Ghosts, Strong Coupling and Accidental Symmetries in Massive Gravity

TL;DR

This paper investigates the strong coupling phenomena in massive gravity, linking it to a ghost degree of freedom, and explains the Vainshtein mechanism through ghost exchange, contrasting it with other models like DGP.

Contribution

It provides a novel interpretation of the strong self-interaction in massive gravity as related to a ghost degree of freedom and clarifies the Vainshtein mechanism's underlying physics.

Findings

Strong coupling linked to a ghost degree of freedom.

Vainshtein mechanism explained via ghost exchange.

Differences between massive gravity and DGP model in recovery process.

Abstract

We show that the strong self-interaction of the scalar polarization of a massive graviton can be understood in terms of the propagation of an extra ghost-like degree of freedom, thus relating strong coupling to the sixth degree of freedom discussed by Boulware and Deser in their Hamiltonian analysis of massive gravity. This enables one to understand the Vainshtein recovery of solutions of massless gravity as being due to the effect of the exchange of this ghost which gets frozen at distances larger than the Vainshtein radius. Inside this region, we can trust the two-field Lagrangian perturbatively, while at larger distances one can use the higher derivative formulation. We also compare massive gravity with other models, namely deconstructed theories of gravity, as well as DGP model. In the latter case we argue that the Vainshtein recovery process is of different nature, not involving a ghost degree of freedom.