Effects of modified gravity on B-mode polarization

TL;DR

This paper investigates how modified gravity theories influence B-mode polarization in the cosmic microwave background, focusing on lensing effects and gravitational wave propagation, and constrains the gravitational wave speed using observational data.

Contribution

It identifies the distinct impacts of modified gravity on B-modes and constrains the gravitational wave speed to within 10% of the speed of light.

Findings

Gravitational wave speed is consistent with the speed of light.

Lensing contribution to B-modes depends on anisotropic stress.

Inflationary peak location is affected by gravitational wave speed.

Abstract

We explore the impact of modified gravity on B-modes, identifying two main separate effects: lensing and propagation of tensor modes. The location of the inflationary peak of the BB spectrum depends on the speed of gravitational waves; the amplitude of the lensing contribution depends on the anisotropic stress. We single out these effects using the quasi-static regime and considering models for which the background and the growth of matter perturbations are standard. Using available data we obtain that the gravitational wave speed is compatible with the speed of light and constrained to within about 10%.