Shortcuts in Domain Walls and the Horizon Problem

TL;DR

This paper investigates how gravitons can take shortcuts through extra dimensions in a domain wall universe, potentially addressing the horizon problem by allowing faster causal contact than photons confined to the brane.

Contribution

It demonstrates the conditions under which gravitational shortcuts occur in a dynamical membrane universe with scalar bulk matter, highlighting their potential cosmological significance.

Findings

Shortcuts for gravitons are possible under mild conditions.

In some universes, shortcuts are small; in others, they are significant.

Shortcuts could help solve the horizon problem.

Abstract

We consider a dynamical membrane world in a space-time with scalar bulk matter described by domain walls. Using the solutions to Einstein field equations and Israel conditions we investigate the possibility of having shortcuts for gravitons leaving the wall and returning subsequently. As it turns out, they usually appear under mild conditions. In the comparison with photons following a geodesic inside the brane, we verify that shortcuts exist. For some Universes they are small, but there are cases where shortcuts are effective. In these cases we expect them to play a significant role in the solution of the horizon problem.