Brane New World

TL;DR

This paper investigates a brane-world cosmological model with a strongly coupled conformal field theory, revealing that CFT effects significantly suppress tensor perturbations on small scales, contrasting with previous predictions.

Contribution

It provides a quantum mechanical treatment of matter effects in brane cosmology using AdS/CFT, showing suppression of tensor perturbations due to conformal anomalies.

Findings

CFT strongly suppresses metric perturbations on small scales

The model yields a de Sitter geometry driven by conformal anomaly

Tensor perturbations are smaller than previous estimates neglecting matter effects

Abstract

We study a Randall-Sundrum cosmological scenario consisting of a domain wall in anti-de Sitter space with a strongly coupled large $N$ conformal field theory living on the wall. The AdS/CFT correspondence allows a fully quantum mechanical treatment of this CFT, in contrast with the usual treatment of matter fields in inflationary cosmology. The conformal anomaly of the CFT provides an effective tension which leads to a de Sitter geometry for the domain wall. This is the analogue of Starobinsky's four dimensional model of anomaly driven inflation. Studying this model in a Euclidean setting gives a natural choice of boundary conditions at the horizon. We calculate the graviton correlator using the Hartle-Hawking ``No Boundary'' proposal and analytically continue to Lorentzian signature. We find that the CFT strongly suppresses metric perturbations on all but the largest angular scales. This is true independently of how the de Sitter geometry arises, i.e., it is also true for four dimensional Einstein gravity. Since generic matter would be expected to behave like a CFT on small scales, our results suggest that tensor perturbations on small scales are far smaller than predicted by all previous calculations, which have neglected the effects of matter on tensor perturbations.