Measuring the modified gravitational waves propagation beyond general relativity from CMB observations

TL;DR

This paper investigates how modifications to gravitational wave propagation, specifically a nonstandard friction term, affect observable CMB polarization spectra and parameter constraints, providing new tests for theories beyond general relativity.

Contribution

It introduces a framework to constrain modified gravitational wave propagation effects from CMB data, focusing on the impact of a friction term on tensor spectra and inflation parameters.

Findings

Friction term alters the tensor power spectrum and B-mode polarization.

Constraints on the tensor-to-scalar ratio are significantly affected.

The inflation consistency relation is modified by the friction term.

Abstract

In modified gravity theories, the gravitational waves propagation are presented in nonstandard ways. We consider a friction term different from GR and constrain the modified gravitational waves propagation from observations. The modified gravitational waves produce anisotropies and polarization which generate measurable tensor power spectra. We explore the impact of the friction term on the power spectrum of B-modes and the impact on the constraints on the other parameters (e.g., $r$ or $A_t$) when $\nu_0$ is allowed to vary in the Monte Carlo analyses from Planck+BK18 datasets. If we assume the result of the scalar perturbations is unchanged, the inflation consistency relation alters with the friction term. In the $\Lambda$CDM+$r$+$\nu_0$ model, the tensor-to-scalar ratio and the amplitude of tensor spectrum are influenced obviously.