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
