On the Observational Characteristics of Inhomogeneous Cosmologies: Undermining the Cosmological Principle

TL;DR

This thesis explores inhomogeneous cosmological models that retain the Copernican principle, showing they can fit observational data while being significantly different from standard homogeneous models.

Contribution

It generalizes the Ehlers-Geren-Sachs theorem to inhomogeneous spacetimes with isotropic radiation for all observers, maintaining the Copernican principle.

Findings

Existence of inhomogeneous models consistent with observational constraints

Models can be significantly inhomogeneous yet isotropic for all observers

Retains the Copernican principle in inhomogeneous cosmologies

Abstract

This thesis concerns the compatibility of inhomogeneous cosmologies with our present understanding of the universe. It is a problem of some interest to find the class of all relativistic cosmological models which are capable of providing a reasonable `fit' to the universe. This thesis, in some respects, is part of this process. We consider Stephani models, which are a generalisation of the standard Friedmann-Lemaitre-Robertson-Walker (FLRW) models, which can be thought of as FLRW models with acceleration and pressure gradients. Thus these models generalise the `dust' assumption of standard cosmology. The crucial aspect of this work is the retention of the Copernican principle -- an assumption regarded by many as crucial to cosmology. It states that we are not at a special location in the universe. This is a vital aspect of the original work in this thesis: consideration of an inhomogeneous model, while retaining the Copernican principle has, as far as the author is aware, not been considered in detail before. We start by generalising the Ehlers-Geren-Sachs Theorem to identify the class of inhomogeneous spacetimes which allow an isotropic radiation field for all observers in the spacetime. We then investigate observational and physical aspects of these models from all observer locations. We conclude that there exist spacetimes which conform to present observational constraints (especially anisotropy constraints) for any location in the spacetime, while at the same time being significantly inhomogeneous; ie, not `almost-FLRW'.