The Isotropy and Homogeneity of the Universe

Summary

Abstract

I discuss current observational limits on the inhomogeneity and the isotropy of the universe. Isotropy observations come from the COBE differential microwave radiometer. COBE results are consistent with prior estimates based on cosmic nucleosynthesis. The COBE results on the present structure can be used to limit the range of background density, in particular the closure of the described universe.

Examples from the literature are given whereby a 30 eV massive neutrino simultaneously fits both the observed structure on small scales, and the level of observed quadrupole anisotropy. Further simulations are needed to verify these theoretical fits to the observations.

This paper is dedicated to Charles Misner on his sixtieth birthday.

Introduction

In 1966, prompted by the apparent anisostropic distribution of the three or four then known QSOs, Charles Misner began investigating the behavior of anisotropic universes. These had been studied before, by Kasner [1], Zel'dovich [2], Thorne [3], Taub [4], but Misner's development was a tour de force combining differential geometry, classical mechanics, and astrophysics. One track of his research led to the Mixmaster universe [5], a closed 3-spherical universe in which the ratios of the principal circumferences oscillate as the universe expands and recollapses. This oscillation can lead to very large horizon lengths in particular directions, and gave the hope of explaining the horizon problem. The Mixmaster results led directly to Hamiltonian cosmology and the Quantum Cosmology research effort. The more astrophysical branch of this research [6] developed into studies of dissipation in anisotropic Bianchi type I cosmologies.