On Time-dependent Backgrounds in Supergravity and String Theory

TL;DR

The paper explores time-dependent backgrounds in string theory through three complementary avenues: de Sitter deformations of gauge/gravity dualities, analytic continuation of static solutions, and exact worldsheet constructions. It demonstrates how introducing a Hubble parameter $H$ in confining gauge theories can restore chiral symmetry and how deformations of the Maldacena–Nuñez background yield regular time-dependent geometries with distinct chirality phases, including a possible phase transition at $H_{ m critical}$. It also develops analytic-continuation techniques, refined by T-duality to preserve real fluxes, and presents iD-brane and Milne/WZW models that illuminate how winding modes can experience non-singular propagation near cosmological singularities. Together, these results offer concrete, controllable frameworks for studying time dependence in string theory, while also highlighting limitations for phenomenological cosmology and the need for further understanding of stability and UV/IR behavior in deformed backgrounds.

Abstract

Time-dependent solutions of supergravity and string theory are studied. The examples are obtained from de Sitter deformation of gauge/gravity dualities, analytical continuation of static solutions, and ``exactly solvable'' worldsheet models. Among other things, it is shown that turning on a Hubble parameter in the background of a confining gauge theory in four dimensions can restore chiral symmetry. Some of the solutions obtained from analytical continuation have the interpretation of a universe with a bounce separating a big bang from a big crunch singularity. In the worldsheet context, it is argued why string propagation close to a Milne-type cosmological singularity might be physically non-singular.

Figures (1)

• Figure 1: Penrose diagram for the metric \ref{['tdbh']}. Depending on the sign of $k$, this represents a black hole or a cosmological background.