I How would dark matter aggregate?

[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.

 

[Ken G]

That first article raises more questions in my mind. First of all, the article claims that the speed of the dark matter particles was expected to be a few millimeters per second, but now they are saying 9 km/s. How they think 5 orders of magnitude difference in speed could have been hidden all this time is really beyond me, but an even deeper question is, how do they think they can go from a thermal speed to a temperature? From where I'm sitting, the temperature is still proportional to the particle mass, even if you know the thermal speed, so how do they claim a temperature of 10,000 K? They seem to be assuming the particle mass is like that of a proton, but where do they get that from?

 

[nikkkom]

That first article raises more questions in my mind. First of all, the article claims that the speed of the dark matter particles was expected to be a few millimeters per second, but now they are saying 9 km/s. How they think 5 orders of magnitude difference in speed could have been hidden all this time is really beyond me

DM particles are not directly detectable (yet?), thus no direct measurements of their velocities exist.

but an even deeper question is, how do they think they can go from a thermal speed to a temperature? From where I'm sitting, the temperature is still proportional to the particle mass

Wrong.
Temperature is simply the mean kinetic energy. A "soup" of particles whizzing around with 1eV of mean kinetic energy corresponds to temperature of ~11000 Kelvin.

To go from temperature to velocity (or vice versa), you also need to know the mass (or distribution of masses) of the particles. Lighter particles move faster at the same temperature.

If we assume that DM particles were at equilibrium with the rest of the plasma early during Big Bang, then, if DM particles are heavy, they moved (relatively) slowly at decoupling; and if they are light (like ordinary neutrinos are), they moved much faster. In both cases their velocities then decrease during expansion of the Universe.

For neutrinos, unless we missed something in our BB models, expected decoupling time is ~1 second after BB, expected temperature of neutrinos today is 1.95 K, which is ~0.2meV of kinetic energy. Since neutrino rest masses are comparable to this energy, it means that neutrinos are still moving relativistically and can't explain dark matter observations.

https://en.wikipedia.org/wiki/Cosmic_neutrino_background

 

[Ken G]

DM particles are not directly detectable (yet?), thus no direct measurements of their velocities exist.

Of course, yet they know the matter distribution fairly well, and the claim was there was an expectation of a few mm/s of speed. From where does such a remarkably slow speed come? Seems quite unlikely to me.

Wrong.
Temperature is simply the mean kinetic energy. A "soup" of particles whizzing around with 1eV of mean kinetic energy corresponds to temperature of ~11000 Kelvin.

No, I am not wrong. If what you know is the speed (9 km/s), then you need a mass to get a temperature. Obviously, if you have the energy per particle instead, then you will have a temperature, but the article reports on an inference of 9 km/s, not 1 eV.

If we assume that DM particles were at equilibrium with the rest of the plasma early during Big Bang, then, if DM particles are heavy, they moved (relatively) slowly at decoupling; and if they are light (like ordinary neutrinos are), they moved much faster. In both cases their velocities then decrease during expansion of the Universe.

Exactly my point, read my post again. The claims in that article would allow us to determine that the dark matter particle mass is about that of a proton, so if they feel the consistency of the mass of dwarf galaxy haloes can tell us that, it would seem to be a result of vastly greater significance than saying that dark matter temperature estimates are "warming up." Why would they not instead report the bombshell discovery of the mass of the dark matter particle? That's what I'm puzzled about.

 

[.Scott]

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.

First, thanks for the links.

I do have a problem with the article. For example, it states:

The team found that each galaxy seemed to contain the same amount of dark matter: roughly 30 million times the mass of the Sun. They say this is no coincidence. Instead, it represents the minimum amount of dark matter needed for a stable clump to hang together.

Presuming their math is right, the correct statement would be: The amount of DM found in these galaxies correspond to a DM temperature of 10K°C. What should not be implied is that 10K°C is a common temperature for DM - only that it is the temperature that formed these structures.

The reason that this is important is that a mass of DM does not readily transfer thermal energy to other DM masses. So gravitational structures will act as DM prisms - separating out DMs of different velocities not unlike flowing water can separate pebbles and sand into separate layers.
I imagine one DM layer, at 10K°C, visited our galaxy or was separated out by our galaxy's gravitation and then formed the clumping seen in this article.

 

[nikkkom]

First, thanks for the links.

I do have a problem with the article. For example, it states:Presuming their math is right, the correct statement would be: The amount of DM found in these galaxies correspond to a DM temperature of 10K°C. What should not be implied is that 10K°C is a common temperature for DM - only that it is the temperature that formed these structures.

The reason that this is important is that a mass of DM does not readily transfer thermal energy to other DM masses. So gravitational structures will act as DM prisms - separating out DMs of different velocities not unlike flowing water can separate pebbles and sand into separate layers.
I imagine one DM layer, at 10K°C, visited our galaxy or was separated out by our galaxy's gravitation and then formed the clumping seen in this article.

The "sorting" happens only above certain scale.

For example, at 10000K, and with DM particle mass of 2 GeV it has average thermal velocity of ~10km/s and would travel some 30 light years during each billion years (not taking into account that in the past their velocity was higher).

It means that all thermal inhomogeneities in DM below ~100 ly are erased: if you'd be able to "see" DM sky like we can today observe CMB sky, you would see some patches of sky having some DM temperature fluctuations, but you (and any other place) would receive streams of DM particles both from "cold spots" and from "warm spots", making temperature of DM particles flying through your neighborhood to be the average of sky "DM temperature".

 

[Chronos]

If DM does have weak interactions with ordinary matter I would anticipate DM could aggregate via temperature transfer to ordinary mattar. This could be a slow process, but, would suggest its temperature should tend to be lower in regions where it accretes more matter over long periods of time.

 

[haruspex]

If DM does have weak interactions with ordinary matter I would anticipate DM could aggregate via temperature transfer to ordinary mattar. This could be a slow process, but, would suggest its temperature should tend to be lower in regions where it accretes more matter over long periods of time.

Even purely gravitational interactions would lead to some transfer.

 

[Wolfgang Konle]

The normal matter in a galaxy, having shed energy as radiation, is thermodynamically cooler (i.e. less KE) than the dark matter zipping past it.

The thermodynamic term "temperature" is based on an equilibrium achieved by interactions (I. e. particle collisions). For dark matter such an interaction is unknown, therefor I think "temperature" is not defined for dark matter.

 

[haruspex]

The thermodynamic term "temperature" is based on an equilibrium achieved by interactions (I. e. particle collisions). For dark matter such an interaction is unknown, therefor I think "temperature" is not defined for dark matter.

Then ignore that term and just use my clarification "less KE".

 

[Ken G]

One thing to bear in mind is that the kinetic energy per particle is set by the virial theorem, so the more heat the baryonic gas loses to radiation, the higher its kinetic energy. But temperature only includes the energy of random motion, not the orbital energy associated with global angular momentum. So in a spiral galaxy, you have a lot of the kinetic energy in the baryons in a form that does not contribute to their temperature, and in that way, you can get the temperature to drop by radiating. That would not be possible in an elliptical galaxy, but if we are talking about spirals, then yes, the baryons that radiate will end up with less kinetic energy per particle than the dark matter-- except in the core bulge of the galaxy where there is not a propensity of angular momentum, and most of the energy does show up in the temperature. There the baryons should be hotter than the dark matter, since dark matter seems to be nearly isothermal, so has a kinetic energy per particle that is typical of the halo of the whole galaxy. Then the dark matter pressure that supports it against its own gravity, in a fluid picture (so with locally isotropic velocities), comes not from a temperature gradient, but rather from a density gradient.