Light propagation in statistically homogeneous and isotropic universes with general matter content

Understand

We derive the relationship of the redshift and the angular diameter distance to the average expansion rate for universes which are statistically homogeneous and isotropic and where the distribution evolves slowly, but which have otherwise arbitrary geometry and matter content.

• The relevant average expansion rate is selected by the observable redshift and the assumed symmetry properties of the spacetime.
• We show why light deflection and shear remain small.
• We write down the evolution equations for the average expansion rate and discuss the validity of the dust approximation.