The linear growth rate of structure in Parametrized Post Friedmannian Universes

TL;DR

This paper explores how modifications to general relativity can be constrained by analyzing the growth rate of cosmic structure, providing a framework to test theories of dark energy and gravity with future observations.

Contribution

It derives consistency conditions for modified gravity theories with second derivatives, linking physical requirements to observational constraints without needing a complete theory.

Findings

Derived restrictions on parameters governing structure growth

Identified physical conditions for stable, metric, gauge-invariant theories

Highlighted potential of small-scale structure measurements to constrain gravity models

Abstract

A possible solution to the dark energy problem is that Einstein's theory of general relativity is modified. A suite of models have been proposed that, in general, are unable to predict the correct amount of large scale structure in the distribution of galaxies or anisotropies in the Cosmic Microwave Background. It has been argued, however, that it should be possible to constrain a general class of theories of modified gravity by focusing on properties such as the growing mode, gravitational slip and the effective, time varying Newton's constant. We show that assuming certain physical requirements such as stability, metricity and gauge invariance, it is possible to come up with consistency conditions between these various parameters. In this paper we focus on theories which have, at most, 2nd derivatives in the metric variables and find restrictions that shed light on current and future experimental constraints without having to resort to a (as yet unknown) complete theory of modified gravity. We claim that future measurements of the growth of structure on small scales (i.e. from 1-200 h^{-1} Mpc) may lead to tight constraints on both dark energy and modified theories of gravity.