Emergent Cosmological Expansion in Scalar-Tensor Theories of Gravity

TL;DR

This paper investigates how scalar-tensor theories of gravity can produce cosmological expansion by modeling sub-horizon regions as weak-field expansions and joining them, revealing that perturbative scalar fields align with Friedmann solutions, but screening mechanisms cause deviations.

Contribution

It introduces a novel approach to modeling large-scale cosmology in scalar-tensor theories by combining local weak-field regions, highlighting the impact of screening mechanisms on cosmological behavior.

Findings

Perturbative scalar fields reproduce Friedmann cosmology.

Screening mechanisms suppress deviations from general relativity.

Chameleon mechanism causes significant deviations from Friedmann solutions.

Abstract

We consider the emergence of large-scale cosmological expansion in scalar-tensor theories of gravity. This is achieved by modelling sub-horizon regions of space-time as weak-field expansions around Minkowski space, and then subsequently joining many such regions together to create a statistically homogeneous and isotropic cosmology. We find that when the scalar field can be treated perturbatively, the cosmological behaviour that emerges is well modelled by the Friedmann solutions of the theory. When non-perturbative screening mechanisms occur this result no longer holds, and in the case of scalar fields subject to the chameleon mechanism we find significant deviations from the expected Friedmann behaviour. In particular, the screened mass no longer contributes to the Klein-Gordon equation, suppressing deviations from general relativistic behaviour.