Signatures of graviton masses on the CMB

TL;DR

This paper explores how massive and multiple gravitons influence the B modes of the CMB, revealing unique spectral features and potential observational signatures that could indicate new physics beyond standard gravity.

Contribution

It analytically and numerically investigates the effects of massive and multiple gravitons on the CMB B modes, including the impact of graviton speed, mass, and instabilities.

Findings

Massive gravitons cause a plateau at low l in the B-mode spectrum.

Changes in graviton speed shift the CMB acoustic peaks.

Presence of multiple gravitons introduces characteristic spectral shifts.

Abstract

The impact of the existence of gravitons with non-vanishing masses on the B modes of the Cosmic Microwave Background (CMB) is investigated. We also focus on putative modifications to the speed of the gravitational waves. We find that a change of the graviton speed shifts the acoustic peaks of the CMB and then could be easily constrained. For the case of massive gravity, we show analytically how the B modes are sourced in a manner differing from the massless case leading to a plateau at low $l$ in the CMB spectrum. We also study the case when there are more than one graviton, and when pressure instabilities are present. The latter would occur in doubly coupled bigravity in the radiation era. We focus on the case where a massless graviton becomes tachyonic in the radiation era whilst a massive one remains stable. As the unstable mode decouples from matter in the radiation era, we find that the effects of the instability is largely reduced on the spectrum of B-modes as long as the unstable graviton does not grow into the non-linear regime. In all cases when both massless and massive gravitons are present, we find that the B-mode CMB spectrum is characterised by a low $l$ plateau together with a shifted position for the first few peaks compared to a purely massive graviton spectrum, a shift which depends on the mixing between the gravitons in their coupling to matter and could serve as a hint in favour of the existence of multiple gravitons.