Cosmology: The search for twenty-four (or more) functions

Abstract

We enumerate the $4(1+F)+2S$ independent arbitrary functions of space required to describe a general relativistic cosmology containing an arbitrary number of noninteracting fluid ($F$) and scalar fields ($S$). Results are also given for arbitrary space dimension and for higher-order gravity theories, where the number increases to $16+4F+2S$. Both counts are subject to assumptions about whether the dark energy is a cosmological constant. A more detailed analysis is provided when global homogeneity is assumed and the functions become constants. This situation is also studied in the case where the flat and open universes have compact spatial topologies. This changes the relative generalities significantly and places new constraints on the types of expansion anisotropy that are permitted. The most general compact homogeneous universes containing Friedmann models are spatially flat and described by $8+4F+2S$ constants. Comparisons are made with the simple six-parameter lambda-CDM model and physical interpretations provided for the parameters needed to describe the most general cosmological models.