Giulia Cusin (Département de Physique Théorique and Center for Astroparticle Physics, University of Geneva, Switzerland)
We introduce a new formalism to study perturbations of Hassan-Rosen bigravity theory, around general backgrounds for the two dynamical metrics. In particular, we derive the general expression for the mass term and we explicitly compute it for some cosmological settings. We study in detail tensor perturbations in branch-one bigravity using this formalism. We show that the tensor sector is affected by a late-time instability, which sets-in when the mass matrix becomes not positive definite.