Problems with Tachyon Inflation

TL;DR

This paper critically examines string theory tachyon condensation as a mechanism for cosmological inflation, highlighting significant theoretical challenges and limitations in simple models, especially regarding reheating and the nature of the tachyon field.

Contribution

It identifies fundamental issues with tachyon-based inflation models, particularly their inability to produce realistic reheating and their requirement for super-Planckian densities, contrasting with hybrid inflation models.

Findings

Inflation in simple tachyon models occurs only at super-Planckian densities.

Reheating is problematic due to the non-oscillatory nature of the tachyon field.

Hybrid inflation models with tachyons avoid these issues.

Abstract

We consider cosmological consequences of string theory tachyon condensation. We show that it is very difficult to obtain inflation in the simplest versions of this theory. Typically, inflation in these theories could occur only at super-Planckian densities, where the effective 4D field theory is inapplicable. Reheating and creation of matter in models where the tachyon potential V(T) has a minimum at infinitely large T is problematic because the tachyon field in such theories does not oscillate. If the universe after inflation is dominated by the energy density of the tachyon condensate, it will always remain dominated by the tachyons. It might happen that string condensation is responsible for a short stage of inflation at a nearly Planckian density, but one would need to have a second stage of inflation after that. This would imply that the tachyon played no role in the post-inflationary universe until the very late stages of its evolution. These problems do not appear in the recently proposed models of hybrid inflation where the complex tachyon field has a minimum at T << M_p.