What is Equation of state for the Dark Matter?

Dmitry67 · May 15, 2009

Just to clarify, I am NOT talking about the Dark Energy, I am talking about the Dark Matter
w=?

Chalnoth · May 16, 2009

Dmitry67 said: ↑

Just to clarify, I am NOT talking about the Dark Energy, I am talking about the Dark Matter
w=?

w = 0 for dark matter. It's pressureless.

Dmitry67 · May 16, 2009

No, it is pressureless because it does not interact with our matter.
I can imagine a 'dark radiation', which is 'pressureless', but has w=1/3
But yes, probably DM now is a cold gas with w=0
Not sure about the early ages of our Universe still

Chalnoth · May 16, 2009

Dmitry67 said: ↑

No, it is pressureless because it does not interact with our matter.
I can imagine a 'dark radiation', which is 'pressureless', but has w=1/3
But yes, probably DM now is a cold gas with w=0
Not sure about the early ages of our Universe still

It doesn't interact with itself either. At least not much.

Dmitry67 · May 16, 2009

Chalnoth said: ↑

It doesn't interact with itself either. At least not much.

The same is true for the light, and still it has w=1/3

cjl · May 16, 2009

Yep. Regular matter has w ~ 0 as well - it's because it has in essence no kinetic energy compared to its total energy. It doesn't really matter whether it interacts or not. Relativistic matter would have an equation of state parameter such that 0 < w < 1/3, with the exact value depending on the kinetic energy. It approaches 1/3 as its' kinetic energy becomes much greater than its rest energy.

Chalnoth · May 16, 2009

Dmitry67 said: ↑

The same is true for the light, and still it has w=1/3

That comes about from its relativistic motion, though. If the typical velocity of dark matter particles was also near the speed of light, it too would behave like that.

Chalnoth · May 16, 2009

cjl said: ↑

Yep. Regular matter has w ~ 0 as well - it's because it has in essence no kinetic energy compared to its total energy. It doesn't really matter whether it interacts or not. Relativistic matter would have an equation of state parameter such that 0 < w < 1/3, with the exact value depending on the kinetic energy. It approaches 1/3 as its' kinetic energy becomes much greater than its rest energy.

Well, it's also because it's currently at low density. Normal matter experiences quite a lot of pressure at higher densities. But on cosmic scales this effect is completely negligible.

Dmitry67 · May 16, 2009

So as DM is captured around the Galaxies, DM particles have typical velocity in a range of 100km/s. This is equialent to a high temperature if their mass is = mass of proton. But it is logical to assume that DM has the same temperature as other relic sorts of matter, about 3K. Then DM particles must be very light - even lighter then electrons.

Chalnoth · May 16, 2009

Dmitry67 said: ↑

So as DM is captured around the Galaxies, DM particles have typical velocity in a range of 100km/s. This is equialent to a high temperature if their mass is = mass of proton. But it is logical to assume that DM has the same temperature as other relic sorts of matter, about 3K. Then DM particles must be very light - even lighter then electrons.

Er, well, no. The velocities of these particles within galaxies and galaxy clusters is actually irrelevant as far as this determination is concerned. The dark matter particles get heated by their fall into those gravitational potential wells. Dark matter particles that haven't fallen into such wells are actually probably much cooler than the CMB, because the photons get lots of energy dumped into them by various processes, but as the dark matter loses its ability to interact early-on, it doesn't get this extra energy.

What happens is that the earlier the dark matter decouples from the normal matter, the lower the temperature. Usually this means that higher-mass dark matter particles end up with lower temperatures, but it is rather model-dependent.