Superstring cosmology

Aspects of superstring cosmology are reviewed with an emphasis on the cosmo logical implications of duality symmetries in the theory. The string effect ive actions are summarized and toroidal compactification to four dimensions reviewed. Global symmetries that arise in the compactification are discuss ed and the duality relationships between the string effective actions are t hen highlighted. Higher-dimensional Kasner cosmologies are presented and in terpreted in both string and Einstein frames, and then given in dimensional ly reduced forms. String cosmologies containing both non-trivial Neveu-Schw arz/Neveu-Schwarz and Ramond-Ramond fields are derived by employing the glo bal symmetries of the effective actions. Anisotropic and inhomogeneous cosm ologies in four dimensions are also developed. The review concludes with a detailed analysis of the pre-big bang inflationary scenario. The generation of primordial spectra of cosmological perturbations in such a scenario is discussed. Possible future directions offered in the Horava-Witten theory a re outlined. (C) 2000 Elsevier Science B.V. All rights reserved.