Dark Matter: what is it

[vincentm]

Can someone explain to me what is it or what is theorized to be dark matter. What effects does it have in regards to the cosmos.

Thank you,
vm

[Lisa!]

I think if you do a search for dark matter, you can find more threads. (http://www.physicsforums.com/showthread.php?t=76921)

It seems that dark matter is so popular around here these days. :wink:

[vincentm]

Thanks lisa!

[Chronos]

In a universe that submits to Newtonian gravity and GR principles, dark matter is the missing mass necessary to explain: large scale structures in the universe; what holds galactic clusters together; galactic rotational curves; gravitational lensing; WMAP power spectrum, and a few other puzzling observational artifacts. Aside from that, DM is just another epicycle.

[Lisa!]

Thanks lisa!
Thanks to people who answered my question.
(Happy belated birthday ,if I'm not mistaking)

[Mike2]

So if anyone were to come up with a possible theory for Dark Matter, he would have to compare it with observation. So I have to wonder how much mass is needed to produce the rotation curves and the extra lenzing effects that we observe... how much more matter is needed, compared to a galaxy, to produce the constant rotation curve, and how must it be distributed to produce the effect? Thanks.

[Nacho]

I know this is a question that is hard to quantify .. I have absolutely no way to even start thinking about solving it.

Say you have a diffuse mass that has quite a bit more relative motion against the mass contained in an average galaxy. What would the gravitational effect of that mass be on that galaxy as it passed it by?

I know that problem is poorly defined .. make assumptions where you feel free to.

[Mike2]

So if anyone were to come up with a possible theory for Dark Matter, he would have to compare it with observation. So I have to wonder how much mass is needed to produce the rotation curves and the extra lenzing effects that we observe... how much more matter is needed, compared to a galaxy, to produce the constant rotation curve, and how must it be distributed to produce the effect? Thanks.
Surely they had some distribution in mind in order to suggest that it flattens rotation curves, right? I even hear recently that they think there may even be some DM in the center of galaxies. What did they use for a model to even come up with the suggesting that extra mass was needed?

[SpaceTiger]

So if anyone were to come up with a possible theory for Dark Matter, he would have to compare it with observation. So I have to wonder how much mass is needed to produce the rotation curves and the extra lenzing effects that we observe... how much more matter is needed, compared to a galaxy, to produce the constant rotation curve, and how must it be distributed to produce the effect?

It varies, but galaxies like the Milky Way seem to require about ten times as much dark matter as luminous matter. Most of this matter is in the outer halo, however, so if you were to just look at the dark matter interior to the sun, it would make up less than half of the total -- that is, the sun's motion is probably dominated by gravitation from baryonic matter.

[SpaceTiger]

Say you have a diffuse mass that has quite a bit more relative motion against the mass contained in an average galaxy. What would the gravitational effect of that mass be on that galaxy as it passed it by?

If the smaller mass (the galaxy) got close enough to the larger one, it would be tidally disrupted and you would likely see streams of stars and gas. Something like this:

.Colliding Galaxies (http://www.universetoday.com/am/uploads/2004-0917galaxy-full.jpg)

except you would only see one disrupted galaxy.

[SpaceTiger]

Surely they had some distribution in mind in order to suggest that it flattens rotation curves, right?

There are a variety of theoretical models for the dark matter profiles. A simple one that gives a flat rotation curve is the "isothermal sphere":

\rho=\frac{v^2}{4\pi r^2G}

where v is the rotation speed of the galaxy and r is the distance from the center. Numerical simulations imply a profile slightly different from this (called the NFW profile):

\rho \propto \frac{r_s}{r(1+\frac{r}{r_s})^2}

where r_s is a free parameter (the scale radius). It's still not clear what the best fit model is, but it's almost certainly not as simple as an isothermal sphere. It's also not clear that it should be universal (i.e. that all dark matter halos should have the same profile).


I even hear recently that they think there may even be some DM in the center of galaxies. What did they use for a model to even come up with the suggesting that extra mass was needed?

Most models assume that there is some dark matter in the centers of galaxies (for example, the formulae I gave above), but that it's of much lower density than the luminous matter. To my knowledge, the only possible observational evidence for dark matter at the centers of galaxies is the excess gamma-radiation and the "haze" in the WMAP data near the galactic center. These are being interpreted as an annihilation signature of, perhaps, neutralinos. That's still very speculative at this point, however.

[Mike2]

There are a variety of theoretical models for the dark matter profiles. A simple one that gives a flat rotation curve is the "isothermal sphere":
\rho=\frac{v^2}{4\pi r^2G}
where v is the rotation speed of the galaxy and r is the distance from the center. Numerical simulations imply a profile slightly different from this (called the NFW profile):
\rho \propto \frac{r_s}{r(1+\frac{r}{r_s})^2}
where r_s is a free parameter (the scale radius). It's still not clear what the best fit model is, but it's almost certainly not as simple as an isothermal sphere. It's also not clear that it should be universal (i.e. that all dark matter halos should have the same profile).
Most models assume that there is some dark matter in the centers of galaxies (for example, the formulae I gave above), but that it's of much lower density than the luminous matter. To my knowledge, the only possible observational evidence for dark matter at the centers of galaxies is the excess gamma-radiation and the "haze" in the WMAP data near the galactic center. These are being interpreted as an annihilation signature of, perhaps, neutralinos. That's still very speculative at this point, however.
I genuinely appreciate the help. So is the DM halo about 10 times as massive as the lumious matter?

I'd like to ask again if there is some evidence or proof that eliminates baryonic matter, such as too much scattering by baryonic matter for any distribution?

I'm seriously tempted to consider the energy density of the Unruh radiation applied to the acceleration due to gravity? It would seem like an easy calculation to find out. First find the energy density of this assumed Unruh radiation. This would involve an integral of Planck's density spectrum over all frequencies. I've looked at this, and I think I can find a definiate integral formula to accomplish this. This would give us an energy formula at temperature. That energy can be converted to mass, and the additional gravitational effects could be calculated from that. But you'd have to find the acceleration for the Unruh formula at a given radius from the galactic center. I suppose one could use Newton's inverse squared law as a good approximation. Then apply the equation for the Unruh temperature. Then one could construct an integral over all space of this extra mass density produced by the Unruh effect applied to acceleration due to gravity.

I suppose this might seem like a very small effect; but that's a lot of space, and I've not done the calculation yet. Not only that, but once you have a first approximation, then you'd have to do it all over again since now you have to take into account the existence of this first approximation results. Your galaxy just acquired more mass, so it will produce more gravitational acceleration that you realized, which requires another iteration of the process. I suppose you'd have to do this 4 or 5 times to see how quickly the series converged.

I suppose I could do it all, but I'd need help with conversion of units and some prelimiary approximation to a galaxy luminous mass distribution. Eh?

[SpaceTiger]

I genuinely appreciate the help. So is the DM halo about 10 times as massive as the lumious matter?

Yeah, see above.



I'd like to ask again if there is some evidence or proof that eliminates baryonic matter, such as too much scattering by baryonic matter for any distribution?

Nucleosynthesis and the cosmic microwave background both suggest that the dark matter can't be baryonic. Also, there is some evidence of dark matter that is separated from luminous matter:

Clowe et al. 2003 (http://arxiv.org/abs/astro-ph/0312273)


But you'd have to find the acceleration for the Unruh formula at a given radius from the galactic center. I suppose one could use Newton's inverse squared law as a good approximation.

