Brane Induced Gravity: From a No-Go to a No-Ghost Theorem

TL;DR

This paper demonstrates that brane induced gravity theories are free of ghosts and unitarity violations, resolving previous concerns and supporting their viability for large-scale modifications of gravity.

Contribution

It provides a detailed proof that the ghost mode in brane induced gravity is non-dynamical, ensuring unitarity and stability of the theory.

Findings

The induced Einstein-Hilbert term does not threaten unitarity.

The ghost mode is non-dynamical and not part of the physical Hilbert space.

Brane induced gravity can be consistent and ghost-free at large scales.

Abstract

Numerous claims in the literature suggest that gravity induced on a higher co-dimensional surface violates unitarity in the weak coupling regime. However, it remained unclear, why a conserved source localized on this surface and giving rise to an induced gravity term at low energies would absorb and emit the associated ghost, given a consistent source-free theory. In this article it is shown that the appearance of the induced Einstein Hilbert term does not threaten the unitarity of the theory. The physics arguments behind this statement are presented in a semi-covariant language, but the detailed proof is given using Dirac's constraint analysis. It is shown that the would-be ghost highlighted in previous works is non-dynamical and therefore not associated with a state in the Hilbert space. As a result of these investigations, brane induced gravity goes without a ghost, opening an exciting window of opportunity for consistent deformations of gravity at the largest observable distances.