The Global Structure of the Colliding Bubble Braneworld Universe

Abstract

The one-brane Randall-Sundrum model offers an example of a model with an `infinite' extra dimension in which ordinary gravity is recovered at large distances and the usual (3+1)-dimensional cosmology at late cosmic times. This is possible because the `bulk' has the geometry of anti de Sitter space, the curvature length ℓ of which delineates the (3+1)-dimensional behavior at large distances from the (4+1)-dimensional behavior at short distances. This spacetime, however, possesses a past Cauchy horizon on which initial data must be specified in a natural and convincing way. A more complete story is required that singles out some set of initial conditions to resolve the `bulk' smoothness and horizon problems. One such complete story is offered by the colliding bubble braneworld universe, where bubbles filled with AdS ⁵ nucleate from dS ⁵ or M ⁵ through quantum tunnelling. A pair of such colliding bubbles forms a Randall-Sundrum-like universe in the future of the collision. Because of the symmetry of bubbles produced through quantum tunnelling, the resulting universe is spatially homogeneous and isotropic at leading order, and the perturbations at the next order are completely well defined and calculable. In this contribution we discuss the possible global structure of such a spacetime.