Nonsingular multidimensional cosmologies without fine tuning

TL;DR

This paper presents exact multidimensional cosmological solutions inspired by superstring theory that avoid singularities and do not require fine tuning, suggesting the universe may have bypassed a quantum gravity-dominated phase.

Contribution

It introduces nonsingular bouncing solutions in multidimensional models with dilaton fields, highlighting their natural emergence without fine tuning and their implications for early universe cosmology.

Findings

Nonsingular bouncing solutions found for certain parameter ranges.

Internal scale factor and dilaton tend to constants at late times.

Solutions support the idea that the universe avoided a quantum gravity stage.

Abstract

Exact cosmological solutions for effective actions in D dimensions inspired by the tree-level superstring action are studied. For a certain range of free parameters existing in the model, nonsingular bouncing solutions are found. Among them, of particular interest can be open hyperbolic models, in which, without any fine tuning, the internal scale factor and the dilaton field (connected with string coupling in string theories) tend to constant values at late times. A cosmological singularity is avoided due to nonminimal dilaton-gravity coupling and, for D > 11, due to pure imaginary nature of the dilaton, which conforms to currently discussed unification models. The existence of such and similar solutions supports the opinion that the Universe had never undergone a stage driven by full-scale quantum gravity.