A Fake Instability in String Inflation

Understand

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.