Anisotropic Scalar-Tensor Cosmologies

TL;DR

This paper develops a method to derive solutions for anisotropic scalar-tensor cosmologies, showing how these models evolve towards isotropy and general relativity, with applications to string theory and initial singularity analysis.

Contribution

It introduces a general solution derivation method for anisotropic scalar-tensor cosmologies with perfect fluids, extending to string effective actions and analyzing isotropization.

Findings

Models approach isotropy and general relativity over time.

Solutions provided for stiff fluid, radiation, and vacuum cases.

Initial singularity behavior discussed with shear and scalar-tensor effects.

Abstract

We examine homogeneous but anisotropic cosmologies in scalar-tensor gravity theories, including Brans-Dicke gravity. We present a method for deriving solutions for any isotropic perfect fluid with a barotropic equation of state ($p\propto\rho$) in a spatially flat (Bianchi type~I) cosmology. These models approach an isotropic, general relativistic solution as the expansion becomes dominated by the barotropic fluid. All models that approach general relativity isotropize except for the case of a maximally stiff fluid. For stiff fluid or radiation or in vacuum we are able to give solutions for arbitrary scalar-tensor theories in a number of anisotropic Bianchi and Kantowski-Sachs metrics. We show how this approach can also be used to derive solutions from the low-energy string effective action. We discuss the nature, and possibly avoidance of, the initial singularity where both shear and non-Einstein behavior is important.