Massive Gravity in Minkowski Space via Gravitational Higgs Mechanism

TL;DR

This paper presents a model of massive gravity in Minkowski space achieved through a gravitational Higgs mechanism with all diffeomorphisms spontaneously broken, avoiding non-unitary modes by including higher derivative terms and tuning parameters.

Contribution

It introduces a novel setup where all diffeomorphisms are spontaneously broken, resulting in a unitary massive gravity in Minkowski space without non-unitary propagating modes.

Findings

Massive gravity in Minkowski space achieved with all diffeomorphisms broken.

Higher derivative terms enable decoupling of non-unitary modes.

Selective breaking of spatial diffeomorphisms yields only transverse propagating modes.

Abstract

In gravitational Higgs mechanism graviton components acquire mass in the process of spontaneous breaking of diffeomorphisms (via scalar vacuum expectation values). Recently, gravitational Higgs mechanism was discussed in the context of obtaining massive gravity directly in four dimensions. We discuss a setup, with all diffeomorphisms spontaneously broken by scalars, where gravitational Higgs mechanism results in massive gravity in Minkowski space with no non-unitary propagating modes. This is achieved by including higher derivative terms in the scalar sector. Decoupling the non-unitary mode (trace of the graviton) then requires tuning a single parameter, namely, the cosmological constant. We also discuss a setup where only spatial diffeomorphisms are broken. In this case we obtain massive gravity in Minkowski space with only transverse modes propagating, so the mode count is the same as for a massless graviton.