Question

Why are the cosmological parameters in Friedmann equations constant?

Answer 1

Besides the density and gravitationconstant G, the equation contains the Hubble parameter H, a scalingparameter R, and a factor k which is called the curvature parameter.Einstein considered adding another term, the famous (or infamous)cosmological constant which would produce a static universe.

Answer 2

The Hubble parameter can change over time if other parts of the equation are time dependent (in particular the mass density, the vacuum energy, or the spatial curvature). Evaluating the Hubble parameter at the present time yields Hubble's constant which is the proportionality constant of Hubble's law. Applied to a fluid with a given equation of state, the Friedmann equations yield the time evolution and geometry of the universe as a function of the fluid density.