Cosmology of the closed string tachyon

TL;DR

This paper investigates the effects of closed string tachyon condensation on cosmological models within string theory, showing how tachyon dynamics can lead to universe contraction, singularities, or solitonic structures, and how higher-order corrections resolve singularities.

Contribution

It derives a unique perturbative tachyon potential and demonstrates how higher-order corrections resolve spacetime singularities in null tachyon backgrounds.

Findings

Tachyon drives accelerated contraction of the universe.

Higher-order corrections resolve spacetime curvature singularities.

Tachyon condensation can generate or terminate cosmological time flow.

Abstract

The spacetime physics of bulk closed string tachyon condensation is studied at the level of a two-derivative effective action. We derive the unique perturbative tachyon potential consistent with a full class of linearized tachyonic deformations of supercritical string theory. The solutions of interest deform a general linear dilaton background by the insertion of purely exponential tachyon vertex operators. In spacetime, the evolution of the tachyon drives an accelerated contraction of the universe and, absent higher-order corrections, the theory collapses to a cosmological singularity in finite time, at arbitrarily weak string coupling. When the tachyon exhibits a null symmetry, the worldsheet dynamics are known to be exact and well-defined at tree level. We prove that if the two-derivative effective action is free of non-gravitational singularities, higher-order corrections always resolve the spacetime curvature singularity of the null tachyon. The resulting theory provides an explicit mechanism by which tachyon condensation can generate or terminate the flow of cosmological time in string theory. Additional particular solutions can resolve an initial singularity with a tachyonic phase at weak coupling, or yield solitonic configurations that localize the universe along spatial directions.