Critical Density: What Is It and Where Does It Come From?

[LSulayman]

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!

Does that answer your question?

 

[LSulayman]

Yes it does! A rather simple answer, but very logical actually.

 

[Chalnoth]

I think I'd also like to point out that the critical density depends upon the rate of expansion. A larger rate of expansion requires a greater density for spatial flatness. Specifically:

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