Aren't these stars in approximate free-fall? I would think that the dominant effect that made their frame non-inertial would be rotation.

[Mike2]

Nucleosynthesis and the cosmic microwave background both suggest that the dark matter can't be baryonic. Also, there is some evidence of dark matter that is separated from luminous matter:
Clowe et al. 2003 (http://arxiv.org/abs/astro-ph/0312273)
If the major contribution of DM were photons, then it would not scatter light itself. But then I suppose you'd see a microwave signature of this effect, right? If 10 times the galaxy mass were evenly distributed throughout and around the galaxy, then I wonder how hot it would be?

In the conclusion section of the paper you cite, one finds: "Adopting big-bang nucleosynthesis limits on the mean baryonic mass of the universe excludes most of this mass from being baryons in cold, condensed structures." They do not rule out an even distribution throughout space.

If the ZPE is the CC, then it is not scattering light since we can see distant galaxies through it. If the same ZPE is causing DM effects due to the Unruh effect applied to the acceleration of gravity, then we should not expect it to scatter light either, since it too is a ZPE effect. This would have the same low scattering cross section as WIMPs.

Aren't these stars in approximate free-fall? I would think that the dominant effect that made their frame non-inertial would be rotation. That would be the Unruh effect felt by individual stars and planet, the atoms within stars, etc. I've seen calculations that show this to be very small indeed (I don't remember where I saw it). But what I'm after is the Unruh effect on space itself due to accelerated reference frames caused by gravity. Again, this would be small; but integrating over vast amounts of space (and doing multiple iterations) may have an accumulative effect of the scale of DM.

:blushing: Oh, by the way, my thought for the day is to consider whether this Unruh effect may be the very cause of particle creation during inflation so that there is no need to suppose a Higgs boson and a false vacuum. We may already have all the physics we need. For particle creation during inflation is accompanied with very fast accelerated expansion so that horizons are small and the Unruh effect (=Hawking radiation) is more pronounced. Not only that but when the universe was small and expanding very fast, the gravitational field was more intense and perhaps this caused greater accelerated reference frames to produce a much higher Unruh effect. Though perhaps you need more of an accelerated expansion to produce accelerated reference frames with respect to others to produce this Unruh effect than you need a gravitational well. For at the beginning every frame might be at the same gravitational level so that neighboring space is not accelerating wrt other regions of space. But at least the temperature profile is there. Its expansion was accelerating very fast, its gravitational field was very strong, and the temperature was very high. Do we have a more specific temperature curve verses expansion rate or gravity strength for the inflationary epoch? If we start from a singularity and assume exponential rate of expansion, can we get a particle creation verses temperature model that ultimately match the CMB? I think that would be just a matter of how you adjust your scale factors to give how fast it is growing for a given overall size.

[SpaceTiger]

If the major contribution of DM were photons

Photons can't be the dark matter. Radiation can't be bound to potential wells for long. Besides, we can see it. :smile:


In the conclusion section of the paper you cite, one finds: "Adopting big-bang nucleosynthesis limits on the mean baryonic mass of the universe excludes most of this mass from being baryons in cold, condensed structures." They do not rule out an even distribution throughout space.

I think the point there was that we might not be able to see cold, condensed structures (in this particular system), whereas we could see diffuse gas. Nucleosythesis constraints don't depend on present-day clumping.


That would be the Unruh effect felt by individual stars and planet, the atoms within stars, etc. I've seen calculations that show this to be very small indeed (I don't remember where I saw it). But what I'm after is the Unruh effect on space itself due to accelerated reference frames caused by gravity. Again, this would be small; but integrating over vast amounts of space (and doing multiple iterations) may have an accumulative effect of the scale of DM.

Perhaps you can explain further. My understanding of the Unruh effect is that accelerating frames view the quantum vacuum as having a net temperature. Are you suggesting that the vacuum is viewing itself as having a net temperature and that space is curving as a result of the contributions to the stress-energy tensor? If so, why would the quantum vacuum not also be in free-fall?


Oh, by the way, my thought for the day is to consider whether this Unruh effect may be the very cause of particle creation during inflation so that there is no need to suppose a Higgs boson and a false vacuum. We may already have all the physics we need. For particle creation during inflation is accompanied with very fast accelerated expansion so that horizons are small and the Unruh effect (=Hawking radiation) is more pronounced. Not only that but when the universe was small and expanding very fast, the gravitational field was more intense and perhaps this caused greater accelerated reference frames to produce a much higher Unruh effect.

Again, I'm prone to question your use of the term "acceleration" here. During inflation, space itself is undergoing accelerated expansion, but spacetime is still locally Lorentzian.

[Mike2]

Perhaps you can explain further. My understanding of the Unruh effect is that accelerating frames view the quantum vacuum as having a net temperature. Are you suggesting that the vacuum is viewing itself as having a net temperature and that space is curving as a result of the contributions to the stress-energy tensor? If so, why would the quantum vacuum not also be in free-fall?
As I understand the Unruh effect, accelerating objects feel a temperature because the ZPE is not invariant wrt acceleration. It's Lorentzian and invariant wrt to velocities, but not acceleration. OK. I simply thought it was fair to apply the equivalence principle wherein there is no distinction between acceleration and gravitation. I also think that there is nothing particular to the object that is causing the temperature rise, but it is the accelerating reference frame that "feels" the temperature whether there is an object in that accelerating frame or not. Correct me if I'm wrong, but it seems that DM is distributed as though the gravity it causes around it has some sort of weight itself, right? This argue for some sort of Unruh effect. I don't believe this Unruh effect produces permanant particles (unless there is an horizon) because an accelerating observer could feel a temperature and presume collision with particles. But if he were to stop and go back at constant speed, he would never see those particles again, right? So it seems applicable only to accelerating systems of which gravity is one.


Again, I'm prone to question your use of the term "acceleration" here. During inflation, space itself is undergoing accelerated expansion, but spacetime is still locally Lorentzian.
Yes, I'm not real clear yet about all this. But it seems that if one region of space is accelerating wrt a second region, then the second would have to conclude that the first is feeling a temperature (due to the Unruh effect) so that the second should observe that the first has a higher ZPE than itself so that there must be some particles created in the second region to account for the temperature. I think it is equivalent to also think in terms of Hawking radiation. If the Universe is accelerating very fast, then there will be a much smaller cosmological event horizon which separates virtual particles out of the ZPE to create permatant particles during inflation.

I think all this is the same as saying that the ZPE is greater in curved space than or flat space, or that curved space creates permanant particles out of the ZPE.

Added a couple of hours later:
Just a moment,... now I'm thinking that DM can not be any sort of permanant matter whatsoever. For if it were, then it would gravitate towards the host galaxy and eventually concentrate there. But AFAIK we don't observe older galaxies with this central concentration of DM, but all seem to have the same distribution required to flatten rotation curve and not centrally concentrated DM which would not show this same flatness.

[Nacho]

SpaceTiger,

If the smaller mass (the galaxy) got close enough to the larger one, it would be tidally disrupted and you would likely see streams of stars and gas. Something like this:

Thanks. I see. It would tend to tidally/asymetrically disrupt the galaxy VS symetrically pulling it apart.

