Signatures of graviton masses on the CMB

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.

Instability section updated. Some references added

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• GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral
• Resummation of Massive Gravity
• Beyond the Cosmological Standard Model
• Dark Energy after GW170817: dead ends and the road ahead
• Strong constraints on cosmological gravity from GW170817 and GRB 170817A
• Dark Energy after GW170817 and GRB170817A
• Implications of the Neutron Star Merger GW170817 for Cosmological Scalar-Tensor Theories
• Generalized Galileons: All scalar models whose curved background extensions maintain second-order field equations and stress tensors
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Cited by in corpus (6)

• Testing modified gravity at cosmological distances with LISA standard sirens
• Dark Energy in light of Multi-Messenger Gravitational-Wave astronomy
• Neutron star merger GW170817 strongly constrains doubly coupled bigravity
• Measuring the modified gravitational waves propagation beyond general relativity from CMB observations
• Self-acceleration in scalar-bimetric theories
• Measuring the speed of scalar induced gravitational waves from observations