On massless 4D Gravitons from 5D Asymptotically AdS Space-times

TL;DR

This paper explores the conditions under which four-dimensional massless gravitons can emerge from five-dimensional asymptotically AdS spaces, emphasizing the importance of IR boundary conditions and space termination.

Contribution

It demonstrates that normalizable massless spin-2 modes require the space to end at an IR point with specific boundary conditions, leading to a pure graviton spectrum without additional light modes.

Findings

Massless spin-2 modes exist only with IR space termination.

These modes are localized in the interior of the 5D bulk.

The spectrum can be purely graviton without extra light fields.

Abstract

We investigate the conditions for obtaining four-dimensional massless spin-2 states in the spectrum of fluctuations around an asymptotically $AdS_5$ solution of Einstein-Dilaton gravity. We find it is only possible to have normalizable massless spin-2 modes if the space-time terminates at some IR point in the extra dimension, far from the UV AdS boundary, and if suitable boundary conditions are imposed at the ``end of space.'' In some of these cases the 4D spectrum consists only of a massless spin-2 graviton, with no additional massless or light scalar or vector modes. These spin-2 modes have a profile wave-function peaked in the interior of the 5D bulk space-time. Under the holographic duality, they may be sometimes interpreted as arising purely from the IR dynamics of a strongly coupled QFT living on the AdS boundary.