[Turbot]

That would be the Unruh effect felt by individual stars and planet, the atoms within stars, etc. I've seen calculations that show this to be very small indeed (I don't remember where I saw it). But what I'm after is the Unruh effect on space itself due to accelerated reference frames caused by gravity. Again, this would be small; but integrating over vast amounts of space (and doing multiple iterations) may have an accumulative effect of the scale of DM.Mike2, you may find this interesting:

http://lanl.arxiv.org/PS_cache/gr-qc/pdf/9312/9312027.pdf

Gravity is the unequable flow of time from place to place. It is not that there are two separate phenomena, namely gravity and time and that the one, gravity, affects the other. Rather the theory states that the phenomena we usually ascribe to gravity are actually caused by time’s flowing unequably from place to place.Unruh's approach to gravitation and the vacuum are non-standard, and this 1993 paper (16 years after he proposed the effect named for him) is no exception.

[SpaceTiger]

As I understand the Unruh effect, accelerating objects feel a temperature because the ZPE is not invariant wrt acceleration. It's Lorentzian and invariant wrt to velocities, but not acceleration. OK. I simply thought it was fair to apply the equivalence principle wherein there is no distinction between acceleration and gravitation.

I would think that an application of the equivalence principle would produce the opposite conclusion -- that the objects (or vacuum) were in an inertial frame unless they were being acted upon by a non-gravitational force. The equivalence that you speak of is, I think, that of an accelerating frame to a stationary observer in a uniform gravitational field. Remember, we are not in an inertial frame because (among other things), the surface of the earth is pushing on our feet. If that surface were not there, we would be in free-fall and this would not be equivalent to the artificially accelerating frame.

I should think the same reasoning applies to the accelerating universe.


Correct me if I'm wrong, but it seems that DM is distributed as though the gravity it causes around it has some sort of weight itself, right?

I wouldn't say so. In particular, that paper I cited earlier gives an example of dark matter in apparent absence of anything else to cause gravity.



Just a moment,... now I'm thinking that DM can not be any sort of permanant matter whatsoever. For if it were, then it would gravitate towards the host galaxy and eventually concentrate there. But AFAIK we don't observe older galaxies with this central concentration of DM, but all seem to have the same distribution required to flatten rotation curve and not centrally concentrated DM which would not show this same flatness.

There is a problem in which simulations have difficulty producing the exact central profile of the dark matter expected from observations, but producing flat rotation curves is not a problem in the standard CDM universe.

[Mike2]

There is a problem in which simulations have difficulty producing the exact central profile of the dark matter expected from observations, but producing flat rotation curves is not a problem in the standard CDM universe.
My point is that if DM were permanant particles, then it would all eventually gravitate to the center and there would be none around the edges. This would occur around old stable galaxies. Is this seen in the data?

[Garth]

My point is that if DM were permanant particles, then it would all eventually gravitate to the center and there would be none around the edges. This would occur around old stable galaxies. Is this seen in the data?
The DM particles, whatever they may be, would be in orbit around the galaxy as with any other massive object such as a star etc. If they did interact they could exchange energy and angular momentum and some filter into the centre where they would eventually be absorbed by the central BH, but that might take many Gyrs. For some to lose their orbital energy in this way others would have to be ejected.

Garth

[Mike2]

The DM particles, whatever they may be, would be in orbit around the galaxy as with any other massive object such as a star etc. If they did interact they could exchange energy and angular momentum and some filter into the centre where they would eventually be absorbed by the central BH, but that might take many Gyrs. For some to lose their orbital energy in this way others would have to be ejected.
Garth
If DM orbitted the center like everything else, then they would orbit in a disk in spiral galaxies, like ordinary matter, right? Is this what the deflection maps show or the simulations that produce flat rotation curves?

[EL]

If DM orbitted the center like everything else, then they would orbit in a disk in spiral galaxies, like ordinary matter, right?

In fact it would not, since it does not couple to photons (very much) and hence cannot loose energy through radiation. This means it will not distribute itself as ordinary matter does.

[Garth]

You have to model DM to try and emulate the observed rotation curves and other features of galactic dynamics such as the warp in the Milky Way's disk and its interaction with the Magellanic clouds. The general model is a spherical distribution of DM.

Garth

[Mike2]

In fact it would not, since it does not couple to photons (very much) and hence cannot loose energy through radiation. This means it will not distribute itself as ordinary matter does.
Considering how relatively disperse matter is on the disk (how distant stars are from one another), it seems like the only thing causing the disk structure is gravitational attraction. What does photon coupling have to do with anything?

[EL]

Considering how relatively disperse matter is on the disk (how distant stars are from one another), it seems like the only thing causing the disk structure is gravitational attraction. What does photon coupling have to do with anything?

Ordinary matter can loose energy through radiation. DM can (hardly) not.
This difference gives rise to different distributions.

[SpaceTiger]

A classic paper on the link between galaxy formation and radiative cooling:

Rees & Ostriker 1977 (http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1977MNRAS.179..541R&db_key=AST&d ata_type=HTML&format=&high=4349a261f112796)

[Mike2]

You have to model DM to try and emulate the observed rotation curves and other features of galactic dynamics such as the warp in the Milky Way's disk and its interaction with the Magellanic clouds. The general model is a spherical distribution of DM.
Garth
I would think that if DM were massive particles, then you would get all the various kinds of distributions that we see of the luminous matter. And we would see all sorts of crazy distribution from galaxies that collide. But what I'm understanding is that the DM halo is always just that, a halo, something always evenly distributed at the outer limits of galaxies in more of an evenly distributed manner. I suggest that this contradict DM as being massive particles. It seems more like DM is proportional to the gravitational field in and around galaxies. This suggests more of a phanomina associated with the acceleration of gravity, such as perhaps the Unruh effect. Again, I think the calculation would be comparitively simple in order to find out.

[hellfire]

You should note first that the term Unruh effect is used for an effect that arises in Rindler spacetime which contains a horizon. The gravitational analogue of the Unruh effect is the Hawking effect in a Schwarzschild spacetime, which contains also a horizon. These effects are about thermal distributions of radiation, which make it possible to define a temperature and an entropy for horizons. However, something similar should occur in scenarions without an horizon, due to the difference in the definition of particles between inertial and non-inertial observers, but the radiation field should not be thermal. As far as I understand all this stuff, this radiation field is a bath of photons, and is not localized; it takes place in every point of the vacuum. On the other hand there are some other problems to deal with if you postulate that dark matter is made of photons, most of them already mentioned in this thread.

[Garth]

I would think that if DM were massive particles, then you would get all the various kinds of distributions that we see of the luminous matter. And we would see all sorts of crazy distribution from galaxies that collide. But what I'm understanding is that the DM halo is always just that, a halo, something always evenly distributed at the outer limits of galaxies in more of an evenly distributed manner. I suggest that this contradict DM as being massive particles. It seems more like DM is proportional to the gravitational field in and around galaxies. This suggests more of a phanomina associated with the acceleration of gravity, such as perhaps the Unruh effect. Again, I think the calculation would be comparitively simple in order to find out.My problem with the nature of DM is that it is all so speculative. It has not been identified in the laboratory even after about forty years of intense investigation.

We have a theory-dependent need for DM from analysis of gravitational effects, galaxy rotation curves, cluster dynamics, quasar lensing and large-scale structure formation in the LCDM model. As we try and pin down its properties to be consistent with all these observations we might as well be concocting 'pixie dust'.

