# What is Equation of state for the Dark Matter?

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

Chalnoth
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.

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
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.

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
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
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
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.

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