Equation of State (Cosmology)

In cosmology, the equation of state of a perfect fluid is characterized by a dimensionless number ${\displaystyle w}$, equal to the ratio of its pressure ${\displaystyle p}$ to its energy density ${\displaystyle \rho }$ :

It is closely related to the thermodynamic equation of state and ideal gas law.

The perfect gas equation of state may be written as

where ${\displaystyle \rho _{m}}$ is the mass density, ${\displaystyle R}$ is the particular gas constant, ${\displaystyle T}$ is the temperature and ${\displaystyle C={\sqrt {RT}}}$ is a characteristic thermal speed of the molecules. Thus

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