First find the DM particle, then measure its properties and then see whether those properties fit the observations. Then and only then can we be confident that we know what we are talking about.

If you are going to invent another candidate you are in good company, but it does have to fulfil the relevant criteria.


Garth

[SpaceTiger]

I would think that if DM were massive particles, then you would get all the various kinds of distributions that we see of the luminous matter.

It was just explained why this isn't the case. If you're not understanding, please be specify which parts are confusing you.

[vincentm]

Thanks for the info guys i have a better understanding of what dark matter is theorized to be. Also, is dark matter also factored into the hubble constant regarding the expansion of the universe, mainly the space between galaxies that are receding from one another?

[-Job-]

I was going to propose that if there were more than 3 dimensions of space, then dark matter may be supposed to exist in there, but wouldn't radiation emitted from matter in a fourth or fifth dimension also propagate through our 3 dimensions? I'm thinking it would, so that doesn't solve anything.

[Garth]

I was going to propose that if there were more than 3 dimensions of space, then dark matter may be supposed to exist in there, but wouldn't radiation emitted from matter in a fourth or fifth dimension also propagate through our 3 dimensions? I'm thinking it would, so that doesn't solve anything.
There are theories that suggest Dark Energy is a form of gravitation leaking through from other branes, situated in the other dimensions, but Dark Matter appears to be situated in this universe (brane), we just can't find it, perhaps the next generation of accelerators???

Garth

[SpaceTiger]

Thanks for the info guys i have a better understanding of what dark matter is theorized to be. Also, is dark matter also factored into the hubble constant regarding the expansion of the universe, mainly the space between galaxies that are receding from one another?

The Hubble constant is calculated directly by measuring the distances and redshifts of galaxies, so dark matter doesn't really factor in.

[-Job-]

Garth, on the topic of other dimensions, suppose i live in a two dimensional plane in a three dimensional system. I am able to move in four directions within this plane of mine. I can't experience the third dimension so i don't have proof that it exists. Suppose my plane is parallel to the x and y axis and that it intercepts the z-axis at z=10. I can move around the plane but my z-coordinate is always the same. Consider a particle in this 3D system with coordinate Z=11. This particle is outside my plane, my "universe", so i can't interact with it. However, suppose this same particle is moving so that it intersects my plane. Then, i might be hanging around in my plane and all of the sudden this particle would pop-in and pop-out of existence. I would be able to detect it once its z-coordinate reached z=10, but only for a very short period of time.
Suppose i live in a 3-dimensional cube (or maybe any 3D-body), and that this cube is parallel to the x, y and z axis, and intersects the w-axis (a fourth dimension) at w=10. I can move around in my cube but my w-coordinate never changes. Suppose there's a particle with w-coordinate w=11. This particle is outside my cube so i can't interact with it. Suppose this particle is moving along the w-axis so that it intersects my cube. This particle will intersect my cube at w=10 but be outside it immediatly at w=9. Then, i might be hanging around in my cube and all of the sudden this particle would pop-in and pop-out of existence. If there's more than 3 dimensions how come we don't see particles popping into existence at a point x,y,z and then immediatly vanishing?

[Garth]

Okay - in very general terms this is the idea behind brane theory. That in a higher dimensional space three dimensional + time membranes exist that occasionally collide with each other. The collision is what we experience as a 'big bang' - I say a big bang because there are many of them and consecutive collisons are interspersed by billions of years.

In your description, let's take the 2D plane in the higher 3D space as that is easier to visualise, the plane is infinitely thin in the z-dimension and so the intersection with another particle 'just passing through' will be over in a flash, an infinitely short flash, so would we (the inhabitants of the 2D 'flatland') actually detect it?

In the mutli-brane explanation for DE gravitation is as powerful as the other forces in the other brane but only weakly leaks into our own, except on the largest scales when it appears stronger and is interpreted as DE.

These ideas are good fun and can be dressed up with fancy mathematics, they sometimes make testable predictions (although only ambiguously) and so are just counted as 'scientific'. However, I think they are no more than that, just 'good fun'.

Garth

[Mike2]

You should note first that the term Unruh effect is used for an effect that arises in Rindler spacetime which contains a horizon. The gravitational analogue of the Unruh effect is the Hawking effect in a Schwarzschild spacetime, which contains also a horizon. These effects are about thermal distributions of radiation, which make it possible to define a temperature and an entropy for horizons. However, something similar should occur in scenarions without an horizon, due to the difference in the definition of particles between inertial and non-inertial observers, but the radiation field should not be thermal. As far as I understand all this stuff, this radiation field is a bath of photons, and is not localized; it takes place in every point of the vacuum. On the other hand there are some other problems to deal with if you postulate that dark matter is made of photons, most of them already mentioned in this thread.
I'd have to wonder if they were real photons. For if they were, then after scattering off some accelerating object, it would continue to have effects on nearby non-accelerating objects. Since the nearby non-accelerating objects do not feel the scattered photon (they feel nothing at all), then these particles that cause the temperature of the Unruh effect cannot be real. I suspect they are virtual particle. Perhaps the virtual particle pairs (the ZPE) remain separated for a longer period of time before recombining so that they can make their gravity felt before recombining. Or does an accelerating object have an effect on nearby things. I don't know.

[EL]

I would think that if DM were massive particles, then you would get all the various kinds of distributions that we see of the luminous matter.

I'm sorry but that's not the case, as numerous N-body simulations of DM distributions in galaxies show. The reason is, as mensioned before, that DM doesn't couple to photons very well. (Which is why it is called DM.)

[EL]

My problem with the nature of DM is that it is all so speculative. It has not been identified in the laboratory even after about forty years of intense investigation.

But according to most models we should not have seen it yet. Maybe we'll discover it in LHC, and if we don't there will still be a large piece of the parameter space left to investigate.

First find the DM particle, then measure its properties and then see whether those properties fit the observations. Then and only then can we be confident that we know what we are talking about.

Sure, and that's what so exciting about LHC. If we are lucky, maybe we'll find something.

[Mike2]

I'm sorry but that's not the case, as numerous N-body simulations of DM distributions in galaxies show. The reason is, as mensioned before, that DM doesn't couple to photons very well. (Which is why it is called DM.)
I only read the abstract, IIRC, and it only mentioned how galaxies condensed from the gas left by the BB. But are you trying to tell me that they are using the energy lost by photons in their N-body simulations of galaxy collision models? It seems to me that losing energy by emitting photons (if that were a significant effect) would just cause things to aggregate to the center more easily (as though gravity were stronger for lumious material). If anything I would think that with no photon coupling, the wild gyrations that gravity plays on colliding galaxies would be more pronounce for DM than for baryonic matter, since there is no dampening effect caused by the energy lost from photon emissions. I would expect to see more asymmetric dispersed patterned of DM in colliding galaxies. Is this seen in the data?

[EL]

But are you trying to tell me that they are using the energy lost by photons in their N-body simulations of galaxy collision models?

What? Using energy lost by photons? Collisions?

The thing goes something like this:
When simulating the formation of a galaxy you must use the properties of the matter content as input. The difference between the properties of ordinary matter and DM is that ordinary matter couples to photons and DM does not. This means that ordinary matter can loose energy through radiation and hence more easily accumulate in the centre of the galaxy (in the shape of a disk). DM however cannot loose it's energy in any simple way, and is hence not as willing to lump into a disk, but rather distributes itself in a spherical halo, with the density only depending on the radial distance.

