I How would dark matter aggregate?

[Chalnoth]

I'm guessing here. Assume dark matter was initially spread evenly across the universe as a hot gas. The universe is expanding, so the kinetic energy of dark matter is redshifted. Eventually, the energy is low enough that they form gravitationally-bound clumps. In the clumps the kinetic energy is no longer controlled by the expansion of the universe, but by the virial theorem.

It can't start out spread perfectly evenly. If it was spread out evenly at the start, then it would still be spread out evenly. Instead, there needs to be very small differences in density from place to place in the early universe.

 

[Chronos]

Since it appears likely dark matter has primordial origins [was around before ordinary matter], you are forced to concede it was never uniformly dense, ostensibly due to quantum fluctuations prior to inflation. These density variations show up as tiny anisotrophies in the CMB that are widely believed to have seeded large scale matter structures in the universe today. While dark matter is virtually undetectable in the EM spectrum, researchers assure us it's there. Virtually every galaxy and galactic cluster hints at being embedded in a massive cloud of dark matter. The DM is distributed in a gigantic network called the cosmic web as illustrated here http://www.illustris-project.org/.

 

[Orodruin]

due to quantum fluctuations prior to inflation.

Technically during inflation. Anything from before inflation is diluted into nothing. This is the point of inflation.

 

[Davephaelon]

Here's an interesting new (19 Sept. 16) correlation discovered between radial acceleration curves in galaxies and baryonic matter distribution in galaxies deduced from near-infrared data.

https://arxiv.org/abs/1609.05917

 

[mrspeedybob]

The universe is expanding, so the kinetic energy of dark matter is redshifted. Eventually, the energy is low enough that they form gravitationally-bound clumps.

You beat me to it.

The other thing that I was going to ask is, how do we know dark matter does not radiate? We know it doesn't radiate EM because we can't see it, but what's wrong with the idea of another force or type of radiation that interacts with dark matter and not normal matter. DM had to come from somewhere. If I understand correctly, normal matter condensed out of primordial EM radiation, so it makes sense that there has to be some interactivity between the 2. DM could not have condensed out of EM because EM does not interact with it. Soo, isn't it at least plausible (if not likely) that there's another form of energy that DM does interact with?

 

[phinds]

You beat me to it.

The other thing that I was going to ask is, how do we know dark matter does not radiate? We know it doesn't radiate EM because we can't see it, but what's wrong with the idea of another force or type of radiation that interacts with dark matter and not normal matter. DM had to come from somewhere. If I understand correctly, normal matter condensed out of primordial EM radiation, so it makes sense that there has to be some interactivity between the 2. DM could not have condensed out of EM because EM does not interact with it. Soo, isn't it at least plausible (if not likely) that there's another form of energy that DM does interact with?

You are asking if there is some other whole set of things in physics that no one knows about. Doesn't that seem unlikely to you? What kind of radiation do you image that would not be EM radiation?

 

[mrspeedybob]

The only way we know about dark matter is because of its gravity. If there were a massless particle (like a photon) that didn't interact with the stuff that we and our instruments are made of, it wouldn't be surprising if we didn't know about it. The only way we could would be if it were needed to explain the behavior of dark matter, and that's hard enough to measure.

 

[phinds]

The only way we know about dark matter is because of its gravity. If there were a massless particle (like a photon) that didn't interact with the stuff that we and our instruments are made of, it wouldn't be surprising if we didn't know about it. The only way we could would be if it were needed to explain the behavior of dark matter, and that's hard enough to measure.

And if it didn't interact with "the stuff that we and our instruments are made of" how would it EVER be known to exist and why would it matter since it doesn't interact with anything?

Dark matter is of unknown composition. I think trying to introduce another very hypothetical item to try to explain it doesn't seem very helpful.

 

[collinsmark]

I've read through this thread but I'm surprised not to hear more mention of gravitational waves. Gravitational waves are are the simple answer to the OP's question. (at least according to my high-energy physics textbook). [Edit: I mis-read or mis-remembered what I studied. Corrections made below after double checking.]

Even ignoring baryonic matter for the moment, assuming there are some slight fluctuations in dark matter density in the early universe (i.e., nearly uniform, but not quite), clumps of dark matter would collapse on themselves. But similar to how @haruspex has already mentioned, if our universe obeyed Newtonian gravity, those clumps would merely oscillate indefinitely, getting smaller then bigger, then smaller again and bigger again, and so on and so on, without end. Where did the energy go?

Gravitational waves. A galaxy or even galaxy cluster sized [in terms of mass] chunk of dark matter would produce jaw dropping amounts of gravitational waves from the oscillations as it collapsed and oscillated. These gravitational waves radiate away from the center leaving less energy in the system than when it started.

