| Most of our present knowledge of string theory pertains to time-independent backgrounds. Time-dependent backgrounds are in general much harder to understand, and pose a number of interesting questions.; In the first part of this dissertation, we study light-like singularities in string theory. We discuss physical properties of the parabolic orbifold and the null-brane orbifold of Minkowski space, and we show how a large class of light-like singularities gets repaired by string worldsheet instantons.; The second part of the dissertation is devoted to the study of physical systems related to double analytic continuations of black holes. In particular, we discuss M-theory compactified on a non-supersymmetric interval, which can decay by nucleation of bubbles of nothing. We also study the evolution of spacetimes obtained by a double analytic continuation of Kerr black holes. We compute particle creation in these spacetimes, and explain its relation to non-local deformations of the string worldsheet action.; The main motivation for the work presented in the last part of the dissertation is to gain information about a possible holographic description of de Sitter space. We study the entropy of de Sitter flux compactifications, and the entropy of their deformations containing D-brane domain walls. We find a string scale correspondence point at which the thermodynamic entropy of the spacetime and the statistical entropy on the D-brane domain walls agree up to coefficients of order one. |
