Primordial black holes as dark matter and gravitational waves from bumpy axion inflation

TL;DR

This paper proposes a string theory-inspired inflation model where axion dynamics produce primordial black holes as dark matter and generate a detectable gravitational wave background, linking early universe physics with observable signals.

Contribution

It introduces a novel mechanism involving axion-like fields with cliffs and plateaus that produce PBHs and GWs, consistent with CMB data and testable by future observations.

Findings

PBHs with mass ~10^{-13} M_sun can account for all dark matter

The model predicts an observable SGWB at LISA scales

Distinct non-Gaussian GW signatures can differentiate this scenario

Abstract

We consider a mechanism for producing a significant population of primordial black holes (PBHs) and an observable stochastic gravitational wave background (SGWB) within string theory inspired models of inflation. In this framework where inflaton is identified as a non-compact axion-like field, sub-leading non-perturbative effects can superimpose steep cliffs connected by smooth plateaus onto the underlying axion potential. In the presence of coupling to Abelian gauge fields, the motion of axion on the cliff-like region(s) of its potential triggers a localized production of one helicity state of gauge fields due to the temporary fast-roll of axion around such a feature. In this setup, primordial fluctuations sourced by vector fields exhibit a localized peak in momentum space corresponding to modes that exit the horizon when the axion velocity is maximal. As an application of this general mechanism, we present an example of axion inflation which both matches Planck observations at CMB scales and generates a population of light PBHs ($M_{\rm PBH} \simeq 10^{-13} M_{\odot}$) that can account for all dark matter. In this scenario, the enhanced scalar fluctuations that leads to PBHs also generate an observable SGWB of induced origin at LISA scales. The amplitude and shape of the resulting GW signal inherits specific properties (such as non-Gaussianity and its shape) of its scalar sources that may allow us to distinguish this mechanism from other inflationary scenarios and astrophysical backgrounds. This GW signal together with an observation of PBH distribution at the corresponding scales can thus provide a window to the inflationary dynamics on scales much smaller than those probed by Cosmic Microwave Background (CMB) and Large Scale Structure (LSS) Measurements.