[Edit: The chaotically varying gravitational field adds an additional heating component to the baryonic matter besides that of the adiabatic heating. That heat is then released by the baryonic matter in the form of photons.]

Eventually the oscillations die out when the system becomes virial, and the shape of the clump becomes stable over time. Dark matter particles (assuming WIMPs) have greatly varying speeds relative to each other, but on large scales, the shape of the ensemble remains constant. Since the shape of the clump is now constant, you no longer get appreciable gravitational wave generation.

Add that tiny bit of baryonic matter into the mix, and it doesn't change a whole lot. The baryonic matter just goes along for the ride.

The interesting question is whether the supermassive black holes at the center of galaxies simply came from the mergers of many stellar sized black holes or if the galaxy's initial collapse and oscillations (before virialization) played a more significant role. I don't know enough about the subject to comment on that, so I'll end it there. (It's not really part of the OP's question anyway).

But for a simple answer,

That doesn't explain the loss of energy. If the dark matter started (roughly) uniformly spread over the vastness of space, the KE when trapped in orbit in a galaxy is only half the lost PE. Where did the rest go?

you needn't look any further than gravitational waves heating of the baryionic matter within a chaotically changing gravitational field. They [photons] carried the energy away.

 

[Chronos]

Gravitational waves can only carry off a miniscule amounts of energy, save for interactions between compact bodie [like black holes].

 

[Ken G]

Also, it is not true that density fluctuations will lead to significant contraction automatically. What one expects adiabatically (no energy transport) is simply pressure waves-- that's more or less what sound is. Adding gravity to the mix won't do much, because gravity only gives the compressed gas even more of an excess kinetic energy than adiabatic compression gives it-- it bounces back even more readily. To get a normal gravitational instability, called the Jeans instability, you need heat transport. Indeed, the Jeans instability is a constant temperature instability. But how dark matter would enforce isothermality I have no idea-- maybe conduction? Or some form of dark radiation, as speculated above? The paradox is, it seems to me that to get dark matter to contract first, it needs to be quite good at energy transport, yet we tend to think of it as poor at that. Since I don't know, I regard the OP question as still unanswered here, and perhaps unanswered everywhere.

 

[collinsmark]

Gravitational waves can only carry off a miniscule amounts of energy, save for interactions between compact bodie [like black holes].

Now that I double check, you are right. Allow me to make a correction. The baryonic matter is heated during virialization, due to a chaotically varying gravitational field that's likely to be the main contributor. The bulk of the energy is carried away by the electromagnetic radiation as the baryonic matter radiates photons (rather than being significantly carried by gravitational waves).

Let me just quote from my textbook. Peter Meszaros "The High Energy Universe", Cambridge University Press, 2010, pp. 49, 50.

The most important aspect of the process is that after the epoch of matter-radiation equilibrium, most of the mass-energy is is the form of dark matter, and even before recombination (i.e., before the electrons recombine protons to make neutral atoms), the dark matter dominates the gravitational field of the Universe and also that of any density perturbation in it [20]. Since the dark matter does not "feel" the radiation, regions with an excess of dark matter relative to the background start to slow down and eventually recollapse (see Fig. 4.1). The dark matter is, from all indications, non-relativistic; that is, it has no pressure. Thus when dark matter particles recollapse onto themselves, they are not stopped by their own pressure (they are "collisionless") and they go right through each other. They overshoot, and like a pendulum, eventually they turn around and around again. In the process, the gravitational field varies chaotically, and this acts as a damper on the dark matter motions, which come to a quasi-thermal equlibrium in a few dynamical times satisfying the Virial theorem, which states that twice the kinetic energy of the particles equals their gravitational potential energy. This process is called virialization, the equilibrium outer radius being half the radius at turnaround.

What happens during all of this with baryons? The baryonic gas is a smaller fraction of the total mass than the dark matter (DM) and its dynamics is dominated by the gravitational field of the DM. Thus, the baryons initially follow the DM during the expansion, the turnaround and the early phases of collapse. As the collapse proceeds , the volume ocupied by the DM and the gas decreases and both are adiabatically heated. However, unlike DM, the baryonic gas is collisional (i.e., its atoms have a significant "cross-section" for interacting with each other as the gas density increases in the collapse), and this gives rise to further heating cause by collisions between blobs of baryonic gas leading to shocks which convert the infall kinetic energy into random thermal motion energy of the particles. These thermal gas motions lead to collisions between individual atoms and molecules, which excite their electrons to higher quantum energy levels followed by radiative de-excitation; that is, the emission of photons.​

 

[Ken G]

Yet what that doesn't explain is how dark matter aggregates. Saying dark matter obeys the virial theorem is exactly the problem, that's what requires heat transport to allow contraction because the virial theorem means that contraction induces an excess of kinetic energy over what could sustain the contraction in the long run. So you'll never get a cosmic web of virialized dark matter unless you give the dark matter the ability to transport heat, and that's the part I don't understand.

