Strings in Singular Time-Dependent Backgrounds

TL;DR

Addresses how strings propagate in time-dependent backgrounds with spacelike singularities by leveraging exact CFT coset and orbifold constructions. The paper surveys algebraic, geometric, and dynamical aspects of coset backgrounds and discusses observables, unitarity, and stability, highlighting how these exact models illuminate GR questions. It contributes new 2D cosmology models with Abelian gauge fields described by cosets of SL(2)×U(1) by U(1)×Z, showing gauge fields can alter singularity structure while preserving an exact CFT description. These results provide a controlled framework to test stringy resolutions of spacelike singularities and explore potential holographic interpretations.

Abstract

We review the construction of time-dependent backgrounds with space-like singularities. We mainly consider exact CFT backgrounds. The algebraic and geometric aspects of these backgrounds are discussed. Physical issues, results and difficulties associated with such systems are reviewed. Finally, we present some new results: a two dimensional cosmology in the presence of an Abelian gauge field described within a family of (SL(2)xU(1))/(U(1)xZ) quotient CFTs.

Figures (2)

• Figure 3: a: The two dimensional cosmology where the fundamental domains are marked; the dashed lines are curvature singularities. b: The space after the identification.
• Figure 4: a: The 2-dimensional cosmology with a background gauge field ($\alpha\neq 0$); the dashed lines are curvature singularities and the doted lines are horizons. b: The space after the identification; the dashed lines are time-like curvature singularities -- domain walls -- in the static compact regions. The horizons in a turned into big bang/crunch orbifold singularities, connecting the various regions.