A geometrical instability for ultra-light fields during inflation?

TL;DR

This paper investigates the stability of isocurvature perturbations in supergravity and string inflation models, revealing that ultra-light fields can experience geometrical instability when the background trajectory is a geodesic, requiring non-perturbative analysis.

Contribution

It identifies conditions under which ultra-light fields in inflationary models become geometrically unstable, emphasizing the importance of background trajectory geometry.

Findings

Heavy fields remain stable on the attractor background.

Ultra-light fields can become unstable along geodesic trajectories.

Non-perturbative methods may be needed for full understanding.

Abstract

Westudythebehaviourofisocurvatureperturbationsinnon-linearsigmamodels which naturally emerge in supergravity and string inflationary scenarios. We focus on the case of negatively curved field manifolds which can potentially lead to a geometrical destabilisation of isocurvature modes. We find however that heavy fields are stable when their effective mass is computed on the attractor background solution. On the other hand, we show that ultra-light fields can potentially suffer from a geometrical instability when the background trajectory is a geodesic in field space. In this case a full understanding of the system is likely to require the use of non-perturbative methods.