Brane World Cosmology - Gauge-Invariant Formalism for Perturbation

TL;DR

This paper develops a gauge-invariant formalism for analyzing perturbations in brane-world cosmology, deriving equations that connect bulk and brane perturbations, and highlighting challenges in suppressing certain perturbations for realistic models.

Contribution

It introduces a gauge-invariant framework for perturbations in higher-dimensional brane-world models, including relations between bulk and brane variables and a master equation for perturbations.

Findings

Bulk perturbations reduce to a single hyperbolic master equation.

Boundary conditions link brane stress perturbations to the master variable.

Suppressing massive brane excitations is challenging in realistic models.

Abstract

In the present paper the gauge-invariant formalism is developed for perturbations of the brane-world model in which our universe is realized as a boundary of a higher-dimensional spacetime. For the background model in which the bulk spacetime is $(n+m)$-dimensional and has the spatial symmetry corresponding to the isometry group of a $n$-dimensional maximally symmetric space, gauge-invariant equations are derived for perturbations of the bulk spacetime. Further for the case corresponding to the brane-world model in which $m=2$ and the brane is a boundary invariant under the spatial symmetry in the unperturbed background, relations between the gauge-invariant variables describing the bulk perturbations and those for brane perturbations are derived from Israel's junction condition under the assumption of $\ZR_2$ symmetry. In particular, for the case in which the bulk spacetime is a constant-curvature spacetime, it is shown that the bulk perturbation equations reduce to a single hyperbolic master equation for a master variable, and that the physical condition on the gauge-invariant variable describing the intrinsic stress perturbation of the brane yield a boundary condition for the master equation through the junction condition. On the basis of this formalism it is pointed out that it seems to be difficult to suppress brane perturbations corresponding to massive excitations for a brane motion giving a realistic expanding universe model.