What is Equation of state for the Dark Matter?

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Discussion Overview

The discussion revolves around the equation of state for dark matter, specifically focusing on its properties, interactions, and implications for cosmology. Participants explore various models and ideas related to the behavior of dark matter in different contexts, including its pressure characteristics and temperature considerations.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants assert that dark matter is pressureless with an equation of state parameter w = 0, while others suggest the possibility of a 'dark radiation' with w = 1/3.
  • There is a discussion about the interactions of dark matter, with some claiming it does not interact with normal matter or itself significantly.
  • Participants note that regular matter also has w ~ 0 due to low kinetic energy compared to total energy, and that relativistic matter would have a varying equation of state parameter depending on kinetic energy.
  • One participant mentions that dark matter particles could have typical velocities around 100 km/s, which might imply a high temperature if their mass is similar to protons, but questions the relevance of these velocities for determining temperature.
  • Another participant argues that the temperature of dark matter is likely around 3K, suggesting that dark matter particles must be very light, potentially lighter than electrons.
  • There is a contention regarding the heating of dark matter particles as they fall into gravitational wells, with some suggesting that those not in such wells are cooler than the cosmic microwave background (CMB).
  • Participants discuss the relationship between the mass of dark matter particles and their temperature, indicating that higher-mass particles tend to have lower temperatures, though this is model-dependent.

Areas of Agreement / Disagreement

Participants express differing views on the equation of state for dark matter, with no consensus reached on its properties or implications. Multiple competing models and hypotheses are presented without resolution.

Contextual Notes

Discussions include assumptions about the interactions of dark matter, the relevance of particle velocities, and the dependence of temperature on mass and decoupling processes, which remain unresolved.

Dmitry67
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Just to clarify, I am NOT talking about the Dark Energy, I am talking about the Dark Matter
w=?
 
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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.
 
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
 
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.
 
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
 
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.
 
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.
 
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.
 
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.
 
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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.
 

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