Cosmology of codimension-two braneworlds

We present a comprehensive study of the cosmological solutions of 6D braneworld models with azimuthal symmetry in the extra dimensions, moduli stabilization by flux or a bulk scalar field, and which contain at least one 3-brane that could be identified with our world. We emphasize an unusual property of these models: their expansion rate depends on the 3-brane tension either not at all, or in a nonstandard way, at odds with the naive expected dimensional reduction of these systems to 4D general relativity at low energies. Unlike other braneworld attempts to find a self-tuning solution to the cosmological constant problem, the apparent failure of decoupling in these models is not associated with the presence of unstabilized moduli; rather it is due to automatic cancellation of the brane tension by the curvature induced by the brane. This provides some corroboration for the hope that these models provide a distinctive step toward understanding the smallness of the observed cosmological constant. However, we point out some challenges for obtaining realistic cosmology within this framework.Comment: 30 pages, 4 figures; generalized result for nonconventional Friedmann equation, added referenc

A review of six-dimensional braneworld solutions /

In the last years, brane world scenarios have been studied extensively, but most of these studies have been done in the case of five-dimensional spacetime. It is therefore of interest to investigate which of the particular features observed are proper to one extra dimension and which are generic to any number of dimensions. In this thesis, I present an overview of models and solutions to Einstein's equations for six-dimensional brane world scenarios. Solutions for a simple setup with cylindrically symmetric bulk centered about a three-brane are derived and classified. There are two main kinds of topology: either solutions are compactified in a spherical topology, closed up by another three-brane, or they have a disc topology, which must be terminated by a four-brane. One of the particular features of codimension-two branes is demonstrated, namely that their tension, or vacuum energy, induces a deficit angle in the bulk. Solutions for different arrangements of codimension-one and codimension-two branes are also reviewed. Although the review focuses on topological and cosmological properties of the solutions, models using a field theoretical approach to the brane-world scenario, i.e. considering the brane as a topological defect arising from higher dimensional fields, are also considered