Graceful Exit and Energy Conditions in String Cosmology

TL;DR

This paper investigates conditions for a smooth transition from inflation to radiation dominance in string cosmology, revealing that such a transition requires matter sources that violate classical energy conditions, implying the need for new physics.

Contribution

It provides a generalized phase-space analysis of string cosmology solutions, identifying necessary conditions for graceful exit and clarifying the role of energy condition violations.

Findings

Graceful exit requires sources violating classical energy conditions.

Classical matter sources, except spatial curvature, obey energy conditions hindering exit.

New effective matter phases are necessary for non-singular cosmological transitions.

Abstract

String cosmology solutions are examined in a generalized phase-space including sources representing arbitrary corrections to lowest order string-dilaton-gravity effective action. We find a set of necessary conditions for a graceful exit transition from a dilaton-driven inflationary phase to a radiation dominated era. We show that sources allowing such a transition have to violate energy conditions similar to those appearing in singularity theorems of general relativity. Since familiar classical sources, excepting spatial curvature, obey these energy conditions we conclude that a generic graceful exit in string cosmology requires a new effective phase of matter. Our results clarify and generalize previous analyses and enable us to critically reexamine proposed non-singular cosmologies.