The Cosmic Microwave Background and Scalar-Tensor Theories of Gravity

TL;DR

This paper extends a fundamental theorem in cosmology to scalar-tensor theories, showing that isotropic cosmic microwave background observations imply an isotropic and homogeneous universe, supporting standard cosmological models.

Contribution

It generalizes the Ehlers-Geren-Sachs Theorem from General Relativity to scalar-tensor theories, linking CMB isotropy to universe homogeneity in these alternative gravity models.

Findings

Isotropic CMB implies isotropic and homogeneous spacetime in scalar-tensor theories.

Supports that compatible cosmological models are nearly Friedmann-Lemaitre-Robertson-Walker.

Extends foundational cosmological theorems to alternative gravity frameworks.

Abstract

We show that if all observers see an isotropic cosmic microwave background in an expanding geodesic perfect fluid spacetime within a scalar-tensor theory of gravity, then that spacetime must be isotropic and spatially homogeneous. This result generalizes the Ehlers-Geren-Sachs Theorem of General Relativity, and serves to underpin the important result that any evolving cosmological model in a scalar-tensor theory that is compatible with observations must be almost Friedmann-Lemaitre-Robertson-Walker.