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

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