Constraining the gravitational action with CMB tensor anisotropies

TL;DR

This paper analyzes how tensor anisotropies in the CMB can constrain f(R) gravity theories, emphasizing the importance of background modifications and revealing potential B-mode enhancements relevant for inflationary models.

Contribution

It provides a comprehensive framework for analyzing tensor anisotropies in f(R) gravity, highlighting the significance of background evolution in deviations from General Relativity.

Findings

Background modifications dominate deviations in CMB spectra

Initial conditions in R^n gravity match GR for n≠2

Enhanced B-modes could lower inflationary energy scales

Abstract

We present a complete analysis of the imprint of tensor anisotropies on the Cosmic Microwave Background for a class of f(R) gravity theories within the PPF-CAMB framework. We derive the equations, both for the cosmological background and gravitational wave perturbations, required to obtain the standard temperature and polarization power spectra, taking care to include all effects which arise from f(R) modifications of both the background and the perturbation equations. For R^n gravity, we show that for n different from 2, the initial conditions in the radiation dominated era are the same as those found in General Relativity. We also find that by doing simulations which involve either modifying the background evolution while keeping the perturbation equations fixed or fixing the background to be the Lambda-CDM model and modifying the perturbation equations, the dominant contribution to deviations from General Relativity in the temperature and polarization spectra can be attributed to modifications in the background. This demonstrates the importance of using the correct background in perturbative studies of f(R) gravity. Finally an enhancement in the B-modes power spectra is observed which may allow for lower inflationary energy scales.