Brane motion in a compact space: adiabatic perturbations of brane-bulk coupled fluids

TL;DR

This paper investigates how adiabatic perturbations in a moving brane within a compact space propagate when coupled to bulk fluids, revealing an effective sound speed that can be superluminal under certain conditions, impacting brane-world cosmology.

Contribution

It introduces a model for perturbation propagation on a moving brane coupled to bulk fluids, deriving an effective sound speed influenced by energy densities and equations of state.

Findings

Effective sound speed depends on brane and bulk fluid properties.

Superluminal propagation possible under strong energy condition.

Implications for brane-world cosmology models.

Abstract

When a brane is moving in a compact space, bulk-probing signals originating at the brane can arrive back at the brane outside the lightcone of the emitting event. In this letter, we study how adiabatic perturbations in the brane fluid, coupled to a bulk fluid, propagate in the moving brane. In the non-dissipative regime, we find an effective sound speed for such perturbations, depending on the brane and bulk fluid energy densities, equations of state, and brane speed. In the tight-coupling approximation, the effective sound speed might be superluminal for brane and bulk fluids that satisfy the strong energy condition. This has immediate consequences for brane-world cosmology models.