Undermining the Cosmological Principle: Observational Characteristics of Inhomogeneous Cosmologies

We challenge the widely held belief that the cosmological principle is an obvious consequence of the observed isotropy of the cosmic microwave background radiation (CMBR), combined with the Copernican principle. We perform a detailed study of a class of inhomogeneous perfect fluid cosmological models admitting an isotropic radiation field with a view to assessing their viability as models of the real universe. These spacetimes are distinguished from FLRW universes by the presence of inhomogeneous pressure, which results in an acceleration of the fluid (fundamental observers). We examine their physical, geometrical and observational characteristics \\emph{for all observer positions} in the spacetimes. To this end, we derive \\emph{exact, analytic} expressions for the distance-redshift relations and anisotropies for all observer locations, and compare their predictions with available observational constraints. The isotropy constraints derived from `local' observations (redshift $\\lesssim 1$) are also considered, qualitatively. A crucial aspect of this work is the application of the Copernican principle: for a specific model to be acceptable we demand that it must be consistent with presently available observational constraints (especially anisotropy constraints) for all observer positions. The most important results of the paper are presented as exclusion plots in the 2-D parameter space of the models. We show that there is a region of parameter space not ruled out by the constraints we consider and containing models which are significantly inhomogeneous. It follows immediately from this that the cosmological principle cannot be assumed to hold on the basis of present observational constraints.