Coupling between Galileon and massive gravity with composite metrics

TL;DR

This paper explores how a Galileon scalar field interacts with massive gravity via composite metrics, deriving equations of motion and analyzing perturbations, highlighting potential ghost instabilities.

Contribution

It provides a detailed derivation of equations of motion for coupled Galileon-massive gravity models and examines the stability of linear perturbations.

Findings

Nonminimal coupling excites all 6 degrees of freedom in perturbations.

Potential Boulware-Deser ghost mode identified in the perturbation spectrum.

Analysis informs stability considerations for such coupled theories.

Abstract

We investigate the coupling between a Galileon scalar field and massive gravity through composite metrics. We derive the full set of equations of motion for a flat Friedmann-Robertson-Walker background, and study linear perturbations around it. Generally, the nonminimal coupling with the composite metric will excite all 6 degrees of freedom of the spatial metric perturbations, one of which may correspond to the Boulware-Deser ghost.