Issues in String Cosmology

Liouville string theory is a natural framework for discussing the nonequilibrium evolution of the Universe. It enables non-critical strings to be treated in mathematically consistent manner, in which target time is identified with a world-sheet renormalization-group scale parameter, preserving target-space general coordinate invariance and the existence of an S-matrix. We review our proposals for a unified treatment of inflation and the current acceleration of the Universe. We link the current acceleration of the Universe with the value of the string coupling. In such a scenario, the dilaton plays an essential background rôle, driving the acceleration of the Universe during the present era after decoupling as a constant during inflation. 1 Issues in String Cosmology Formal developments in string theory [1] over the past decade [2], with the discovery of a consistent way of studying quantum domain-wall structures (Dbranes), have opened up novel ways of looking at not only the microcosmos, but also the macrocosmos. In the microcosmos, there are novel ways of compactification, either via the observation [3] that extra dimensions that are large compared to the string scale [3] are consistent with the foundations of string theory, or by viewing our four-dimensional world as a brane embedded in a higher-dimensional bulk spacetime, whose extra dimensions might even be infinite in size [4]. Such ideas are still consistent with the required large hierarchy between the Planck scale and the electroweakor supersymmetry-breaking scale. In this modern approach, fields in the gravitational (super)multiplet of the (super)string are allowed to propagate in the bulk, but not the gauge fields, which are attached to the brane world. In this way, the weakness of gravity compared to the rest of the interactions is a result of the large compact dimensions, and the compactification is not necessarily achieved through the conventional means of closing the extra dimensions up on spatial compact manifolds, but could also involve shadow brane