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

Abstract

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