Radion and Large Scale Anisotropy on the Brane

TL;DR

This paper explores how the radion, a brane displacement mode, influences cosmological perturbations and CMB anisotropies in a brane world model, revealing its potential observational signatures.

Contribution

It identifies two types of radion perturbations and demonstrates their effects on large-scale metric perturbations and CMB anisotropies in a 5D brane setup.

Findings

Radion couples to matter via brane bending, affecting curvature perturbations.

Radion contributes to large-scale CMB anisotropies.

The study provides a framework for observationally testing brane world models.

Abstract

We investigate the effect of the radion on cosmological perturbations in the brane world. The S^1/Z_2 compactified 5D Anti-de Sitter spacetime bounded by positive and negative tension branes is considered. The radion is the relative displacement of the branes in this model. We find two different kinds of the radion at the linear perturbation order for a cosmological brane. One describes a "fluctuation" of the brane which does not couple to matter on the brane. The other describes a "bend" of the brane which couples to the matter. The bend determines the curvature perturbation on the brane. At large scales, the radion interacts with anisotopic perturbations in the bulk. By solving the bulk anisotropic perturbations, large scale metric perturbations and anisotropies of the Cosmic Microwave Background (CMB) on the positive tension brane are calculated. We find an interesting fact that the radion contributes to the CMB anisotropies. The observational consequences of these effects are discussed.