Local conservation law and dark radiation in cosmological braneworld

TL;DR

This paper explores the bulk geometry and dynamics in the Randall-Sundrum braneworld, introducing a local mass concept, analyzing the Weyl charge, and studying black hole formation and brane behavior in the bulk.

Contribution

It presents a novel formulation of local conservation laws, defines the local mass and Weyl charge, and applies these to analyze black hole formation and brane dynamics in the RS scenario.

Findings

Defined a local mass in the bulk related to dark radiation.

Decomposed the Weyl tensor into local mass and bulk energy-momentum.

Showed the brane remains outside the black hole during expansion.

Abstract

In the context of the Randall-Sundrum (RS) single-brane scenario, we discuss the bulk geometry and dynamics of a cosmological brane in terms of the local energy conservation law which exists for the bulk that allows slicing with a maximally symmetric 3-space. This conservation law enables us to define a local mass in the bulk. We show that there is a unique generalization of the dark radiation on the brane, which is given by the local mass. We find there also exists a conserved current associated with the Weyl tensor, and the corresponding local charge, which we call the Weyl charge, is given by the sum of the local mass and a certain linear combination of the components of the bulk energy-momentum tensor. This expression of the Weyl charge relates the local mass with the projected Weyl tensor, $E_{\mu\nu}$, which plays a central role in the geometrical formalism of the RS braneworld. On the brane, in particular, this gives a decomposition of the projected Weyl tensor into the local mass and the bulk energy-momentum tensor. Then, as an application of these results, we consider a null dust model for the bulk energy-momentum tensor and discuss the black hole formation in the bulk. We investigate the causal structure by identifying the locus of the apparent horizon and clarify possible brane trajectories in the bulk. We find that the brane stays always outside the black hole as long as it is expanding. We also find an upper bound on the value of the Hubble parameter in terms of the matter energy density on the brane, irrespective of the energy flux emitted from the brane.