Massive cosmological scalar perturbations

TL;DR

This paper analyzes cosmological scalar perturbations in a bi-metric gravity model, revealing that adiabatic perturbations are not constant at super horizon scales, which challenges its use in inflationary descriptions.

Contribution

It provides the first detailed study of scalar perturbations in Hassan-Rosen bi-metric gravity, highlighting the behavior of entropy and adiabatic modes.

Findings

Adiabatic perturbations are not constant at super horizon scales.

Entropy perturbations can be generated from adiabatic modes.

The model cannot describe the inflationary epoch effectively.

Abstract

We study the cosmological perturbations of the new bi-metric gravity proposed by Hassan and Rosen \cite{hasan} as a representation of massive gravity. The mass term in the model, in addition of 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 super horizon 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.