Isotropization in Brane Gas Cosmology

The Standard Big Bang (SBB) cosmology has become an extremely successful model that has been well tested by experiment. However, the model is incomplete. The underlying theory of classical general relativity and the description of matter as an ideal gas breaks down at the high temperatures of the early Universe, and the solutions of the theory in fact have an initial singularity. Moreover, SBB does not address many important cosmological questions such as the observed homogeneity, spatial flatness, and the origin of structure in the Universe. Cosmological inflation (see e.g. [1,2] for textbook treatises and [3,4] for shorter reviews) builds on SBB cosmology providing a solution to some of these issues, but it (at least in the context of scalar field-driven inflation) suffers from the same initial singularity problem [5] and other conceptual problems [4], which indicate that inflation cannot be the complete story of early Universe cosmology. In recent years, many models motivated by string theory and M-theory have emerged as possible solutions to the outstanding problems of early Universe and inflationary cosmology (see e.g. [6,7] for recent but incomplete reviews). Beginning with the work on Pre-Big-Bang cosmology [8,9] it was realized that a dynamical dilaton should play an important role in the very early Universe. More recently, models have become prominent in which our Universe consists of a 3-brane embedded in a higher dimensional bulk space, with the standard model constrained to live on the brane [10–15]. Although these models can resolve a number of issues, such as the hierarchy problem, they introduce several other difficulties in the process. For example, large extra dimensions should be explained by classical general relativity, and it has been shown this results in problems stabilizing the brane [16]. More importantly, in most of these models the six “extra” spatial dimensions are taken to be compactified, a priori, with no explanation provided for how this