Nature of singularities in anisotropic string cosmology

TL;DR

This paper investigates the types and conditions of singularities in anisotropic string cosmological models with Gauss-Bonnet terms, identifying scenarios with and without nonsingular solutions and highlighting the significance of determinant singularities.

Contribution

It provides a detailed classification of singularities in anisotropic string cosmology models, contrasting dilaton-driven and modulus-driven cases, and explores conditions for nonsingular solutions.

Findings

In dilaton-driven models, singularities are inevitable during evolution.

In modulus-driven models, nonsingular solutions exist asymptotically.

Determinant singularities may lead to overall singular solutions.

Abstract

We study nature of singularities in anisotropic string-inspired cosmological models in the presence of a Gauss-Bonnet term. We analyze two string gravity models-- dilaton-driven and modulus-driven cases-- in the Bianchi type-I background without an axion field. In both scenarios singularities can be classified in two ways- the determinant singularity where the main determinant of the system vanishes and the ordinary singularity where at least one of the anisotropic expansion rates of the Universe diverges. In the dilaton case, either of these singularities inevitably appears during the evolution of the system. In the modulus case, nonsingular cosmological solutions exist both in asymptotic past and future with determinant $D=+\infty$ and D=2, respectively. In both scenarios nonsingular trajectories in either future or past typically meet the determinant singularity in past/future when the solutions are singular, apart from the exceptional case where the sign of the time-derivative of dilaton is negative. This implies that the determinant singularity may play a crucial role to lead to singular solutions in an anisotropic background.