The Tachyon at the End of the Universe

TL;DR

This paper demonstrates that tachyon condensates can replace spacelike singularities in certain string theory spacetimes, providing a potential resolution to singularities and insights into the beginning or end of time.

Contribution

It introduces explicit examples of tachyon condensates in cosmological and black hole spacetimes, analyzing their effects using worldsheet methods and showing their robustness and finiteness.

Findings

Tachyon condensates replace singularities in specific string backgrounds.

String amplitudes remain finite and perturbative in the presence of tachyon condensates.

The resulting state resembles a thermal Hartle-Hawking state with low energy density.

Abstract

We show that a tachyon condensate phase replaces the spacelike singularity in certain cosmological and black hole spacetimes in string theory. We analyze explicitly a set of examples with flat spatial slices in various dimensions which have a winding tachyon condensate, using worldsheet path integral methods from Liouville theory. In a vacuum with no excitations above the tachyon background in the would-be singular region, we analyze the production of closed strings in the resulting state in the bulk of spacetime. We find a thermal result reminiscent of the Hartle-Hawking state, with tunably small energy density. The amplitudes exhibit a self-consistent truncation of support to the weakly-coupled small-tachyon region of spacetime. We argue that the background is accordingly robust against back reaction, and that the resulting string theory amplitudes are perturbatively finite, indicating a resolution of the singularity and a mechanism to start or end time in string theory. Finally, we discuss the generalization of these methods to examples with positively curved spatial slices.