(1+3) Covariant Dynamics of Scalar Perturbations in Braneworlds

TL;DR

This paper develops a covariant framework for analyzing scalar perturbations in Randall-Sundrum braneworld cosmology, revealing new modes and providing solutions relevant for CMB studies.

Contribution

It introduces a complete set of frame-independent equations for scalar perturbations in braneworlds, including new non-local degrees of freedom and their implications.

Findings

Identifies new modes from non-local degrees of freedom.

Provides solutions in high and low-energy regimes.

Derives covariant line of sight equations for CMB anisotropies.

Abstract

We discuss the dynamics of linear, scalar perturbations in an almost Friedmann-Robertson-Walker braneworld cosmology of Randall-Sundrum type II using the 1+3 covariant approach. We derive a complete set of frame-independent equations for the total matter variables, and a partial set of equations for the non-local variables which arise from the projection of the Weyl tensor in the bulk. The latter equations are incomplete since there is no propagation equation for the non-local anisotropic stress. We supplement the equations for the total matter variables with equations for the independent constituents in a cold dark matter cosmology, and provide solutions in the high and low-energy radiation-dominated phase under the assumption that the non-local anisotropic stress vanishes. These solutions reveal the existence of new modes arising from the two additional non-local degrees of freedom. Our solutions should prove useful in setting up initial conditions for numerical codes aimed at exploring the effect of braneworld corrections on the cosmic microwave background (CMB) power spectrum. As a first step in this direction, we derive the covariant form of the line of sight solution for the CMB temperature anisotropies in braneworld cosmologies, and discuss possible mechanisms by which braneworld effects may remain in the low-energy universe.