The pattern of growth in viable f(R) cosmologies

TL;DR

This paper investigates the growth patterns of large-scale structures in viable f(R) gravity models, revealing observable scale-dependent features that differ from LCDM and can be tested with future weak lensing surveys.

Contribution

It provides an analytical study of structure growth in f(R) models in both Jordan and Einstein frames, demonstrating their equivalence and physical interpretation, and discusses observational prospects.

Findings

Growth of structure is enhanced in f(R) models.

No small-scale instabilities are found in the models.

Observable scale-dependent patterns can be detected with future surveys.

Abstract

We study the evolution of linear perturbations in metric f(R) models of gravity and identify a potentially observable characteristic scale-dependent pattern in the behavior of cosmological structures. While at the background level viable f(R) models must closely mimic LCDM, the differences in their prediction for the growth of large scale structures can be sufficiently large to be seen with future weak lensing surveys. While working in the Jordan frame, we perform an analytical study of the growth of structures in the Einstein frame, demonstrating the equivalence of the dynamics in the two frames. We also provide a physical interpretation of the results in terms of the dynamics of an effective dark energy fluid with a non-zero shear. We find that the growth of structure in f(R) is enhanced, but that there are no small scale instabilities associated with the additional attractive "fifth force". We then briefly consider some recently proposed observational tests of modified gravity and their utility for detecting the f(R) pattern of structure growth.