Gravity in Randall-Sundrum Brane World Revisited

TL;DR

This paper examines the subtleties in gauge fixing when computing the graviton propagator in the Randall-Sundrum brane world, highlighting issues with the graviscalar component and implications for quantum coupling and higher curvature effects.

Contribution

It identifies gauge fixing subtleties affecting the graviton propagator and shows that the graviscalar cannot be gauged away in the ultra-violet, impacting quantum coupling and fine-tuning.

Findings

The graviton propagator diverges near the brane in Gaussian normal coordinates.

Allowed gauge transformations do not eliminate the graviscalar coupling at high energies.

Higher curvature terms in the bulk tend to delocalize gravity in warped backgrounds.

Abstract

We point out some subtleties with gauge fixings (which sometimes include the so-called ``brane bending'' effects) typically used to compute the graviton propagator on the Randall-Sundrum brane. In particular, the brane, which has non-vanishing tension, explicitly breaks some part of the diffeomorphisms, so that there are subtleties arising in going to, say, the axial gauge or the harmonic gauge in the presence of (non-conformal) matter localized on the brane. We therefore compute the graviton propagator in the gauge where only the graviphoton fluctuations are set to zero (the diffeomorphisms necessary for this gauge fixing are intact), but the graviscalar component is untouched. We point out that in the Gaussian normal coordinates (where the graviscalar component vanishes on the brane) the graviton propagator blows up in the ultra-violet near the brane. In fact, the allowed gauge transformations, which do not lead to such ultra-violet behavior of the graviton propagator, are such that the coupling of the graviscalar to the brane matter cannot be gauged away in the ultra-violet. Because of this, at the quantum level, where we expect various additional terms to be generated in the brane world-volume action including those involving the graviscalar, fine-tuning (which is independent of that for the brane cosmological constant) is generically required to preserve consistent coupling between bulk gravity and brane matter. We also reiterate that in such warped backgrounds higher curvature terms in the bulk are generically expected to delocalize gravity.