Brane gases in the early Universe

Abstract

Over the past decade it has become clear that fundamental strings are not the only fundamental degrees of freedom in string theory. D-branes are also part of the spectrum of fundamental states. In this paper we explore some possible effects of D-branes on early Universe string cosmology, starting with two key assumptions: firstly that the initial state of the Universe corresponded to a dense, hot gas in which all degrees of freedom were in thermal equilibrium, and secondly that the topology of the background space admits one-cycles. We argue by t-duality that in this context the cosmological singularities are not present. We derive the equation of state of the brane gases and apply the results to suggest that, in an expanding background, the winding modes of fundamental strings will play the most important role at late times. In particular, we argue that the string winding modes will only allow four space-time dimensions to become large. The presence of brane winding modes with p > 1 may lead to a hierarchy in the sizes of the extra dimensions.

Stabilization of extra dimensions at tree level

“…From (16) we find that the zero mode energy and pressure 1 of the string gas in D − 1 compact dimensions can be written as, E = 3µN (3) e λ + 3µM (3) e −λ + 6µN (6) e ν + 6µM (6) e −ν ,…”

“…A possible resolution to this problem was suggested in [1] (see also [2]) and has since been generalized to include more realistic background geometries and the inclusion of branes [3,5,6]. The authors of [1] argued that, by considering the dynamics of the winding and momentum modes of the strings, a new symmetry specific of string theory, namely t-duality, could eliminate the big-bang singularity and also explain the dimensionality of space-time.…”

Isotropization in brane gas cosmology

“…The most important result to emerge from these works is a dynamical mechanism, tied to the existence of string winding modes, which yields a nonsingular cosmology and may explain why at most three spatial dimensions can become large if the initial state is chosen to correspond to a Universe which is small in all spatial directions. This work has been generalized to include the cosmological effects of p-brane gases and leads to the current model of Brane Gas Cosmology (BGC) [21]. In BGC, the Universe starts analogous to the SBB picture, i.e.…”

Moduli entrapment with primordial black holes

“…To leading order, the effects of the interactions of the modulus with the black hole gas ρ BH can be described as an environment-induced mass term, similar to the interactions discussed in [25], and more recently to the chameleon fields [38] (see also [39]). The presence of this new mass term ∝ ρ BH suggests a new method for capturing the scalar in a potential minimum.…”