Here is something that may be interesting to look into.
String Gas Cosmology (arXiv:0808.0746)
An interesting review in String Gas cosmology by Brandenberger. Any attempt to incorporate stringy idea into cosmological model is interesting. The starting point is always crucial.
First of all, as the review points out there are several problems that is associated with string cosmology. One needs to consider some form of scalar field and in general in particle physics we consider Higgs field which is responsible for spontaneous symmetry breaking. However, Higgs is not appropriate in the case of cosmology (inflationary models) as it is too massive (possible somewhere between (115-150 Gev). In order to really understand string cosmology completely, we need a complete perturbative description of string theory. In the absence of these, the ideas that Brandenberger and company uses is the idea of T-duality and stringy winding modes in order to understand the string cosmology. So what Brandenberger and company do is to couple a classical background such as graviton and the other dilaton field to gas of strings. This may be extended to other basic degrees of freedom in string theory such as branes. In this way Brandenberger et al., claims that they are using the key feature of string theory into the cosmology. As we know string has three kinds of states, momentum modes (center of mass mode), oscillatory mode (fluctuation of the strings) and winding mode (counting the number of times that the string). The interesting aspect is that the background of string cosmology is non singular as it never exceeds the Hagedron temperature. I dont have much illuminating things to say about this rather than quoting Brandenberger.
Claim(not verbatim)
If we start evolution as a dense gas of strings in a space in which all dimensions are string scale tori the gas of strings in which all dimensions are string scale tori then there are dynamical arguments according to which only three of the spatial dimensions can become large.So string gas cosmology can shed light to why we just see three extra dimensions and not the compactified dimension.
I am not entirely convinced by this argument although I concede that I havent paid attention to the dynamics. Brandenberger also hopes that string gas can explain the orgin of the scale invariant spectrum of cosmological fluctuations. A signature of the SG scenario is the slight blue tilt in the spectrum of gravitational waves which is predicted by the models. More string gas cosmology later…
