String gas cosmology

A critical review and summary of string gas cosmology is presented. A pedagogical derivation of the effective action starting from string theory, emphasizing the necessary approximations that must be invoked, is included. Working in the effective theory, that at late times it is not possible to stabilize the extra dimensions by a gas of massive string winding modes is demonstrated. Additional string gases are considered that contain so-called enhanced symmetry states. These string gases are very heavy initially, but drive the moduli to locations that minimize the energy and pressure of the gas. Both classical and quantum gas dynamics are considered, where in the former the validity of the theory is questionable and some fine-tuning is required, but in the latter a consistent and promising stabilization mechanism that is valid at late times is found. In addition, string gases provide a framework to explore dark matter, presenting alternatives to the cold dark matter model recently considered by Gubser and Peebles. Also quantum trapping with string gases as a method for including dynamics on the string landscape is discussed.