# Geometrical Destabilisation of Ultra-Light Axions in String Inflation

## TL;DR

This paper investigates how ultra-light axions in Fibre Inflation models become destabilized due to geometrical effects, highlighting the need for non-perturbative methods to fully understand inflationary dynamics.

## Contribution

It provides a detailed analytical and numerical analysis showing the consistent negative mass-squared of ultra-light axions during inflation, revealing a geometrical destabilization mechanism.

## Key findings

- Ultra-light axions become tachyonic during inflation.
- Geometrical destabilization affects isocurvature perturbations.
- Perturbative analysis is insufficient for full understanding.

## Abstract

We perform a detailed analytical and numerical analysis of the multi-field evolution of Fibre Inflation and show that, regardless of the microscopic realisation of the model, the mass-squared of one of the two ultra-light axions becomes always negative during inflation. This implies that the corresponding isocurvature perturbations experience a potential geometrical destabilisation which seems to bring the system away from the perturbative regime. Therefore we conclude that a full understanding of the inflationary evolution of Fibre Inflation can be achieved only via a non-perturbative analysis where the backreaction of tachyonic isocurvature perturbations is properly taken into account.