A general theory of linear cosmological perturbations: bimetric theories

TL;DR

This paper develops a comprehensive framework for analyzing linear cosmological perturbations in bimetric gravity theories, identifying the most general invariant action and parameter space, with a focus on non-derivative interactions and comparison to massive bigravity.

Contribution

It introduces a general parametrised action for linear perturbations in bimetric theories, extending previous work and clarifying the structure of these models.

Findings

Derived the most general diffeomorphism-invariant action for bimetric perturbations.

Identified the number of free parameters characterizing these theories.

Compared the results with existing massive bigravity models.

Abstract

We implement the method developed in [1] to construct the most general parametrised action for linear cosmological perturbations of bimetric theories of gravity. Specifically, we consider perturbations around a homogeneous and isotropic background, and identify the complete form of the action invariant under diffeomorphism transformations, as well as the number of free parameters characterising this cosmological class of theories. We discuss, in detail, the case without derivative interactions, and compare our results with those found in massive bigravity.