Nonsingular Dilaton Cosmology in the String Frame

TL;DR

This paper introduces a modified string frame dilaton gravity theory with higher derivative terms, resulting in nonsingular cosmological solutions that smoothly transition from superinflation to expansion, addressing the pre-big-bang graceful exit problem.

Contribution

It constructs a new gravity model with higher derivatives that yields nonsingular, bounded curvature solutions, providing a natural resolution to the pre-big-bang cosmology's graceful exit.

Findings

All solutions are nonsingular with bounded curvature invariants.

Existence of solutions with a smooth transition from superinflation to Friedmann expansion.

Addresses the graceful exit problem in pre-big-bang cosmology.

Abstract

We consider the theory obtained by adding to the usual string frame dilaton gravity action specially constructed higher derivative terms motivated by the limited curvature construction of [MukhanovET1992a] and determine the spatially homogeneous and isotropic solutions to the resulting equations of motion. All solutions of the resulting theory of gravity with these symmetries are nonsingular and all curvature invariants are bounded. For initial conditions inspired by the pre-big-bang scenario solutions exist which correspond to a spatially flat Universe starting in a dilaton-dominated superinflationary phase with ${\dot H} > 0$ and having a smooth transition to an expanding Friedmann Universe with ${\dot H} < 0$. Hence, the graceful exit problem of pre-big-bang cosmology is solved in a natural way.