Inhomogeneous cosmologies, the Copernican principle and the cosmic microwave background: More on the EGS theorem

TL;DR

This paper explores inhomogeneous cosmological models consistent with the Copernican principle, showing that isotropic CMB observations do not necessarily imply a homogeneous universe, and discusses models in General Relativity and scalar-tensor theories.

Contribution

It demonstrates that the cosmological principle is not a direct consequence of the Copernican principle and constructs inhomogeneous models consistent with isotropic CMB observations.

Findings

Inhomogeneous models can satisfy the Copernican principle.

Isotropy of the CMB does not imply spatial homogeneity.

Models are developed in both General Relativity and scalar-tensor theories.

Abstract

We discuss inhomogeneous cosmological models which satisfy the Copernican principle. We construct some inhomogeneous cosmological models starting from the ansatz that the all the observers in the models view an isotropic cosmic microwave background. We discuss multi-fluid models, and illustrate how more general inhomogeneous models may be derived, both in General Relativity and in scalar-tensor theories of gravity. Thus we illustrate that the cosmological principle, the assumption that the Universe we live in is spatially homogeneous, does not necessarily follow from the Copernican principle and the high isotropy of the cosmic microwave background.