The Fate of the Radion in Models with Metastable Graviton

TL;DR

This work analyzes gravity localization in brane-world models with metastable gravitons, focusing on the radion associated with the negative-tension brane. It derives the 4D effective action for the radion and gravitons, showing that a ghost-like radion (negative kinetic term) is necessary to recover the correct Einstein gravity at intermediate scales, with brane bending equivalence explained via radion dynamics. In the GRS limit, the graviton zero mode is non-normalizable and Einstein gravity emerges from a KK tower plus radion, yet the radion's ghost implies potential instabilities at very large distances, signaling the need for stabilization or model modifications. The study highlights the delicate interplay between radion dynamics, energy conditions, and gravity localization in brane-world scenarios.

Abstract

We clarify some general issues in models where gravity is localized at intermediate distances. We introduce the radion mode, which is usually neglected, and we point out that its role in the model is crucial. We show that the brane bending effects discussed in the literature can be obtained in a formalism where the physical origin is manifest. The model violates positivity of energy due to a negative tension brane, which induces a negative kinetic term for the radion. The very same effect that violates positivity is responsible for the recovery of conventional Einstein gravity at intermediate distances.