13 - Cosmological principles

Summary

INTRODUCTION

I first met Dennis Sciama in 1974 whilst I was still an undergraduate. At our first meeting he told me about the challenge of explaining the large scale regularity of the Universe, along with other of its unusual features, like the existence of galaxies and its proximity to a state of “zero binding energy” that we now tend to call “flatness”, without making special assumptions about initial conditions. Many of these issues remain a continuing focus of attention in cosmology. Here, my intention is to review a number of cosmological ‘principles’ and their interaction with a variety of cosmological developments that have taken place over the period during which Dennis has worked on cosmology. The talk on which this article is based formed a small part of these Proceedings which celebrate the huge contribution that Dennis has made and continues to make to general relativity, cosmology and astrophysics. Besides Dennis' personal contributions and those of his students, that of so many of his former students (and their students) exhibits the non-linear amplification in their effectiveness that was always created by the collaborations and contacts between them that have been catalysed by their shared associations with Dennis.

THE PERFECT COSMOLOGICAL PRINCIPLE

In 1948 Bondi, Gold and Hoyle (Bondi and Gold, 1948; Hoyle 1948) proposed a powerful cosmological symmetry principle which they called the ‘Perfect Cosmological Principle’.