# Extending applicability of bimetric theory: chameleon bigravity

## TL;DR

This paper introduces an environment-dependent extension of bimetric massive gravity that maintains stability, avoids the Higuchi bound, and remains phenomenologically viable across cosmic history, offering a partial UV completion.

## Contribution

It proposes a minimal, environment-dependent extension of bimetric gravity that improves theoretical consistency and observational compatibility without relying on the Vainshtein mechanism.

## Key findings

- Model is stable on cosmological scales
- Circumvents the Higuchi bound
- Maintains a high strong coupling scale

## Abstract

This article extends bimetric formulations of massive gravity to make the mass of the graviton to depend on its environment. This minimal extension offers a novel way to reconcile massive gravity with local tests of general relativity without invoking the Vainshtein mechanism. On cosmological scales, it is argued that the model is stable and that it circumvents the Higuchi bound, hence relaxing the constraints on the parameter space. Moreover, with this extension the strong coupling scale is also environmentally dependent in such a way that it is kept sufficiently higher than the expansion rate all the way up to the very early universe, while the present graviton mass is low enough to be phenomenologically interesting. In this sense the extended bigravity theory serves as a partial UV completion of the standard bigravity theory. This extension is very generic and robust and a simple specific example is described.