Singularity free dilaton-driven cosmologies and pre-little-bang

TL;DR

This paper presents inhomogeneous, singularity-free dilaton-driven cosmological models where curvature invariants remain bounded, replacing the big bang with a maximal curvature phase, suggesting a pre-little-bang scenario in string cosmology.

Contribution

The authors construct explicit inhomogeneous solutions with bounded curvature, demonstrating non-singular cosmologies and introducing the concept of pre-little-bang phases in string theory.

Findings

Curvature invariants are bounded and regular in the models.

Geodesics are complete, indicating no physical singularities.

Existence of high-curvature, non-singular 'little bang' phases.

Abstract

There are no reasons why the singularity in the growth of the dilaton coupling should not be regularised, in a string cosmological context, by the presence of classical inhomogeneities. We discuss a class of inhomogeneous dilaton-driven models whose curvature invariants are all bounded and regular in time and space. We prove that the non-space-like geodesics of these models are all complete in the sense that none of them reaches infinity for a finite value of the affine parameter. We conclude that our examples represent truly singularity-free solutions of the low energy beta functions. We discuss some symmetries of the obtained solutions and we clarify their physical interpretation. We also give examples of solutions with spherical symmetry. In our scenario each physical quantity is everywhere defined in time and space, the big-bang singularity is replaced by a maximal curvature phase where the dilaton kinetic energy reaches its maximum. The maximal curvature is always smaller than one (in string units) and the coupling constant is also smaller than one and it grows between two regimes of constant dilaton, implying, together with the symmetries of the solutions, that higher genus and higher curvature corrections are negligible. We argue that our examples describe, in a string cosmological context, the occurrence of ``little bangs''(i.e. high curvature phases which never develop physical singularities). They also suggest the possibility of an unexplored ``pre-little-bang'' phase.