Acausality of Massive Gravity

TL;DR

This paper demonstrates that the ghost-free massive gravity model, despite removing the Boulware-Deser ghost, exhibits superluminal shock wave solutions, indicating acausality in the theory.

Contribution

It reveals that the well-known ghost-free massive gravity model is acausal due to superluminal solutions, challenging its physical consistency.

Findings

Presence of superluminal shock wave solutions

Acausality arises from the constraint removing the ghost

Challenges the physical viability of the model

Abstract

We show, by analyzing its characteristics, that the ghost-free, 5 degree of freedom, Wess--Zumino massive gravity model admits superluminal shock wave solutions and thus is acausal. Ironically, this pathology arises from the very constraint that removes the (sixth) Boulware-Deser ghost mode.