Equation of state (cosmology)

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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 ${\displaystyle \rho }$: $${\displaystyle w\equiv {\frac {p}{\rho }}.}$$ It is closely related to the thermodynamic equation of state and ideal gas law.