Graviton propagation in ghost-free massive gravity

TL;DR

This paper proves that in ghost-free massive gravity, the helicity-2 gravitational mode always propagates on the lightcone, regardless of background, which is crucial for observational tests of the theory.

Contribution

It establishes that the helicity-2 mode in ghost-free massive gravity always propagates on the metric lightcone in the high-frequency limit for any background.

Findings

Helicity-2 mode always propagates on the metric lightcone.

Helicity-1 and -0 modes have different characteristics on general backgrounds.

Propagation properties are relevant for observational tests.

Abstract

We consider the ghost-free dRGT massive gravity with two of its three possible mass terms. This theory has five gravitational degrees of freedom. On Minkowski spacetime these modes have helicity-2, -1 and -0 and propagate on the Minkowski lightcone in the high-frequency limit. However for a general background the degrees of freedom corresponding to the helicity-1 and -0 modes have characteristics different to that of the metric lightcone. Here we prove in all generality, that the two degrees of freedom corresponding to the helicity-2 mode always propagate on the metric lightcone for any background in the high-frequency limit, which has significant relevance for current and future observational tests of the theory.