When do colliding bubbles produce an expanding universe?

TL;DR

This paper investigates the conditions under which collisions of branes in higher-dimensional spacetime can produce an expanding universe, analyzing symmetric and asymmetric collisions through analytical and numerical methods.

Contribution

It provides a detailed analysis of brane collision scenarios, identifying key conditions for universe expansion, and confirms findings with numerical simulations of thick-wall brane collisions.

Findings

Symmetric brane collisions often lead to a collapsing universe unless pre-collision expansion is large.

Pre-existing expansion from negative curvature or positive cosmological constant favors expansion after collision.

Numerical results support the analytical thin-wall approximation conclusions.

Abstract

It is intriguing to consider the possibility that the Big Bang of the standard (3+1) dimensional cosmology originated from the collision of two branes within a higher dimensional spacetime, leading to the production of a large amount of entropy. In this paper we study, subject to certain well-defined assumptions, under what conditions such a collision leads to an expanding universe. We assume the absence of novel physics, so that ordinary (4+1) -dimensional Einstein gravity remains a valid approximation. It is necessary that the fifth dimension not become degenerate at the moment of collision. First the case of a symmetric collision of infinitely thin branes having a hyperbolic or flat spatial geometry is considered. We find that a symmetric collision results in a collapsing universe on the final brane unless the pre-existing expansion rate in the bulk just prior to the collision is sufficiently large in comparison to the momentum transfer in the fifth dimension. Such prior expansion may either result from negative spatial curvature or from a positive five-dimensional cosmological constant. The relevance of these findings to the Colliding Bubble Braneworld Universe scenario is discussed. Finally, results from a numerical study of colliding thick-wall branes is presented, which confirm the results of the thin-wall approximation.