A Braneworld Universe From Colliding Bubbles

TL;DR

This paper proposes a natural mechanism for the formation of a homogeneous and isotropic braneworld universe through bubble collisions in a higher-dimensional space, addressing initial condition issues in braneworld cosmology.

Contribution

It introduces a bubble collision model in a 5D spacetime that naturally produces a homogeneous and isotropic brane universe, linking bubble dynamics to cosmological initial conditions.

Findings

A collision of true vacuum bubbles creates a brane with symmetric properties.

The model guarantees homogeneity and isotropy of the universe on the brane.

Quantum corrections can lead to observable cosmological perturbations.

Abstract

Much work has been devoted to the phenomenology and cosmology of the so-called braneworld universe, where our (3+1)-dimensional universe lies on a brane surrounded by a (4+1)-dimensional bulk spacetime that is essentially empty except for a negative cosmological constant and the various modes associated with gravity. For such a braneworld cosmology, the difficulty of justifying some preferred initial conditions inevitably arises. The various proposals for inflation restricted to the brane only partially explain the homogeneity and isotropy of the resulting braneworld universe because the homogeneity and isotropy of the bulk must be assumed. We propose a mechanism by which a brane surrounded by AdS space arises naturally so that the homogeneity and isotropy of both the brane and the bulk are guaranteed. We postulate an initial false vacuum phase of (4+1)-dimensional Minkowski or de Sitter space subsequently decaying to a true vacuum of anti-de Sitter space, assumed discretely degenerate. This decay takes place through bubble nucleation. When two bubbles of the true AdS vacuum collide, a brane (or domain wall) inevitably forms between the two AdS phases. We live on this brane. The SO(3,1) symmetry of the collision geometry ensures the three-dimensional spatial homogeneity and isotropy of the universe on the brane as well as of the bulk. In the semi-classical limit, this symmetry is exact. We sketch how the leading quantum corrections translate into cosmological perturbations.