I How would dark matter aggregate?

[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.