String Propagation through a Big Crunch/Big Bang Transition

TL;DR

This paper studies how classical and quantum strings propagate through cosmological bounces, revealing conditions under which strings become highly excited and implications for early universe models like ekpyrotic/cyclic scenarios.

Contribution

It classifies cosmological singularities based on string behavior and analyzes string excitation during bounces, linking anisotropic space-times to plane waves and cosmological perturbations.

Findings

Strings remain well-behaved near certain singularities.

Highly excited string states emerge when wavelengths are below the string scale.

Long wavelength modes are mildly excited, affecting cosmological perturbations.

Abstract

We consider the propagation of classical and quantum strings on cosmological space-times which interpolate from a collapsing phase to an expanding phase. We begin by considering the classical propagation of strings on space-times with isotropic and anisotropic cosmological singularities. We find that cosmological singularities fall into two classes, in the first class the string evolution is well behaved all the way up to the singularity, whilst in the second class it becomes ill-defined. Then assuming the singularities are regulated by string scale corrections, we consider the implications of the propagation through a `bounce'. It is known that as we evolve through a bounce, quantum strings will become excited giving rise to `particle transmutation'. We reconsider this effect, giving qualitative arguments for the amount of excitation for each class. We find that strings whose physical wavelength at the bounce is less that $\sqrt{\alpha'}$ inevitably emerge in highly excited states, and that in this regime there is an interesting correspondence between strings on anisotropic cosmological space-times and plane waves. We argue that long wavelength modes, such as those describing cosmological perturbations, will also emerge in mildly excited string scale mass states. Finally we discuss the relevance of this to the propagation of cosmological perturbations in models such as the ekpyrotic/cyclic universe.