Geometrical Destabilisation of Ultra-Light Axions in String Inflation
  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.
