A Comment on Brane Bending and Ghosts in Theories with Infinite Extra Dimensions

TL;DR

This paper critiques models with infinite extra dimensions, showing that brane bending mechanisms introduce ghosts that invalidate the models unless these ghosts are eliminated, leading to conflicts with General Relativity.

Contribution

It demonstrates that brane bending does not resolve ghost issues in infinite extra dimension models and highlights the need for new mechanisms.

Findings

Brane bending introduces ghosts that cancel unwanted graviton polarizations.

Eliminating ghosts makes 4D gravity inconsistent with General Relativity.

Ghosts are linked to violations of positive-energy conditions.

Abstract

Theories with infinite volume extra dimensions open exciting opportunities for particle physics. We argued recently that along with attractive features there are phenomenological difficulties in this class of models. In fact, there is no graviton zero-mode in this case and 4D gravity is obtained by means of continuum bulk modes. These modes have additional degrees of freedom which do not decouple at low energies and lead to inconsistent predictions for light bending and the precession of Mercury's perihelion. In a recent papers, [hep-th/0003020] and [hep-th/0003045] the authors made use of brane bending in order to cancel the unwanted physical polarization of gravitons. In this note we point out that this mechanism does not solve the problem since it uses a {\it ghost} which cancels the extra degrees of freedom. In order to have a consistent model the ghost should be eliminated. As soon as this is done, 4D gravity becomes unconventional and contradicts General Relativity. New mechanisms are needed to cure these models. We also comment on the possible decoupling of the ghost at large distances due to an apparent flat-5D nature of space-time and on the link between the presence of ghosts and the violation of positive-energy conditions.