The Phantom Divide in String Gas Cosmology

TL;DR

String gas cosmology naturally violates the Null Energy Condition, allowing crossing of the phantom divide, but non-perturbative string effects prevent multiple crossings, aligning with observational constraints.

Contribution

This paper demonstrates that while string gas cosmology can cross the phantom divide, non-perturbative effects prohibit multiple crossings, resolving a key cosmological debate.

Findings

String gas cosmology violates the Null Energy Condition.

Crossing the phantom divide is possible but limited.

Non-perturbative effects prevent multiple phantom crossings.

Abstract

One of the main virtues of string gas cosmology is that it resolves cosmological singularities. Since the Universe can be approximated by a locally asymptotically de Sitter spacetime by the end of the inflationary era, a singularity theorem implies that these cosmologies effectively violate the Null Energy Condition [not just the Strong Energy Condition]. We stress that this is an extremely robust result, which does not depend on assuming that the spatial sections remain precisely flat in the early Universe. This means, however, that it must be possible for string cosmologies to cross the recently much-discussed "phantom divide" [from w < -1 to w > -1, where w is the equation-of-state parameter]. This naturally raises the question as to whether the phantom divide can be crossed again, to account for recent observations suggesting that w < -1 at the present time. We argue that non-perturbative string effects rule out this possibility, even if the NEC violation in question is only "effective".