# Critical density

## Main Question or Discussion Point

What exactly does the critical density mean?
In articles they keep comparing the desity of the universe to the critical density and what the consequences are but nobody explains what exactly the critical density is and where it comes from.

Nabeshin
The critical density is the density required to have a spatially FLAT universe. It comes from... well, requiring that the universe be spatially flat!

$$\rho_c = {3 H_0^2 \over 8\pi G}$$