Ultra slow-turn inflation

TL;DR

This paper introduces the ultra slow-turn inflation class, where decreasing turn rates stabilize models that would otherwise be unstable due to tachyonic isocurvature perturbations, with implications for supergravity and string-inspired models.

Contribution

It identifies a new class of inflation models called ultra slow-turn, demonstrating how decreasing turn rates can prevent instabilities in multi-field inflation.

Findings

Exponential decrease in turn rate stabilizes isocurvature perturbations.

Several supergravity and string-inspired models are classified as ultra slow-turn.

Total entropy perturbation correctly indicates model stability.

Abstract

In standard multi-field models, tachyonic isocurvature perturbations generally indicate the presence of an instability. We revisit the stability of some known counterexamples and show that, in a certain class of models that we call ultra slow-turn, an exponentially decreasing turn rate can shut off this potential instability. We argue that the stability of a given model can be correctly inferred by the total entropy perturbation, even if the effective mass squared of the isocurvature perturbation is negative. Several recent supergravity- or string-inspired models such as fibre inflation, SL(2,$\mathbb{Z}$) attractors and modular inflation fall into the ultra slow-turn class.