D-branes in a Big Bang/Big Crunch Universe: Nappi-Witten Gauged WZW Model

TL;DR

This paper investigates D-branes in a cosmological model with big bang/big crunch singularities and closed time-like curves, showing that D-branes remain well-behaved despite these spacetime pathologies.

Contribution

It classifies D-branes in the Nappi-Witten gauged WZW model and demonstrates their regularity in singular and causally complex regions.

Findings

D-brane metrics do not exhibit singularities.

Wave functions on D-branes are well-behaved near singularities.

D-branes remain consistent in regions with closed time-like curves.

Abstract

We study D-branes in the Nappi-Witten model, which is a gauged WZW model based on (SL(2,R) x SU(2)) / (U(1) x U(1)). The model describes a four dimensional space-time consisting of cosmological regions with big bang/big crunch singularities and static regions with closed time-like curves. The aim of this paper is to investigate by D-brane probes whether there are pathologies associated with the cosmological singularities and the closed time-like curves. We first classify D-branes in a group theoretical way, and then examine DBI actions for effective theories on the D-branes. In particular, we show that D-brane metric from the DBI action does not include singularities, and wave functions on the D-branes are well behaved even in the presence of closed time-like curves.