is a positive cosmological constant, which is a vacuum energy (an energy density constant in both space and time, and which does not change when the space expands).

[robinson]

So no matter how much space/time expands, there is always more energy to cause more expansion?

Is that really what you are saying?

[DeiRenDopa]

'Dark energy' (DE) = cosmological constant (CC, a.k.a. Λ)?

This is one of the hottest topics in observational cosmology today! :)

If it turns out that DE is not CC, then perhaps CC is, in fact zero (and the particle/quantum physics issue becomes less troubling ... it's still a huge question, of course).

What, other than, Λ, could DE be? Several things have been proposed, and several tests have already been done, to try to distinguish between the alternatives. For example, one test involves looking for a change in DE with time - CC, being constant, is not expected to change with time. As far as I know, no firm conclusions have yet been reached regarding this question ... the interpretations of the observations do not rule out anything, yet.

[sol invictus]

So no matter how much space/time expands, there is always more energy to cause more expansion?

Expansion, in ordinary physics as in GR, does not generally require any energy source at all. Acceleration does.

Is that really what you are saying?

The solution is a kind of run-away - the more the space expands the more ordinary matter and energy density dilutes while the vacuum energy density is unchanged. Therefore the late-time solution is dominated by vacuum energy.

A very good model for this is an icy hilltop. Galaxies are objects near the top of the hill, held together by their gravitational self-attraction. They are also weakly bound to each other, but if they are too far apart the slope of the hill (that's dark energy) breaks the bond and they go flying off down it. After a while, only the one at the very top remains, all alone.

That's our future.

[Reality Check]

Again, can you quote a source for those statements? Especially the DE part.

Read any paper on dark energy - you will see that it is an energy.
Read any paper on gravity - you will see that it is a force.
An energy cannot be the opposite to a force. You need another force to be opposite to a force.
As far as I can see (and I am probably wrong), Dark Energy may be considered as the source of a force just like mass and energy is the source of gravity. In that case it is that force that is the opposite of gravity.

I doubt that there is a paper that states such a basic concept and applies it to dark energy and gravity. If you believe that such a paper exists then go look for it. There are a number of papers that use "antigravity" as a shorthand for the effects of Dark Energy but as I pointed out before this is not common usage.


Now we are back to the question. Is this a Universal expansion? Or just between clusters of Galaxies?

It is an expansion of the universe but local conditions can override it, e.g. the singularity in the middle of a black hole does not expand, the gravitational pull of a star is much greater than the expansion and it is not until you get out to the scale of galactic clusters that local conditions do not matter.


Again, what is the source of the energy causing acceleration? And what is space/time?

The source of the expansion is the original explosion of space/time (the Big Bang). Space/time is space + time. Space is the three dimensions that we measure the lengths, widths and heights of objects with. These are usually denoted by x, y and z. Time is denoted by time. A point in space/time is denoted by a set of 4 numbers - (x,y,z,t).


If time is expanding, does that mean a second gets larger, or smaller? Or in other words, does time expanding mean it gets longer? Or shorter? And how does that effect the speed of light? Particle/wave functions? Matter? Gravity?
Actually that is an interesting question. I have been reading Roger Penrose's 'The Road to Reality' lately so spacetime comes to mind when talking about the universe. But I think that the expansion is just space not time.

[sol invictus]

There are a number of papers that use "antigravity" as a shorthand for the effects of Dark Energy but as I pointed out before this is not common usage.

It's not common because it's misleading. I think when you say antigravity, most people think of a force just like gravity but repulsive. The force due to dark energy is not like that.

The ordinary gravitational force between two objects is proportional to the product of their masses, and it always acts on each object equally and in the opposite directions (attracting them together). Because the forces are equal in magnitude, if one object is much more massive than the other it will barely move (while the other orbits or falls onto it).

Dark energy gives rise to a repulsive force - so in that sense it's like antigravity - but when it acts on something the force is proportional only to the mass of that object, and moreover it pushes each object away from some central point (the origin of coordinates), not away from each other. It does not act equally on two separated objects, and the direction of the force is not opposite (generally). All objects accelerate away from the origin with a rate that only depends on their distance to the origin, and not on their mass or any other mass.

[robinson]

Expansion, in ordinary physics as in GR, does not generally require any energy source at all. Acceleration does.

We know that. The question is, if Galaxies are accelerating away from each other, and according to theory (based on observations of distant luminous bodies), they are, and the farther away the faster they are accelerating, where does that energy come from? Just to negate the gravitational attraction would require a huge amount of energy, but to accelerate entire Galaxies, that is, as was stated, a huge amount of energy.

So much that if converted to mass, it is 15 times more mass than all the visible matter in the Universe. This means, if the theory is correct, that most of the Universe is some unknown invisible energy, which acts against the force of gravity, and if we believe our instruments, and our understanding of mass and gravity, it is accelerating everything.

Away from everything else. Which is back to the questions, (which I don't expect anyone to know the answer to, as was pointed out, this is a huge question), where does that energy come from? How is this unknown energy accelerating almost every massive object we can see, faster and faster?

Is it the nature of space/time itself to just cause everything to fly apart, contrary to gravity? Why does it only seem to be effecting Galaxies?

The glacier analogy is flawed of course. Gravity is the source of the acceleration in that example.

Are there other theories that explain all this, without needing a mysterious unknown energy?

It is all quite fascinating. Isn't it?

[robinson]

As I side note, this medium, quoting and writing back and forth, especially without a plethora of emoticons or other methods of transmitting tone, often leads to misunderstanding, especially when humor, subtle at times, is being used.

