Testing gravity with the cosmic microwave background: constraints on modified gravity with two tensorial degrees of freedom

TL;DR

This paper tests a modified gravity theory with two tensorial degrees of freedom using Planck CMB data, providing the first observational constraints on this model that is indistinguishable from ΛCDM at background level.

Contribution

It introduces a cosmological test for a specific modified gravity model with no scalar mode, constraining its parameter using CMB data.

Findings

Constraint on the parameter β: -0.047 to -0.028 at 68% confidence

First observational test of this modified gravity model

Model indistinguishable from ΛCDM at background level

Abstract

We provide a cosmological test of modified gravity with two tensorial degrees of freedom and no extra propagating scalar mode. The theory of gravity we consider admits a cosmological model that is indistinguishable from the $\Lambda$CDM model at the level of the background evolution. The model has a single modified-gravity parameter $\beta$, the effect of which can be seen in linear perturbations, though no extra scalar mode is propagating. Using the Boltzmann code modified to incorporate the present model, we derive the constraints $-0.047 < \beta < -0.028$ at 68$\%$ confidence from Planck CMB data. Since our modified gravity model can hardly be constrained by the Solar System tests and gravitational-wave propagation, our result offers the first observational test on the model.