**2.4. Critical density and the density parameter**

The spatial geometry is flat if *k* = 0. For a given *H*, this
requires that
the density equals the critical density

Densities are often measured as fractions of
_{c} :

The quantity is known as the density parameter, and can be applied to individual types of material as well as the total density.

The present value of the Hubble parameter is still not that well known, and is normally parametrized as

where *h* is normally assumed to lie in the range
0.5 *h*
0.8. The
present critical density is