A general mass term for bigravity

Abstract

We introduce a new formalism to study perturbations of Hassan-Rosen bigravity\ntheory, around general backgrounds for the two dynamical metrics. In\nparticular, we derive the general expression for the mass term of the\nperturbations and we explicitly compute it for cosmological settings. We study\ntensor perturbations in a specific branch of bigravity using this formalism. We\nshow that the tensor sector is affected by a late-time instability, which sets\nin when the mass matrix is no longer positive definite.\n