Superstring Cosmology -- A Complementary Review

In this review, a number of approaches to superstring cosmology which make use of key features which distinguish string theory from point particle theories are discussed, with particular emphasis on emergent scenarios. One motivation for the discussion is the realization that, in order to describe the evolution of the very early universe, it is necessary to go beyond a conventional effective field theory (EFT) analysis. Some of the conceptual problems of an EFT analysis will be discussed. The review begins with a summary of the criteria for a successful early universe scenario, emphasizing that cosmic inflation is not the only scenario of early universe cosmology which is consistent with current cosmological observations. Bouncing and emergent scenarios as interesting alternatives are introduced. Some realizations of these scenarios from superstring theory are reviewed, e.g. String Gas Cosmology, the Pre-Big-Bang scenario, the Ekpyrotic model, Double Field Theory cosmology and matrix model cosmology. In light of the difficulties in obtaining cosmic inflation from string theory (at the level of EFT), and realizing that there are promising examples of alternative early universe scenarios which are derived from basic principles of superstring theory, one must entertain the possibility that the cosmology emerging from string theory will not involve an extended period of accelerated expansion.