Cosmological tensor perturbations in theories beyond $\Lambda$CDM

We study for the first time a complete analysis of the imprint of tensor anisotropies on the Cosmic Microwave Background for a class of f(R) gravity theories within the CAMB-PPF framework. Herein we present the most relevant equations, both for the cosmological background and gravitational wave perturbations, taking care to include all effects which arise from f(R) modifications. We find that that 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.