Critical Density

Definition

The critical density is

$$\rho_c = \frac{3H^2}{8\pi G}.$$

It is the density required for a spatially flat universe in the simplest Friedmann equation.

Density Parameters

Define

$$\Omega_i = \frac{\rho_i}{\rho_c}.$$

The Friedmann equation can then be written as a relation among $\Omega$ values, which is convenient for comparing matter, radiation, curvature, and dark energy contributions.