# Matter Growth in Imperfect Fluid Cosmology

## TL;DR

This paper explores how extensions of General Relativity can be modeled as imperfect fluids affecting cosmic matter growth, using scalar-tensor theories and observational data to constrain deviations from the standard cosmological model.

## Contribution

It introduces a phenomenological framework for modeling geometric degrees of freedom as imperfect fluids and constrains their effects using RSD data in scalar-tensor cosmologies.

## Key findings

- Constraints on anisotropic pressure and energy flux from RSD data
- Demonstration of imperfect fluid effects in scalar-tensor models
- Potential signatures of deviations from $\\Lambda$CDM

## Abstract

Extensions of Einstein's General Relativity (GR) can formally be given a GR structure in which additional geometric degrees of freedom are mapped on an effective energy-momentum tensor. The corresponding effective cosmic medium can then be modeled as an imperfect fluid within GR. The imperfect fluid structure allows us to include, on a phenomenological basis, anisotropic stresses and energy fluxes which are considered as potential signatures for deviations from the cosmological standard $\Lambda$-cold-dark-matter ($\Lambda$CDM) model. As an example, we consider the dynamics of a scalar-tensor extension of the standard model, the $e_{\Phi}\Lambda$CDM model. We constrain the magnitudes of anisotropic pressure and energy flux with the help of redshift-space distortion (RSD) data for the matter growth function $f \sigma_8$.