 

[Chalnoth]

Yet what that doesn't explain is how dark matter aggregates. Saying dark matter obeys the virial theorem is exactly the problem, that's what requires heat transport to allow contraction because the virial theorem means that contraction induces an excess of kinetic energy over what could sustain the contraction in the long run. So you'll never get a cosmic web of virialized dark matter unless you give the dark matter the ability to transport heat, and that's the part I don't understand.

Virialization doesn't require energy loss. It generally just involves the randomization of the orbits of the various objects/particles so that they're in approximate thermal equilibrium with one another. Dark matter can do this through gravitational interactions which exchange momentum between different particles.

 

[Ken G]

Ah yes, that's a good point-- so all you need is for the dark matter to get a significant overabundance of (negative) gravitational potential energy, and it will want to contract and convert that into kinetic energy as it virializes. That makes sense, so you don't need heat transport, you just need dark matter on a large enough scale, which you certainly have.

 

[.Scott]

Is it necessary for this dark matter to act as a gas? Isn't the presumption that dark matter particles don't even interact with other dark matter particles? If that is so, it is not a gas in the sense or exerting pressure or following other gas laws.

If DM is that inert, then most dark matter particles entering our galaxy will retain galactic escape velocity and eventually pass through; and some will lose enough energy through gravitational effects to stay.

There is one interaction that dark matter should be exceptional at - diving into a black hole. If their trajectory is right, nothing electromagnetic will stop them from diving right in. And, as they enter, they shouldn't be able to radiate much except in the form of gravitational waves.

Speaking of interactions, is DM presumed to be either Fermions (affected by Pauli Exclusion) or Bosons (possibly forming a condensate) ?

 

[haruspex]

Ah yes, that's a good point-- so all you need is for the dark matter to get a significant overabundance of (negative) gravitational potential energy, and it will want to contract and convert that into kinetic energy as it virializes. That makes sense, so you don't need heat transport, you just need dark matter on a large enough scale, which you certainly have.

Virialization doesn't require energy loss. It generally just involves the randomization of the orbits of the various objects/particles so that they're in approximate thermal equilibrium with one another. Dark matter can do this through gravitational interactions which exchange momentum between different particles.

Sure, but that is not going to give you ongoing aggregation, which was what the original question concerned. It just gets you from a static spread at one radius to a chaotic sphere at a smaller one, the lost PE having been turned into KE. I am prepared to take on trust that the gravitational interactions make a chaotic sphere unstable, leading instead to a much flatter structure, but still asymptotically of a great radius.
The answer to that original question appears to be that it indeed does not continue to aggregate, other than by leaking KE to slower moving baryonic matter through the gravitational interactions.

 

[Chalnoth]

Is it necessary for this dark matter to act as a gas? Isn't the presumption that dark matter particles don't even interact with other dark matter particles? If that is so, it is not a gas in the sense or exerting pressure or following other gas laws.

An ideal gas assumes non-interacting particles in thermal equilibrium with one another. So yes, dark matter can be modeled very well as a gas.

Speaking of interactions, is DM presumed to be either Fermions (affected by Pauli Exclusion) or Bosons (possibly forming a condensate) ?

There are dark matter candidate proposals for both of these possibilities. Axions are one possibility for bosons, while the neutralino is a fermion that exists in supersymmetric models and is another candidate.

 

[Chalnoth]

I am prepared to take on trust that the gravitational interactions make a chaotic sphere unstable, leading instead to a much flatter structure, but still asymptotically of a great radius.

I don't think that's accurate. My understanding is that you only get a flattened structure in the presence of friction, so dark matter won't ever do this to any appreciable extent.

 

[haruspex]

I don't think that's accurate. My understanding is that you only get a flattened structure in the presence of friction, so dark matter won't ever do this to any appreciable extent.

I only said I was prepared to believe that. It could be a bit like a spinning block only being stable about two of the three axes. Chaotic gravitational interactions might make a basically spherical shape unstable. Or perhaps it becomes unstable in the presence of the baryonic matter having assumed a disc structure.
If that is not correct then the large scale membranous structure of DM becomes another puzzle.

