Catastrophic Inflation in the Axiverse

Abstract

In the landscape of compactifications of type IIB string theory on O3/O7 orientifolds of Calabi--Yau threefolds, we identify regions of the axion potential in specific geometries that support small-field inflation, under the assumption that the overall Calabi--Yau volume can be stabilized at certain tuned values. At the tuned volume multiple critical points of the axion potential merge in a ``catastrophe'', creating a region with small gradient and small Hessian determinant --- necessary conditions for slow-roll inflation. We describe an algorithm that identifies several explicit examples of these catastrophes that have small field range $f / M_{\rm Pl} \ll 1 $ yet inflate for thousands of efolds. Some examples correctly reproduce the observed spectral tilt but not the amplitude of scalar perturbations, while others reproduce the observed amplitude but with a tilt that is slightly too red. All our examples have negligible tensor perturbations and no significant non-Gaussianity generated during the slow-roll phase. Our results demonstrate that string theory axions can be the source of cosmic inflation without alignment or large field ranges, provided that the Kähler moduli can be stabilized at special values of the volume.