Global Fluctuation Spectra in Big Crunch/Big Bang String Vacua

TL;DR

This paper analyzes scalar fluctuation spectra in Big Crunch/Big Bang string cosmologies, revealing how the spectrum changes across the bounce and the role of whisker regions, with implications for entanglement entropy and spectrum preservation in certain limits.

Contribution

It provides an explicit computation of the fluctuation spectrum change across the bounce in exact superconformal string backgrounds, highlighting the influence of whisker regions and the Milne orbifold limit.

Findings

The fluctuation spectrum is generally altered during the bounce.

The change is characterized by a momentum and time-dependent function Δ.

In the Milne orbifold limit, the spectrum remains unchanged.

Abstract

We study Big Crunch/Big Bang cosmologies that correspond to exact world-sheet superconformal field theories of type II strings. The string theory spacetime contains a Big Crunch and a Big Bang cosmology, as well as additional ``whisker'' asymptotic and intermediate regions. Within the context of free string theory, we compute, unambiguously, the scalar fluctuation spectrum in all regions of spacetime. Generically, the Big Crunch fluctuation spectrum is altered while passing through the bounce singularity. The change in the spectrum is characterized by a function $\Delta$, which is momentum and time-dependent. We compute $\Delta$ explicitly and demonstrate that it arises from the whisker regions. The whiskers are also shown to lead to ``entanglement'' entropy in the Big Bang region. Finally, in the Milne orbifold limit of our superconformal vacua, we show that $\Delta\to 1$ and, hence, the fluctuation spectrum is unaltered by the Big Crunch/Big Bang singularity. We comment on, but do not attempt to resolve, subtleties related to gravitational backreaction and light winding modes when interactions are taken into account.