The Effective Fluid approach for Modified Gravity and its applications

TL;DR

The paper reviews the effective fluid approach for modified gravity, highlighting its advantages in modeling, analysis, and integration into cosmological codes, with applications to various theories.

Contribution

It introduces the effective fluid framework as a versatile tool for describing and analyzing modified gravity models within cosmology.

Findings

Facilitates inclusion of modified gravity models into Boltzmann codes.

Provides insights into dark energy properties like anisotropic stress and sound speed.

Demonstrates applications to $f(R)$, Horndeski, and Scalar-Vector-Tensor models.

Abstract

In this review we briefly summarize the so-called effective fluid approach, which is a compact framework that can be used to describe a plethora of different modified gravity models as general relativity (GR) and a dark energy (DE) fluid. This approach, which is complementary to the cosmological effective field theory, has several benefits as it allows for the easier inclusion of most modified gravity models into the state-of-the-art Boltzmann codes, that are typically hard-coded for GR and DE. Furthermore, it can also provide theoretical insights into their behavior, since in linear perturbation theory it is easy to derive physically motivated quantities such as the DE anisotropic stress or the DE sound speed. We also present some explicit applications of the effective fluid approach with $f(R)$, Horndeski and Scalar-Vector-Tensor models, namely how this approach can be used to easily solve the perturbation equations and incorporate the aforementioned modified gravity models into Boltzmann codes so as to obtain cosmological constraints using Monte Carlo analyses.