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Dmitry67
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Just to clarify, I am NOT talking about the Dark Energy, I am talking about the Dark Matter
w=?
w=?
w = 0 for dark matter. It's pressureless.Dmitry67 said:Just to clarify, I am NOT talking about the Dark Energy, I am talking about the Dark Matter
w=?
It doesn't interact with itself either. At least not much.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
Chalnoth said:It doesn't interact with itself either. At least not much.
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.Dmitry67 said:The same is true for the light, and still it has w=1/3
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.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.
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.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.
The equation of state for dark matter is a mathematical relationship that describes how the pressure and energy density of dark matter are related. It is typically denoted as w, where w = pressure/energy density. This equation helps to quantify the behavior and properties of dark matter.
The equation of state for dark matter is determined through observations and theoretical models. Scientists use data from astronomical observations, such as the rotation curves of galaxies, to infer the amount and distribution of dark matter in the universe. Theoretical models, such as simulations, are also used to understand the behavior of dark matter and its effects on the universe.
The equation of state for dark matter is significant because it helps us understand the properties and behavior of this mysterious substance. It also plays a crucial role in cosmology, as it affects the evolution and structure of the universe. The equation of state can also provide insights into the nature of dark matter and its interactions with other types of matter and energy.
The equation of state for dark matter is currently believed to be constant, meaning it does not change over time or in different regions of the universe. However, as our understanding of dark matter evolves, it is possible that the equation of state may also change. For example, if we discover new types of dark matter particles or interactions, the equation of state may need to be revised.
The equation of state for dark matter is often compared to that of normal matter, which includes atoms and other known particles. The main difference is that normal matter has a positive equation of state, meaning its pressure is directly proportional to its energy density. In contrast, dark matter has a negative equation of state, which means its pressure decreases as its energy density increases. Additionally, normal matter can be compressed and heated, while dark matter is not affected by these processes.