Equation of state in cosmology

Summary

In cosmology, the equation of state of a component of the universe is the ratio $w = p/\rho$ of its pressure to its energy density. Each kind of matter has its own typical value: $w = 0$ for cold matter, $w = 1/3$ for radiation, and $w = -1$ for a cosmological constant. The value of $w$ decides how fast the density of the component dilutes as the universe expands, and so which component dominates at each epoch.