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2.4. Critical density and the density parameter

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

Equation 10 (10)

Densities are often measured as fractions of rhoc :

Equation 11 (11)

The quantity Omega 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

Equation 12 (12)

and when both density and cosmological constant are present the condition for spatial flatness is Omega + OmegaLambda = 1.

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

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