Modifying Gravity at Low Redshift

TL;DR

This paper investigates how modified gravity models with scalar fields affect the growth of cosmological structures, especially at low redshifts, by analyzing the evolution of density perturbations and gravitational potentials.

Contribution

It introduces a framework to study the impact of a time-varying scalar field on structure growth and gravitational slip in modified gravity models.

Findings

Growth rate of matter perturbations can deviate from GR predictions.

The gravitational slip function varies with redshift in these models.

Scalar field effects become significant at low redshift.

Abstract

We consider the growth of cosmological perturbations in modified gravity models where a scalar field mediates a non-universal Yukawa force between different matter species. The growth of the density contrast is altered for scales below the Compton wave-length of the scalar field. As the universe expands, the Compton wave-length varies in time in such a way that scales which were outside the range of the scalar field force may feel it at a lower redshift. In this case, both the exponent $\gamma$ measuring the growth of Cold Dark Matter perturbations and the slip function representing the ratio of the two Newtonian potentials $\psi$ and $\phi$ may differ from their values in General Relativity at low redshift.