Problems with Tachyon Inflation

TL;DR

This paper assesses string-theory tachyon condensation as a driver of inflation in brane inflation scenarios, showing that realistic inflation is hard to achieve in the simplest tachyon models. It derives that slow-roll inflation near the tachyon maximum requires $H^2 = \tau_3/(3 M_p^2)$ with $H^2 \\gg |m^2|$, but gravitational-wave bounds $H/M_p \\lesssim 3.6 \\times 10^{-5}$ together with EFT constraints $v \\gg 1$ render this scenario incompatible with producing the observed perturbations, limiting tachyon inflation to at best a brief Planck-density stage. Consequently, the last 60 e-folds of observable inflation cannot originate from tachyon condensation in these models. The authors conclude that viable post-inflation cosmology likely requires either a separate late-stage inflation or hybrid inflation models with a finite minimum at $|T| \\ll M_p$, while the tachyon condensate could only be a dark-matter candidate in finely tuned setups.

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.