Dissipation and nonlocality in a general expanding braneworld universe

TL;DR

This paper investigates how scalar and tensor perturbations evolve in an expanding braneworld universe, revealing dissipation and nonlocal effects caused by interactions with bulk gravitons, with implications for cosmological observations.

Contribution

It introduces a framework to analyze dissipation and nonlocality in a general expanding braneworld using a four-dimensional perspective coupled to bulk gravitons.

Findings

Dissipation rates vary across cosmological epochs.

Bulk gravitons can be reflected and reabsorbed, causing nonlocal effects.

Near-brane approximation captures purely nonlocal bulk effects.

Abstract

We study the evolution of both scalar and tensor cosmological perturbations in a Randall-Sundrum braneworld having an arbitrary expansion history. We adopt a four dimensional point of view where the degrees of freedom on the brane constitute an open quantum system coupled to an environment composed of the bulk gravitons. Due to the expansion of the universe, the brane degrees of freedom and the bulk degrees of freedom interact as they propagate forward in time. Brane excitations may decay through the emission of bulk gravitons which may escape to future infinity, leading to a sort of dissipation from the four dimensional point of view of an observer on the brane. Bulk gravitons may also be reflected off of the curved bulk and reabsorbed by the brane, thereby transformed into quanta on the brane, leading to a sort of nonlocality from the four dimensional point of view. The dissipation and the nonlocality are encoded into the retarded bulk propagator. We estimate the dissipation rates of the bound state as well as of the matter degrees of freedom at different cosmological epochs and for different sources of matter on the brane. We use a near-brane limit of the bulk geometry for the study when purely nonlocal bulk effects are encountered.