Abstract
The standard picture of the Universe is of a system highly Isotropic and spatially homogeneous on scales larger than the observed clustering scale of local luminous matter. Evidence for this is derived from the isotropy of the microwave background radiation and the abundance of helium. In this picture the Universe is represented by a Friedman-Robertson-Walker (FRW) model with small perturbations. However, while exact homogeneity and isotropy imply a FRW model, observations of approximate homogeneity and isotropy in a finite region do not, as we shall see, imply approximate FRW behaviour for all time even locally. We therefore find that the universe need not be of FRW type in order to satisfy the observational constraints. This has important consequences for proposed explanations of approximate homogeneity and isotropy. Such explanations have been focussed on (i) the choice of initial conditions; (ii) dynamical dissipation of anisotropy and imhomogeneity and (iii) the anthropic principle. Under (i) I shall discuss the role of gravitational entropy and I shall mention Mach’s Principle. (Quantum initial conditions are considered by Hartle in this volume.) I shall also comment on the anthropic explanation. Investigations of the behaviour of certain cosmological models of non-FRW type suggest that these approaches are unlikely to be successful.
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