A Fake Instability in String Inflation

TL;DR

This paper clarifies that a perceived instability in Fibre Inflation models is unphysical, showing that proper variables reveal the decay of axionic isocurvature perturbations, thus confirming the single-field nature of the inflationary dynamics.

Contribution

It demonstrates that the apparent tachyonic instability in Fibre Inflation is due to ill-defined variables and is resolved when using correct entropy perturbations.

Findings

Axionic isocurvature perturbations decay during inflation

The inflationary dynamics are effectively single-field

The perceived instability is an artifact of variable choice

Abstract

In type IIB Fibre Inflation models the inflaton is a Kaehler modulus which is kinetically coupled to the corresponding axion. In this setup the curvature of the field space induces tachyonic isocurvature perturbations normal to the background inflationary trajectory. However we argue that the associated instability is unphysical since it is due to the use of ill-defined entropy variables. In fact, upon using the correct relative entropy perturbation, we show that in Fibre Inflation axionic isocurvature perturbations decay during inflation and the dynamics is essentially single-field.