I see it all the time in other threads, where somebody responds with a heated and lengthy tirade, when all along the comment that set them off, was only humor, which is obvious to some, but clearly not to the poor mind busy responding, with such fervor and serious intent.

Considering the nature of the topic, vast theoretical musings about events and energies, trillions and gazillions of miles away, billions of years in the past, it isn't the sort of thing to burst a blood vessel over.

[sol invictus]

]The question is, if Galaxies are accelerating away from each other, and according to theory (based on observations of distant luminous bodies), they are, and the farther away the faster they are accelerating, where does that energy come from?

It is simply potential energy being converted to kinetic energy, just like an object sliding or rolling down a hill.

So much that if converted to mass, it is 15 times more mass than all the visible matter in the Universe.

Yes, roughly. But remember that the mass density in a galaxy, let alone on a planet, is far higher than the average density.

This means, if the theory is correct, that most of the Universe is some unknown invisible energy, which acts against the force of gravity, and if we believe our instruments, and our understanding of mass and gravity, it is accelerating everything.

It acts against the gravitational attraction of galaxies, but it doesn't act against gravity - it IS gravity. It's the gravitational force due to the dark energy acting on the galaxy, if you want to think of it that way.

Away from everything else. Which is back to the questions, (which I don't expect anyone to know the answer to, as was pointed out, this is a huge question), where does that energy come from? How is this unknown energy accelerating almost every massive object we can see, faster and faster?

It comes naturally out of quantum mechanics. The difficulty is that the number one gets that way is much too large. So from that point of view, it is not at all surprising that there is such a force - the surprise is why the force is so weak.

Is it the nature of space/time itself to just cause everything to fly apart, contrary to gravity? Why does it only seem to be effecting Galaxies?

Again, it's not contrary to gravity, it IS gravity. It only affects structures which are large enough, because locally it is an extremely weak force, and in any structure smaller than a galaxy cluster the local gravitational attraction is stronger than it.

The glacier analogy is flawed of course. Gravity is the source of the acceleration in that example.

Nonsense - it is not flawed, at least not in that way. Gravity is the source of the acceleration here as well.

Are there other theories that explain all this, without needing a mysterious unknown energy?

Yes there are, but they all have problems, and they are all more complex than simply adding a cosmological constant to the Einstein-Hilbert action.

It is all quite fascinating. Isn't it?

Yes, it is.

[INRM]

I thought I remember hearing something about negative energy being produced by a "compressed state" or something like that...

[BeAChooser]

Just to muddy the water ...

http://books.google.com/books?id=fLfd-oJRQIcC&pg=PA227&lpg=PA227&dq=negative+energy+narlikar&source=web&ots=ive1I4nfzl&sig=DNV3o7RadnkHLm4rYtgZ52uVMjU&hl=en "A Different Approach to Cosmology: From a Static Universe Through the Big Bang Towards Reality, by Fred Hoyle, Geoffrey Burbidge and Jayant V. Narlikar, 2000 ... snip ... Negative energy fields are inherently explosive. Concentrated locally they produce violent events, offering a possibility of explaining a wide range of observations - radiogalaxies, QSOs and active galactic nuclei. When uniformly distributed, a negative energy field exerts a negative pressure that shows itself in our view in the expansion of the universe."

:D

[Dancing David]

That part is not correct.

Space doesn't expand uniformly, because the equation for it depends on the local energy density. Within structures smaller than galaxy clusters (which are millions of lightyears across) it isn't expanding at all - it's actually collapsing, or doing something more complicated. But averaged over the observable universe - on the scale of which galaxy clusters are quite small - it is expanding.


Man I love that, some pundits (which is a generous term) complain that modern physics tries to much to simplify things (mainly because they want philospohy to enter in.) And then i read a statement like that which is more what physics is about. At this one scale and observation we see something that we currently classify as the expansion of space time, however at another scale and set of observations we see something that looks different.

It would appear that the last things physics tries to do is simplify things.

Just so cool, on one scale we have general relaticity and at another we have QM, and there are some things that are likely to be true that just go against human inuition and perception. (Black holes seem to really upset some people. Just as relativity did during the 20s, 30s, 40s of the last century and in fact it still bother some people).

[Dancing David]

I thought I remember hearing something about negative energy being produced by a "compressed state" or something like that...


Well it is one of those counter intuitive things, especially whne gravity is mentioned.

But here is an example of what I think some are saying when they say there isn't really negative energy, dependeing on a scalar point.

You have rock at the surface of the earth. If you carry it up a hill, you are adding potential energy to it. That is positive energy from the scalar point of the surface of the earth.

You then dig a hole for your rock, say 10 meters down and let the rock fall, it expresses it's potential energy and falls to the bottom of the hole, thud. Equilibrium established. It is now at a negative potential energy regards the surface again. So this is negative energy from the scalar point of the surface of the earth.

Now let us add a higher hill and a mine shaft to the picture. To take our rock from the top of the hill to the top of the higher hill will be adding 'positive' energy from teh scale of the surface.

Now our rock which is at the bottom of the hole, say the mine shaft is right next to the hole, we can drop the rock from the hole down the shaft, which will lower it's potential energy even more.

Now wait, here we have the rock in the hole, it has negative energy comkpared to the surface of the earth, but in relation to the bottom of the mine shaft it has positive energy.

This is what the scale zero point thing is about. The hill is a positive energy state until we consider the higher hill, then it becomes negative. The surface of the earth is zero, unless we scale from the top of the hill, then it is negative, or the bottom of the hole, then it is positive. The bottom of the hole is negative unless we consider the bottom of the mine shaft, then it is positive.

So when we talk about potential energy and psoitive states and negative states it is very imporatnt to consider what you are scaling from and to.