A geometrical instability for ultra-light fields during inflation?

Understand

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.