Shortcuts in a Dynamical Universe and the Horizon Problem

TL;DR

This paper investigates how gravitational signals traveling through extra dimensions in a brane-world model could create shortcuts, potentially solving the horizon problem if such effects were significant before nucleosynthesis.

Contribution

It explicitly constructs geodesics in a brane-world scenario and shows that gravitational shortcuts could influence causal structure, offering a novel approach to the horizon problem.

Findings

Shortcuts exist but are negligible today.

Effects could be significant before nucleosynthesis with high redshifts.

Potential relevance for probing extra dimensions during inflation.

Abstract

We consider the dynamics of a FRW brane in a purely AdS background, from the point of view of the bulk, and explicitly construct the geodesical behaviour of gravitational signs leaving and subsequently returning to the brane. In comparison with photons following a geodesic inside the brane, we verify that shortcuts exist, though they are extremely small for today's Universe. However, we show that at times just before nucleosynthesis, if high redshifts were available, the effect could be sufficiently large to solve the horizon problem. Assuming an inflationary epoch in the brane evolution, we argue that the influence of those signs trought the extra dimension in the causal structure cannot be neglected. This effect could be relevant for probing the extra dimension in inflationary scenarios.