On scale-free extensions of massive (bi-)gravity

TL;DR

This paper introduces a scale-free bigravity model where the mass parameter is a dynamical scalar, maintaining ghost-freedom and exploring its cosmological implications including late-time acceleration and inflation.

Contribution

It presents a novel scale-free bigravity framework with a dynamical mass scalar, preserving ghost-freedom and analyzing its cosmological and strong coupling properties.

Findings

The model remains ghost-free with a dynamical mass scalar.

It supports cosmological solutions including late-time acceleration.

The scalar can act as an inflaton during early universe inflation.

Abstract

We discuss a scale-free model of bigravity, in which the mass parameter of the standard bigravity potential is promoted to a dynamical scalar field. This modification retains the ghost-free bigravity structure, in particular it remains free of the Boulware-Deser ghost. We investigate the theory's interaction structure, focusing on its consistent scaling limits and strong coupling scales. Furthermore we explore the model's quadratic action, both around generic background configurations and paying special attention to cosmological backgrounds and to the associated background evolution. Finally we consider the possibility of realizing a phase of late-time acceleration as well as a quasi-de Sitter inflationary stage at early times, when the promoted "mass scalar" becomes the inflaton.