Sharp turns in axion monodromy: primordial black holes and gravitational waves

TL;DR

This paper introduces a supergravity-based mechanism for generating sharp turns during inflation, leading to enhanced scalar fluctuations, primordial black holes, and detectable gravitational waves with resonant features.

Contribution

It presents the first concrete supergravity model of axion monodromy with multiple sharp turns, producing observable primordial black holes and gravitational waves.

Findings

Enhanced scalar power spectrum with resonant features

Production of very light primordial black holes

Large gravitational wave spectra detectable by upcoming surveys

Abstract

Large turns in multifield inflation can lead to a very rich phenomenology, but are difficult to realise in supergravity, and typically require large field space curvatures. In this work, we present a mechanism to realise multiple sharp turns, and therefore strong non-geodesic trajectories, from transient violations of slow-roll without the requirement of large field space curvatures in supergravity inflation. Such turning rates can strongly source the adiabatic fluctuations, resulting in an enhanced scalar power spectrum with resonant features and a large peak amplitude. If the growth of the scalar power spectrum at small scales is large enough, primordial black holes can be produced in abundance. These large scalar fluctuations induce a characteristic large spectrum of gravitational waves for a wide range of frequencies, which inherits the resonant features. We illustrate this mechanism in a supergravity model of axion monodromy, which provides the first concrete model to realise such resonant features. The model can sustain inflation for around 60 e-folds, leading to considerable production of very light primordial black holes, and large gravitational wave spectra, which could be detectable by multiple upcoming gravitational wave surveys. For the set of parameter we consider, large oscillations occur at all scales. This represents a challenge for the model at large scales and motivates further investigation to reconcile this class of models with Planck data.