 

[collinsmark]

If there is any residual angular momentum after virialization, the clump of dark matter would form an oblate spheroid [Edit: technically, a halo having oblate spheroid-ish symmetry]. My understanding is that this agrees with observation in our Milky Way galaxy; that is, even though the baryonic matter is mostly in the shape of a flat disk, the dark matter halo part is much thicker: not quite spherical, but much closer to spherical than the baryonic matter (i.e., the dark matter halo is oblate spheroid shaped).

 

[newjerseyrunner]

You're looking at it backwards. The dark matter doesn't clump around galaxies, the galaxies form in dark matter clumps. There is five times as much dark matter as matter. I like to think of galaxies as little blobs bobbing around a huge dark matter cloud.

 

[.Scott]

You're looking at it backwards. The dark matter doesn't clump around galaxies, the galaxies form in dark matter clumps. There is five times as much dark matter as matter. I like to think of galaxies as little blobs bobbing around a huge dark matter cloud.

That's unclear. DM doesn't clump as well as the regular stuff. Except with black holes, it does elastic collisions - or no collisions at all. So it would be much better at intergalactic (or inter-clump) cruising.

On the other hand, we have one example of a 99% dark matter galaxy and no certain examples of a similar regular matter galaxy. Makes me wonder if that 99% DM galaxy is a fragment separated from another galaxy.

 

[Ken G]

It is widely thought that you need dark matter clumping to get baryonic matter clumping to happen faster. One of the reasons the dark matter model rose to the fore is that models of pure baryonic clumping could not get galaxies to appear fast enough. So the "cosmic web" is very much a dark matter phenomenon, that pulls the baryons into compliance. It seems the dark matter clumping can be explained simply by virialization on large enough scales such that there is enough gravitational potential energy to convert into kinetic as the contraction plays out. Why it makes filaments instead of spherical clumps is another thing we need to understand-- I don't know why the contraction is more two-dimensional than three dimensional.

 

[nikkkom]

And if it didn't interact with "the stuff that we and our instruments are made of" how would it EVER be known to exist and why would it matter since it doesn't interact with anything?

For example, if we detect that DM is cooling, that would be an indication there are some significant self-interactions in DM sector. There can be "dark photons"

Dark matter is of unknown composition. I think trying to introduce another very hypothetical item to try to explain it doesn't seem very helpful.

Well, we do want eventually understand what that stuff is. Just leaving it as "unknown uncharged heavy particles" is not good enough.

 

[phinds]

For example, if we detect that DM is cooling, that would be an indication there are some significant self-interactions in DM sector. There can be "dark photons"

But since we don't know what DM is, why posit an extra unknown to explain a characteristic of something we already don't know enough about?

Well, we do want eventually understand what that stuff is. Just leaving it as "unknown uncharged heavy particles" is not good enough.

I agree.

 

[nikkkom]

I visualize it as follows.

Imagine an expanding universe with uniform, motionless (in comoving coordinates) dust. This is not gravitationally stable. Dust will start to clump, start to "fall into" randomly shaped regions. If dust is non-interacting, dust particles will accelerate, pass through the center of "their" overdense region, then decelerate. Ones which move a bit faster than on average will escape from this clump... and fall into the next, while expansion of Universe makes their velocity relative to this other clump smaller.

Dust particles which do not escape, they fall back into the clump, and will generally stay orbiting it. (Some will still escape on further revolutions).

With initially motionless dust, the result is very clumpy.

If initially dust does have some random velocities, not too large, the clumps would still form but they can't be small: dust moves too fast to form small ones. The "hotter" it is, the larger this lower clump size limit.

Evidently, the observed DM in the actual Univrse does clump only on large scales (at least galaxy-scale). It should be possible to estimate its random velocity distribution from this.

 

[nikkkom]

But since we don't know what DM is, why posit an extra unknown to explain a characteristic of something we already don't know enough about?

I just explained to you that if, in your words, "it didn't interact with 'the stuff that we and our instruments are made of'", there are still possibilities to get more information about it indirectly. It's possible to indirectly observe that DM's random velocity ("temperature") is decreasing. If something like this will be seen, it needs to be explained.

 

[Ken G]

For example, if we detect that DM is cooling, that would be an indication there are some significant self-interactions in DM sector. There can be "dark photons".

Yes, it would be interesting to be able to determine if the degree of dark matter clumping is pure virialization at a given scale, or if it requires some additional heat loss to get that much contraction. I don't know what the models are saying about that at present.

 

[Chronos]

The temperature of dark matter remains an interesting issue in cosmology. The traditional 'cold' dark matter paradigm is under scrutiny as a consequence of new data and simulations. These discussions may be of interest http://www.nature.com/news/2006/060206/full/news060206-1.html, Dark matter warms up and http://arxiv.org/abs/1604.07409, Substructure and galaxy formation in the Copernicus Complexio warm dark matter simulations.