| We study classical and quantum features of cosmological models based on superstring theories. In the first part of this work, we consider the emergence of chaos during the collapse of the universe to a big crunch, which is a potential problem for recently proposed models in which cosmological history is cyclic. We describe two mechanisms by which chaos can be avoided. The first requires a matter component with an equation of state w > 1. The second mechanism, which we term "controlled chaos," requires the spacetime to satisfy a set of topological conditions, expressed in terms of its de Rham cohomology. We present techniques to systematically find solutions with controlled chaos and classify all solutions for the heterotic superstring theory. In the second part of this work, we turn to the problem of string pair production in time-dependent spacetimes. Through studying a specific background corresponding to two colliding gravitational waves, we show that the spectrum and production rate for string pairs differ significantly from point particle pairs. These results suggest unique signatures of string physics that may persist in cosmological spacetimes. |
