Massive cosmological scalar perturbations

We study the cosmological perturbations of the new bimetric gravity proposed by Hassan and Rosen [J. High Energy Phys. 02 (2012) 126] as a representation of massive gravity. The mass term in the model, in addition to ensuring ghost freedom for both metrics, causes the two scale factors to mix at the cosmological level, and this affects the cosmological perturbation of the model. We find two combinations corresponding to the entropy and adiabatic perturbations of the theory. In this sense we show that the adiabatic perturbations could be a source for the entropy perturbations. So in addition to the adiabatic perturbations, entropy perturbations can also be present in this theory. We also show that the adiabatic perturbations are not constant at the superhorizon scales, implying that the theory could not be used to describe the inflationary epoch, even if it can impose some corrections to the standard inflationary scenarios.