[Mike2]

What? Using energy lost by photons? Collisions?
The thing goes something like this:
When simulating the formation of a galaxy you must use the properties of the matter content as input. The difference between the properties of ordinary matter and DM is that ordinary matter couples to photons and DM does not. This means that ordinary matter can loose energy through radiation and hence more easily accumulate in the centre of the galaxy (in the shape of a disk). DM however cannot loose it's energy in any simple way, and is hence not as willing to lump into a disk, but rather distributes itself in a spherical halo, with the density only depending on the radial distance.
OK, so we both understand that losing energy due to photon emission make orbits decay so it more easily clumps in the center. I fail to see why this should help in the process of forming a disk. Perhaps a few sentences would clear this up for me, thank you.

But my point is still open, have they specifically, or can they, looked at how DM would distribute itself in the case of galaxy collisions? We already know that this produces wild distribution of baryonic matter. So would the same be true for DM. I remember a simulation on TV where the moon is depicted as forming from the collision of two planets at just the right angle, at just the right speed. There are two spherically shaped distributions. So in galactic terms, it should be possible to form a DM galactic "moon" by the same process, right?

In any case, it should be possible to study wild galactic collision distributions to see if DM are particles or if it is not, right? If DM does not have wild distributions due to the same gravitational effects on permanant particles, then perhaps it is a second order gravitational effect like the Unruh effect applied to the acceleration due to gravity.

[Chronos]

Dark matter is thought to be mostly collisionless, even with itself. Gravitational collapse by products, like stars, do not result because particle interactions are too weak to form dense clumps like ordinary matter. They also do not emit photons, as El noted, hence the name 'dark' matter.

[Garth]

But according to most models we should not have seen it yet.
Yes and I have some very shy pixies in my house. I have looked for them but cannot find them, I know they are there because they keep hiding my odd socks. People have suggested that if they are really there then I would have found them by now but I answer, "They are so shy that we should not have seen them yet"!

Garth

[EL]

OK, so we both understand that losing energy due to photon emission make orbits decay so it more easily clumps in the center. I fail to see why this should help in the process of forming a disk. Perhaps a :zzz: few sentences would clear this up for me, thank you.

Well I would say the most intuitive scenario would be the forming of a disk, due to that the pregalactic clump of matter always has some angular momentum. However I can not give a detailed explanation for why the dark matter doesn't distribute itself as a disk too. Probably it would if we just waited for a very long time. I guess the radiation losses helps the ordinary matter to form disks much faster.
However there's a lot of people who have studied this in detail, and all simulations show that if DM exists it would distribute itself more as a spherical halo.


But my point is still open, have they specifically, or can they, looked at how DM would distribute itself in the case of galaxy collisions?
...
In any case, it should be possible to study wild galactic collision distributions to see if DM are particles or if it is not, right?

I don't know very much about galaxy collisions, so I'm afraid I can't answer these questions. But of course it would in principle be possible to simulate what DM distributions we should find after collisions, and compare this to data. However I'm not aware of if this has been done.

[EL]

Yes and I have some very shy pixies in my house. I have looked for them but cannot find them, I know they are there because they keep hiding my odd socks. People have suggested that if they are really there then I would have found them by now but I answer, "They are so shy that we should not have seen them yet"!
Garth

:smile: :tongue2: :rofl:
(What's a "pixie"?)

[SpaceTiger]

In any case, it should be possible to study wild galactic collision distributions to see if DM are particles or if it is not, right?

Galaxy collisions and dense clusters are, theoretically, an excellent testing ground for dark matter theories. Since the baryons are coupled to one another by non-gravitational forces, they may become separated from the dark matter, which can only interact gravitationally. If this happens, then we can use gravitational lensing to to compare the mass peaks to the light peaks. This is exactly what was done in Clowe et al. 2003 (linked earlier in the thread).

Unfortunately, there are very few systems in which this kind of analysis can be done, so I wouldn't say that they have yielded definitive proof. However, results are so far consistent with particle dark matter theories.

[Garth]

:smile: :tongue2: :rofl:
(What's a "pixie"?)
I'll be able to tell you once I've caught one, but what I do know already is that as I have tried to see and photograph them and cannot they must be very dark. Also, because I've set up an infrared night vision camera, which has not caught them, they must be very small, light and non-interacting. So I am just waiting for my infrared night vision microscope camera to arrive and then I will see them.

They also have irksome heavy cousins – the Higgs Fairies - who try to stop things moving around. They rob you of energy in the morning, you know when your 'get-up-and-go' has 'got-up-and-gone'? They make freezer draws stick when the ice-cream is falling on the ground and filing cabinet draws jam when you have to look something up; they make it hard to move the furniture and generally slow you down when you are in a hurry...:yuck:

Garth

[EL]

I'll be able to tell you once I've caught one, but what I do know already is that as I have tried to see and photograph them and cannot they must be very dark. Also, because I've set up an infrared night vision camera, which has not caught them, they must be very small, light and non-interacting. So I am just waiting for my infrared night vision microscope camera to arrive and then I will see them.


What if you find some other reason for your odd socks? Maybe it's just your old washing machine which doesn't work the way you suspect it to do? Have you really watched closely so that it doesn't change the color of one sock during the laundry?
But probably you are right, since if the color changing washing machine was the correct answer, it doesn't solve the problems like why my glasses never are where I left them and why my wallet always gets empty so quickly.

[Mike2]

Galaxy collisions and dense clusters are, theoretically, an excellent testing ground for dark matter theories. Since the baryons are coupled to one another by non-gravitational forces, they may become separated from the dark matter, which can only interact gravitationally. If this happens, then we can use gravitational lensing to to compare the mass peaks to the light peaks. This is exactly what was done in Clowe et al. 2003 (linked earlier in the thread).
Unfortunately, there are very few systems in which this kind of analysis can be done, so I wouldn't say that they have yielded definitive proof. However, results are so far consistent with particle dark matter theories.
I think they need to rewrite that paper. It's so filled with qualifying clauses, I can't tell what they are trying to say.

Are they saying that one would expect the mass centroid to be coincident with the light centroid, but they find through lensing effects that the mass centroid in not coincident with the light centroid due to the invisible dark matter contribution? What paragraph did they say that exactly.

[SpaceTiger]

What paragraph did they say that exactly.

Several places, but you can check the abstract if you're just looking for a statement like that:

The observed offsets of the lensing mass peaks from the peaks of the dominant visible mass component (the X-ray gas) directly demonstrate the presence, and dominance, of dark matter in this cluster

[Mike2]

Several places, but you can check the abstract if you're just looking for a statement like that:
riiiiiiiiight... So does he mean offset in position or amplitude?

So the X-ray sources are assumed to be caused from massive objects like BH's, right? That's why the X-ray portion is considered to contribute more mass than the visible matter, right? Or is there a mechanism for producing these X-rays that does not contribute so much to the mass distribution of the galaxy? Thanks.

