Classical and Quantum Strings in compactified pp-waves and Godel type Universes

TL;DR

This paper investigates classical and quantum string dynamics in supersymmetric pp-wave backgrounds with compactified directions that develop closed null and timelike curves, exploring potential pathologies and effects on the partition function.

Contribution

It analyzes string behavior in U-dual Godel-type universes with CNCs and CTCs, highlighting the impact on the partition function and potential unitarity issues.

Findings

Partition function is significantly affected by CNCs and CTCs.

Strings wrapping around CTCs may introduce pathologies.

Results suggest possible violation of unitarity in these backgrounds.

Abstract

We consider Neveu-Schwarz pp-waves with spacetime supersymmetry. Upon compactification of a spacelike direction, these backgrounds develop Closed Null Curves (CNCs) and Closed Timelike Curves (CTCs), and are U-dual to supersymmetric Godel type universes. We study classical and quantum strings in this background, with emphasis on the strings winding around the compact direction. We consider two types of strings: long strings stabilized by NS flux and rotating strings which are stabilized against collapse by angular momentum. Some of the latter strings wrap around CNCs and CTCs, and are thus a potential source of pathology. We analyze the partition function, and in particular discuss the effects of these string states. Although our results are not conclusive, the partition function seems to be dramatically altered due to the presence of CNCs and CTCs. We discuss some interpretations of our results, including a possible sign of unitary violation.