Cosmological perturbations in massive bigravity

TL;DR

This paper analyzes scalar, vector, and tensor cosmological perturbations in ghost-free massive bigravity, identifying conditions for stability and the evolution of perturbations across different regimes.

Contribution

It provides a comprehensive analysis of perturbation evolution in massive bigravity, including analytical solutions and stability conditions for specific parameter choices.

Findings

Viable cosmological backgrounds exist.

Linear perturbations generally exhibit exponential instabilities.

Stable scalar perturbations are possible with specific parameters.

Abstract

We present a comprehensive analysis of classical scalar, vector and tensor cosmological perturbations in ghost-free massive bigravity. In particular, we find the full evolution equations and analytical solutions in a wide range of regimes. We show that there are viable cosmological backgrounds but, as has been found in the literature, these models generally have exponential instabilities in linear perturbation theory. However, it is possible to find stable scalar cosmological perturbations for a very particular choice of parameters. For this stable subclass of models we find that vector and tensor perturbations have growing solutions. We argue that special initial conditions are needed for tensor modes in order to have a viable model.