Loop corrections and graceful exit in string cosmology

TL;DR

This paper investigates how perturbative string loops influence the pre-big-bang cosmological evolution, showing they can enable a smooth transition from superinflation to a decelerating phase, but also induce instabilities at critical couplings.

Contribution

It demonstrates that string loop corrections can facilitate a graceful exit in string cosmology and identifies a critical coupling where a transition to a D-brane dominated phase occurs.

Findings

Loop corrections can drive the evolution into a graceful exit region.

Solutions connect superinflationary phase to a decelerating phase smoothly.

At a critical coupling, a ghost-like instability appears.

Abstract

We examine the effect of perturbative string loops on the cosmological pre-big-bang evolution. We study loop corrections derived from heterotic string theory compactified on a $Z_N$ orbifold and we consider the effect of the all-order loop corrections to the Kahler potential and of the corrections to gravitational couplings, including both threshold corrections and corrections due to the mixed Kahler-gravitational anomaly. We find that string loops can drive the evolution into the region of the parameter space where a graceful exit is in principle possible, and we find solutions that, in the string frame, connect smoothly the superinflationary pre-big-bang evolution to a phase where the curvature and the derivative of the dilaton are decreasing. We also find that at a critical coupling the loop corrections to the Kahler potential induce a ghost-like instability, i.e. the kinetic term of the dilaton vanishes. This is similar to what happens in Seiberg-Witten theory and signals the transition to a new regime where the light modes in the effective action are different and are related to the original ones by S-duality. In a string context, this means that we enter a D-brane dominated phase.