Non-singular string cosmology in a 2d Hybrid model

TL;DR

This paper demonstrates the construction of non-singular, bouncing string cosmologies in a two-dimensional hybrid model with specific symmetries, avoiding singularities and maintaining perturbative control.

Contribution

It introduces a novel 2D hybrid string model with MSDS symmetry that yields non-singular, perturbative bouncing cosmologies with calculable partition functions.

Findings

Constructed non-singular string cosmologies free of Hagedorn instabilities.

Identified massless thermal states at the self-dual point that connect contracting and expanding phases.

Maintained perturbative string coupling throughout cosmological evolution.

Abstract

The existence of non-singular string cosmologies is established in a class of two-dimensional supersymmetric Hybrid models at finite temperature. The left-moving sector of the Hybrid models gives rise to 16 real (N_4=4) spacetime supercharges as in the usual superstring models. The right-moving sector is non-supersymmetric at the massless level, but is characterized by MSDS symmetry, which ensures boson/fermion degeneracy of the right-moving massive levels. Finite temperature configurations, which are free of Hagedorn instabilities, are constructed in the presence of non-trivial "gravito-magnetic" fluxes. These fluxes inject non-trivial winding charge into the thermal vacuum and restore the thermal T-duality symmetry associated with the Euclidean time circle. Thanks to the unbroken right-moving MSDS symmetry, the one-loop string partition function is exactly calculable beyond any alpha'-approximation. At the self-dual point new massless thermal states appear, sourcing localized spacelike branes, which can be used to connect a contracting thermal Universe to an expanding one. The resulting bouncing cosmology is free of any curvature singularities and the string coupling remains perturbative throughout the cosmological evolution.