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

The critical density
_{c} is
defined as that giving a
spatially-flat geometry, *k* = 0, in the absence of a cosmological
constant. From the Friedmann equation, this implies

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.

A similar definition can be employed for the cosmological constant, giving

and when both density and cosmological constant are present the
condition for spatial flatness is
+
_{} = 1.

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

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