Comment on 4D Lorentz invariance violations in the brane-world

TL;DR

This paper investigates how signals traveling through extra dimensions in brane-world models can arrive earlier than light in our universe, affecting causality and the horizon problem, but their impact is limited by gravitational redshift.

Contribution

It derives conditions for signals to return to the brane and analyzes their propagation, revealing potential violations of 4D Lorentz invariance in brane-world scenarios.

Findings

Signals can arrive earlier than light in 4D perspective

Bulk signals can homogenize the universe but are redshifted

Redshift limits the impact of bulk signals on the horizon problem

Abstract

The brane-world scenario offers the possibility for signals to travel outside our visible universe and reenter it. We find the condition for a signal emitted from the brane to return to the brane. We study the propagation of such signals and show that, as seen by a 4D observer, these signals arrive earlier than light traveling along the brane. We also study the horizon problem and find that, while the bulk signals can travel far enough to homogenize the visible universe, it is unlikely that they have a significant effect since they are redshifted in the gravitational field of the bulk black hole.