[matt.o]

the x-ray source here is the intra-cluster medium of the merging group. it is well known that the ICM contributes most to the baryonic mass in a cluster (around 80-90%), it is also known that the galaxies and dark matter are collisionless during a cluster merger (to a good approximation), whilst the ICM is collisional. This collisional nature leads to a process called ram-pressure stripping, where the ICM of the infalling group is stripped away from the galaxies and dark matter due to its interaction with the ICM of the main cluster. Hence the ICM lags behind the galaxies and DM.
now, if there were no dark matter and all of the gravitational potential (causing the weak lensing) were caused by the baryonic matter, you would expect to see the highest mass concentration where the ram-pressure stripped ICM is, since it makes up ~90% of the baryonic mass in a cluster. the fact that you see the mass concentration coincident with the infalling group galaxies and not with the lagging ICM tells us there must be some other kind of matter, ie dark matter.
note that weak lensing analysis is sensitive to all matter concentrations along the line of sight to the cluster.

[Garth]

But note that if the DM is in the form of IMBH's it too would be largely collisionless. Such IMBH's might be the remnant of an earlier ubiquitous PopIII population, and therefore originally baryonic. The constraint on this is the BBN baryon density which is model dependent on the cosmic expansion factor during the nucleosynthesis epoch.

Garth

[matt.o]

yes, that's what I though you'd say, Garth!

I think IMBH's have been ruled out by lensing surveys. Also remember Helium, lithium and deuterium abundances have observed values very close to those predicted by BBN.

[Mike2]

the x-ray source here is the intra-cluster medium of the merging group. it is well known that the ICM contributes most to the baryonic mass in a cluster (around 80-90%), it is also known that the galaxies and dark matter are collisionless during a cluster merger (to a good approximation), whilst the ICM is collisional. This collisional nature leads to a process called ram-pressure stripping, where the ICM of the infalling group is stripped away from the galaxies and dark matter due to its interaction with the ICM of the main cluster. Hence the ICM lags behind the galaxies and DM.
now, if there were no dark matter and all of the gravitational potential (causing the weak lensing) were caused by the baryonic matter, you would expect to see the highest mass concentration where the ram-pressure stripped ICM is, since it makes up ~90% of the baryonic mass in a cluster. the fact that you see the mass concentration coincident with the infalling group galaxies and not with the lagging ICM tells us there must be some other kind of matter, ie dark matter.
note that weak lensing analysis is sensitive to all matter concentrations along the line of sight to the cluster.
Thanks, that helps a lot. What is the source of the X-rays? And what is the ICM made of? Do you have an on-line source for your information? Thanks again.

[matt.o]

the source of the x-ray radiation comes from the thermal motions of the electrons and ions in the ICM. When a fast moving electron encounters a slow moving ion, it is accelerated due to the ions electric field. This produces Bremsstrahlung radiation (braking radiation). The temperature of the ICM is around 10 million degrees, hence the Bremsstrahlung radiation is in the x-ray band. there is also some line emission from iron, oxygen etc.
the ICM contains mainly ionised hydrogen and helium and electrons. Although the ICM is enriched with metals to about 1/3 of the solar metal abundance.
here are some links that came from "clusters of galaxies" intra-cluster medium;
http://www.astr.ua.edu/keel/galaxies/icm.html
http://www-xray.ast.cam.ac.uk/xray_introduction/Clusters_intro.html
if you need any more go to
http://adsabs.harvard.edu/abstract_service.html
and search for cluster of galaxies, intra-cluster medium, Craig Sarazin, Maxim Markevitch, Alexy Vikhlinin, Christine Jones, Willy Forman, Hans Bohringer Brian McNamara etc.

[Mike2]

the source of the x-ray radiation comes from the thermal motions of the electrons and ions in the ICM. When a fast moving electron encounters a slow moving ion, it is accelerated due to the ions electric field. This produces Bremsstrahlung radiation (braking radiation). The temperature of the ICM is around 10 million degrees, hence the Bremsstrahlung radiation is in the x-ray band. there is also some line emission from iron, oxygen etc.
the ICM contains mainly ionised hydrogen and helium and electrons. Although the ICM is enriched with metals to about 1/3 of the solar metal abundance.
here are some links that came from "clusters of galaxies" intra-cluster medium;
http://www.astr.ua.edu/keel/galaxies/icm.html
http://www-xray.ast.cam.ac.uk/xray_introduction/Clusters_intro.html
if you need any more go to
http://adsabs.harvard.edu/abstract_service.html
and search for cluster of galaxies, intra-cluster medium, Craig Sarazin, Maxim Markevitch, Alexy Vikhlinin, Christine Jones, Willy Forman, Hans Bohringer Brian McNamara etc.
Thanks again. It is delightful to get this level of help.

One question comes to mind about the X-ray ICM. I've not read your references yet, though I plan to do so. But off hand it would seem that X-ray don't necessarily have anything to do with baryonic mass density, but only to do with the velocity of particles. Perhaps the X-rays are produced at the point of collision between galaxies which may have nothing to do with the baryonic distribution. Ya think?

[Garth]

yes, that's what I though you'd say, Garth! Sorry - I'll shut up and just wait patiently.:blushing:

I think IMBH's have been ruled out by lensing surveys. Across what mass ranges?Also remember Helium, lithium and deuterium abundances have observed values very close to those predicted by BBN. Indeed , apparently also concordant with the linear freely coasting model, apart from deuterium which then has to be produced by spallation.

Garth

[SpaceTiger]

Across what mass ranges?

The allowed range is M < 10^4~M_{sun}, except for 0.1 < M < 10~M_{sun}. The former is constrained by (the lack of) globular cluster disruption and the power spectrum of the Lyman alpha forest. The latter is from microlensing.

[Garth]

The allowed range is M < 10^4~M_{sun}, except for 0.1 < M < 10~M_{sun}. The former is constrained by (the lack of) globular cluster disruption and the power spectrum of the Lyman alpha forest. The latter is from microlensing.
Thank you ST.

Garth

[SpaceTiger]

Thanks again. It is delightful to get this level of help.

I apologize for being terse and thanks, matt.o, for stepping in. This semester has been crazy and I think I've been cutting back a bit too much on explanation. :redface:

[matt.o]

Thanks again. It is delightful to get this level of help.
One question comes to mind about the X-ray ICM. I've not read your references yet, though I plan to do so. But off hand it would seem that X-ray don't necessarily have anything to do with baryonic mass density, but only to do with the velocity of particles.
Actually, the density of the ICM can be measured directly from the x-ray emission, assuming spherical symmetry. This is because the emission measure (em)
em \propto \int n_{e}^{2} dl
where n_{e} is the electron density and dl is the length measured along the line of sight.
More importantly, the ICM is thought to respond to the same gravitational potential as the dark matter in the cluster. Hence assuming hydrostatic equilibrium and spherical symmetry of the ICM (which is a fair assumption in a dynamically relaxed cluster) the total mass profiles of the cluster can be measured from the combination of radial profiles of temperature and density of the ICM. That is, we can measure the total mass (including dark matter) within a certain radius using the proporties we measure from the ICM.
Perhaps the X-rays are produced at the point of collision between galaxies which may have nothing to do with the baryonic distribution. Ya think?
Not quite sure what you mean here. Although the main source of heating of the ICM is due to shocks occuring during the formation of the cluster, where super-sonic speeds are common.

