# The singularity problem in brane cosmology

## TL;DR

This paper reviews the development and nature of singularities in brane-bulk systems within 5D Einstein equations, analyzing solutions with various brane geometries and discussing conditions for regularity and physical admissibility.

## Contribution

It characterizes all singular Minkowski brane solutions and explores conditions for regular solutions with nonzero curvature in brane cosmology.

## Key findings

- All singular Minkowski brane solutions are characterized.
- Conditions for regular solutions with nonzero curvature are identified.
- Connections to ambient theory are briefly discussed.

## Abstract

We review results about the development and asymptotic nature of singularities in `brane-bulk' systems. These arise for warped metrics obeying the 5-dimensional Einstein equations with fluid-like sources, and including a brane 4-metric that is either Minkowski or de Sitter or Anti-de Sitter. We characterize all singular Minkowski brane solutions, and look for regular solutions with nonzero curvature. We briefly comment on matching solutions, energy conditions and finite Planck mass criteria for admissibility, and we briefly discuss the connection of these results to ambient theory.