Next Contents Previous

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

Equation 11 (11)

Densities are often measured as fractions of rhoc :

Equation 12 (12)

The quantity Omega 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

Equation 13 (13)

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

Equation 14 (14)