Appearance of Boulware-Deser ghost in bigravity with doubly coupled matter

TL;DR

This paper investigates the stability of ghost-free bigravity theories when matter is coupled to both metrics, revealing that the Boulware-Deser ghost generally reappears, thus constraining models with doubly coupled matter.

Contribution

It demonstrates that ghost freeness in bigravity is compromised by doubly coupled matter, highlighting restrictions on such models and implications for their low-energy effective theories.

Findings

Boulware-Deser ghost reappears with doubly coupled matter

Ghost-free conditions impose strong restrictions on matter coupling models

Potential difficulties in deriving ghost-free bigravity from high-energy theories

Abstract

We discuss the ghost freeness in the case when we add matter coupled to two metrics to the ghost-free bigravity. In this paper we show that the Boulware-Deser ghost generally revives in the presence of doubly coupled matter and that ghost freeness strongly restricts the model of kinetically doubly coupled matter. This result may anticipate difficulties in the attempt to derive the ghost-free bigravity as a low-energy effective theory, starting with a model applicable at high energies.