Time Dependent String Compactification: Towards Bouncing Cosmology

TL;DR

This paper explores how string theory compactifications can lead to bouncing cosmologies by violating the null energy condition in lower dimensions while satisfying it in higher dimensions, using worldsheet symmetries.

Contribution

It demonstrates that certain time-dependent string compactifications can produce lower-dimensional NEC violations compatible with higher-dimensional NEC satisfaction, enabling bouncing cosmologies.

Findings

Lower-dimensional NEC violation enables bouncing cosmology.

Higher-dimensional NEC remains satisfied due to worldsheet symmetry.

Scale-separated solutions are achievable through averaged Einstein-frame conditions.

Abstract

We study the Null Energy Condition (NEC) arising from the Virasoro constraint on the string worldsheet. We then analyze how the NEC in the external spacetime directions emerges under general time-dependent string compactifications. Finally, we exhibit compactifications in which the averaged Einstein-frame condition allows the lower-dimensional description of the external spacetime to violate the NEC, thereby realizing a bouncing cosmology, while the higher-dimensional NEC remains satisfied, as dictated by worldsheet symmetry. We comment on scale-separated solutions obtained through the averaged Einstein-frame condition