[Mike2]

Actually, the density of the ICM can be measured directly from the x-ray emission, assuming spherical symmetry. This is because the emission measure (em)
em \propto \int n_{e}^{2} dl
where n_{e} is the electron density and dl is the length measured along the line of sight.
More importantly, the ICM is thought to respond to the same gravitational potential as the dark matter in the cluster. Hence assuming hydrostatic equilibrium and spherical symmetry of the ICM (which is a fair assumption in a dynamically relaxed cluster) the total mass profiles of the cluster can be measured from the combination of radial profiles of temperature and density of the ICM.
But the paper cited to compare the centroid of baryonic to DM was a cluster with galaxies in the process merging. It talks about "pass through", etc. So I have to wonder if these assumptions of symmetry still hold.

That is, we can measure the total mass (including dark matter) within a certain radius using the proporties we measure from the ICM.
Not quite sure what you mean here. Although the main source of heating of the ICM is due to shocks occuring during the formation of the cluster, where super-sonic speeds are common.
I don't see how you can measure DM this way since it weakly interacts (if at all) with all this radiation going on in the ICM

[matt.o]

But the paper cited to compare the centroid of baryonic to DM was a cluster with galaxies in the process merging. It talks about "pass through", etc. So I have to wonder if these assumptions of symmetry still hold.
You are entirely correct. Assumptions of spherical symmetry and hydrostatic equilibrium don't hold in major mergers. There is really no way to accurately measure the mass in a major merger.
I don't see how you can measure DM this way since it weakly interacts (if at all) with all this radiation going on in the ICM
I am about to go to bed now, so perhaps you could do a search onhttp://adsabs.harvard.edu/abstract_service.html for craig sarazin in order to answer this. Failing this, I could go into more detail at a later date. The basics of it is that both the dark matter and the ICM respond to the same gravitational potential so you can measure the mass of both from the ICM emission.

[Mike2]

You are entirely correct. Assumptions of spherical symmetry and hydrostatic equilibrium don't hold in major mergers. There is really no way to accurately measure the mass in a major merger.
I wonder if it would be better to examine mergers between just two galaxies. But then again, in a bath of ICM, it would probably be too hard to distinguish effects from just the two merging galaxies... unless you can find a cluster that consists of only two galaxies that have merged, not likely.

[matt.o]

firstly, 2 galaxies do not constitute a cluster, although they can be classified as a group. there are things known as fossil groups, ie a single galaxy which appears to have the mass of a group. these are likely to have formed from mergers of 2 or more galaxies.

Oh, and the ICM is not exactly a bath. it has densities of around 10^-2-10^-3 electrons per cm^3, which is less dense than the best vacuums we can create on earth! Plus the ICM is optically thin, hence it doesn't absorb light.

Also, galaxy-galaxy mergers are rare in massive clusters cores due to the high speeds at which the galaxies orbit.

[vincentm]

Okay, first off i wanna say thanks once again, but the math part confuses me more, mainly because i suck at mathematics at this point in my studies.

anyway this is what i have read in regards to DM. Please bare with me:

the stars originally in the halos of galaxies in clusters must currently permeate intergalactic space. Tidal forces between colliding galaxies during the first billion years of the cluster's existence stripped the outer halos of stars. Stripping was effective beyond a radius of about 100,000 light years in a typical galaxy.

From observations of the Doppler shifts of their spectra, we infer that the cluster galaxies move at rather high random velocities. Because we can measire the dimensions of a cluster, we can compute how much mass must be present within the rapidly moving galaxies to contain the expansion. (if this mass were not present, the galaxies would simply fly apart, there would be no cluster.) the result is surprising; the required amount of mass per galaxy is several times as large as that inferred by other types of measurements, usually of nearby galaxies, whose dynamics we can study in suffiecient detail to infer their masses. For example, by measuring the rate at shich a nearby galaxy is roatating, we can infer its mass. We can also measure the velocities of nearby galaxies ina number of isolated close pairs to determine the average mass of the pair.

We can make these statements rather more precise by introducing the mass-luminosity ratio. We measure luminosity directly and, for every unit of luminosity (usually expressed in units of solar luminosity), we can assign a certain number of units of mass (expressed in solar masses). Thus, the sun has a mass -luminosity ratio of 1; the visible regions of the Milky Way galaxy, which consist for the most part of stars less massive and considerably less luminous than the sun, have a mass-luminosity ratio between 200 and 400. Measurements of individial elliptical galaxies yield a mass-luminosity ratio of about 8, although this result is applicable only the central region luminous regions.

By studying radio emmision from meutral hydrogen, scientists have been able to measure the rate at which a spiral galaxy rotates. We can follow to the extreme parts of the galaxy should be more weakly bound. They should therefore experience a weaker centrifugal force and be rotating less rapidly But this is contrary to what is found. It appears from the measurements atht spirals have larger mass-luminosity ratios than we would predict from studying their luminous inner regions More mass must be present than we havepreviously realized. their net mass-luminosity ratios must be about 30 or even larger, Precisely what for this non luminous matter take in the out regions , or halos is not known.

rotation curves probe the outer regions of spiral galaxies, where there is little luminous matter. two different techniques have been used to study ellipticals, which are gas poor and therefore not amenable to rotation curve studies at large distances from the center of the galaxies. X-ray emmision has been discovered around ellipticals. the x-rays are produced by hot gas at about 10 million kelvins, gravitionally confined in the halos of the elliptical galaxies. To confine the gas reuires a considerable amount of mass: it is inferred that the ratio of total mass, including dark halo, to optical luminosity, which comes entirely from the inner regions, may be as large as 50.

Antoerh discovery also indicates a considerable amount of dark matter in the halo of the elliptical. Elliptical galaxies reveal the presence of faint shells on deep photographic plate. These shells extend out two or three times as far as the bulk of the starlight. As many as 20 shells have been discovered around one bright galaxy. the shells appear to be fossil "splashes" remaining from a merger of a smaller satellite galaxy into the core of the elliptical. the spacing of the shells are a measure of the gravitational field, and computer simulations of the merger result in a simple array of concentric shells. Modeling of the shells requires the presence of a massive dark halo.

Classical methods of mass determination, based on optical studies of the luminous inner regions, leave open the possibility of galaxies having considreable amounts of mass in their extended halos. GAlaxies could be very extended indeed, concievably filling most of space with exceedingly tenous halos, In clusters, the halos were stripped during collisions between the galaxies. However the excess mass should stil be present in the intergalactic medium. But the precise form of the dark mass poses a great astrophysical puzzle. the mass cannot be very luminous, or astonomers would be able to observe it directly. It cannot be gaseous, because gas, whether hot or cold, ionized or neutral, is difficult to hide. Many searches have been performed for intergalactic gas. Some gas has been discovered in rich clusters, but not enough to account for the mass discrepancy. Perhaps the most dramatic studies of dark matter in galaxy cluster have merged from the gravitational lensing by the cluster of background galaxies. the gravity field of the cluster bends the background light, acting as a lens, and produces images that are distorted into arcs, This effect was predicted by Albert Einstein but was first detected in the 1970's.

two hypostheses have merged to account for the mass that is inferred to be present in clusters and in galactic halos. One hypothesis argues that the dark mass is baryonic. It might consisst of stars of very low mass, which are so faint that they have escaped detection. Alternatively, many collapsed remnatns perhaps white dwarfs or even blackholes of an early generation of massive stars constitute the hidden mass. A second hypothesis argues that the dark matter is nonbaryonic. it consists of one of the exotic particle species that wearlier hypothesized could exist in sufficientl quantity to yield a substantial fraction of dark matter in halos and in clusters amounts to only 10 percent of the critical density required to reverse the expansion of the universe, if we measure it as the ratio of hypothesized mass to be observed luminosity averaged over a suitably large region of psace.

