A parametrisation of modified gravity on nonlinear cosmological scales

TL;DR

This paper introduces a comprehensive parametrisation framework for modified gravity effects on nonlinear cosmological scales, incorporating screening mechanisms and enabling advanced tests of gravity beyond linear theory.

Contribution

It presents a novel parametrisation of screening mechanisms in scalar-tensor theories for nonlinear structure modeling, extending the linear parametrised post-Friedmannian framework.

Findings

Framework accommodates various screening mechanisms via scale and environment dependence.

Allows embedding of different modified gravity theories into nonlinear cosmological models.

Enables generalised tests of gravity with nonlinear observational data.

Abstract

Viable modifications of gravity on cosmological scales predominantly rely on screening mechanisms to recover Einstein's Theory of General Relativity in the Solar System, where it has been well tested. A parametrisation of the effects of such modifications in the spherical collapse model is presented here for the use of modelling the modified nonlinear cosmological structure. The formalism allows an embedding of the different screening mechanisms operating in scalar-tensor theories through large values of the gravitational potential or its first or second derivatives as well as of linear suppression effects or more general transitions between modified and Einstein gravity limits. Each screening or suppression mechanism is parametrised by a time, mass, and environment dependent screening scale, an effective modified gravitational coupling in the fully unscreened limit that can be matched to linear theory, the exponent of a power-law radial profile of the screened coupling, determined by derivatives, symmetries, and potentials in the scalar field equation, and an interpolation rate between the screened and unscreened limits. Along with generalised perturbative methods, the parametrisation may be used to formulate a nonlinear extension to the linear parametrised post-Friedmannian framework to enable generalised tests of gravity with the wealth of observations from the nonlinear cosmological regime.