15 - Overview

Summary

The first goal of theoretical cosmology is to find a model of the universe, the simplest model, that is in agreement with observational data. The second goal is to explore the range of models that are compatible with observational data, in order to understand whether the simplest model is highly probable, and to understand the full range of cosmological possibilities in epochs that are not constrained by observations. This book describes results and techniques of analysis that pertain to the second goal.

The FL models are widely accepted as meeting the first goal (e.g. Peebles et al. 1991), although some uncertainties remain. First, insufficient evidence is available from redshift and peculiar velocity surveys to convincingly establish the averaging scale over which the universe can be regarded as isotropic and homogeneous. Second, a fully satisfactory theory of the formation of structure (i.e. of galaxies and their distribution in space) in a FL model has not yet been found. Third, the fact that the FL models (with Λ = 0), in particular the flat model, are unstable makes it implausible that the real universe can be approximated by a FL model over its entire evolution up to the present and into the future. Fourth, inflation is motivated by the desire to make a flat FL universe in the present epoch inevitable, or at least highly probable. In attempting to reach this goal one has to work with models more general than FL in the pre–inflation epoch.