Black holes would hav formed as a result of catastrophic stellar explosions, and the ensuing radiation shouldm in principle, be detecable. the current consensus is that if blackholes account for the dark mass in clusters of galaxies they must have fmored sufficiently early in the universe for the cosmological redshift to have hidden the associated optical emission from our observations. at a redshift of, say 10, the protogalactic radiation produced when the massive stellar precursors of the black holes evolved and collapsed would now be visible only in ther infrared region of the spectrum. I the infrared, observations are extrmely difficult because of atmospheric emission (such as the terrestial airglow) and attenuation resulting from a sbosrtion by ozone, water vapor, and other molecules.

White dwarfs or neutron stars are a more conservative choice than blackhole for that matter. They are the only dark matter candidates that we can unambiguosly state must exist, although whether enough actually exist is another matter. If they are to be numerous enough to account for dark matter, white dwarfs must hav been produced by a large number of stars of moderate mass, formed early in the evolution of the galaxy. We cannot exclude such a hypothesis, but we can seek ways to test it. for example, the white dwarfs would have cooled down, but they might still be dimly visible as reddish dwarfs. the ejecta produced when compact remnants, blackholes, neutron stars, or white dwarfs formed would be chemically enriched and would show up in the composition of old stars. Studies suggest that remnants of very massive stars, either black holes or neutron stars, are implausible candidates for the dark matter unless the blackholes are much more massive than ordinary stars, but white dwarfs are a possibility.

Stars of low mass also are a possible source of a small fraction of the dark mass. Stars of very low mass populating the halo of our galaxy would occasionally pass close enough to the sun to be recognizeable. Thet would appear as very faint nearby stars with appreciable proper motions and the high velocities characteristic of their halo origin. Becasue fewsuch objects are seen, the orbits of such stars must restrict them predominantly to the outer halo. Presumably, their orbits are most circular, the dynamical characteristics of these objects would make them distinct from the ordinary stars in our galaxy, which have appreciable velocities in the direction of the center, move in highly elongated orbits. Alternatively, these out halo stars could be "jupiters" essentially invisible giant planets that were not massive enough [less than 0.08 solar mass] to become stars.

if we possed an adequate theory of star formation, we sgould be able to choose between hypotheses of massive versus low mass star formation. Even if low mass stars predominate, there must also have been a considerable number of massive stars in the halo of a newly formed protogalaxy the Processed gases ejected during supernova explosions of the massive stars would accounts for the origin of the neriched intergalactic matter that is observed in rich clusters of galaxies. However, our knowledge of star formation is likely to remain so imprecise that direct observations will be required to determine the form of the dark mass if it is baryonic.

[Mike2]

Oh, and the ICM is not exactly a bath. it has densities of around 10^-2-10^-3 electrons per cm^3, which is less dense than the best vacuums we can create on earth! Plus the ICM is optically thin, hence it doesn't absorb light.
That's interesting... As I understand it, the cosmological constant can be described in terms of the zero point energy and some have calculated how much energy this represents per unit volume. They even suggest how many particles of matter this is per volume. So my question is: how does this ICM density compare with that of the cosmological constant?

Also, I wonder if we can get the energy density from a black body radiation temperature. Thanks.

[matt.o]

I'm not sure that it is worthwhile comparing the density of the ICM to that of the quantum vacuum. My point was just that the ICM is not very dense.

[Chronos]

The average mass density of the universe [conventional] is around 1 hydrogen atom per square meter. The intergalactic medium is thought to be in the range of 10 - 100 H atoms per cubic meter. Intercluster mass density might be ~5 times denser, on average. It is a very difficult thing to model. The classical approach is based on newtonian gravity [most of this stuff does not move fast enough to worry about relativistic effects]. Dark matter is what plugs the gap to explain the apparent gravitational attraction observed, but not otherwise accounted for. DM is a rather unsavory explanation, but is more consistent with observation than the competing theories [MOND in particular]. Finding the DM particle in the lab is, however, a huge issue. It hasn't been done yet. It may also never be feasible. The energies necessary may not be achievable by any known technologies.

[Garth]

It is a very difficult thing to model. The classical approach is based on newtonian gravity [most of this stuff does not move fast enough to worry about relativistic effects]. However the non-linear GR effects of orbiting gravitating matter may be significant.Dark matter is what plugs the gap to explain the apparent gravitational attraction observed, but not otherwise accounted for. DM is a rather unsavory explanation, but is more consistent with observation than the competing theories [MOND in particular]. Finding the DM particle in the lab is, however, a huge issue. It hasn't been done yet. It may also never be feasible. The energies necessary may not be achievable by any known technologies.In which case, if the DM particle is never found, what would be the scientific status of such DM?

Garth

[matt.o]

However, the non-linear effects have no bearing on the ICM, which also requires DM to explain why it hasn't evaporated into inter-cluster space.

[Garth]

However, the non-linear effects have no bearing on the ICM, which also requires DM to explain why it hasn't evaporated into inter-cluster space.
I totally agree - see my post #4 in More about the Cooperstock and Tieu model (http://physicsforums.com/showthread.php?p=803385#post803385)

Garth

[Mike2]

I've gotten the energy density for black body radiation for a given temperature. And we have the Unruh temperature for a given acceleration. But in order to apply this Unruh temperature to the acceleration due to gravity (as required by the equivalence principle. Is this DM) I need to calculate the emount of acceleration at each point for a given mass distribution. I could try to use the inverse square law, but this goes to infinity at r=0. And points in the second iteration would then be inside the new distribution when calculating second order effects. So does anyone have a formula for the acceleration felt by test particles calculated by using the gravitational potential for a given arbitrary mass density formula? Thanks.

I'm seriously tempted to consider the energy density of the Unruh radiation applied to the acceleration due to gravity as possibly the source of Dark Matter? It would seem like an easy calculation to find out. First find the energy density of this assumed Unruh radiation. This would involve an integral of Planck's density spectrum over all frequencies. I've looked at this, and I think I can find a definiate integral formula to accomplish this. This would give us an energy formula at temperature. That energy can be converted to mass, and the additional gravitational effects could be calculated from that. But you'd have to find the acceleration for the Unruh formula at a given radius from the galactic center. I suppose one could use Newton's inverse squared law as a good approximation. Then apply the equation for the Unruh temperature. Then one could construct an integral over all space of this extra mass density produced by the Unruh effect applied to acceleration due to gravity.
I suppose this might seem like a very small effect; but that's a lot of space, and I've not done the calculation yet. Not only that, but once you have a first approximation, then you'd have to do it all over again since now you have to take into account the existence of this first approximation results. Your galaxy just acquired more mass, so it will produce more gravitational acceleration that you realized, which requires another iteration of the process. I suppose you'd have to do this 4 or 5 times to see how quickly the series converged.

[Mike2]

Isn't this calculation worth doing for its own sake? If the Unruh effect of acceleration can be applied to gravitationally accelerated reference frames due to the equivalence principle, then should we try to see how much of an effect this would have?