A novel PBH production mechanism from non-Abelian gauge fields during inflation

TL;DR

This paper proposes a new mechanism for primordial black hole formation during inflation, driven by instabilities in non-Abelian gauge fields coupled to axion-like particles, leading to distinctive gravitational wave signals.

Contribution

It introduces a novel PBH production mechanism via gauge field instabilities during inflation, highlighting its universality and potential observational signatures.

Findings

Scalar perturbations exhibit a known instability leading to PBH formation.

The mechanism is largely insensitive to initial conditions and potential shape.

Spectator ALPs can access the instability region without strong backreaction.

Abstract

We consider the case of axion-like particles (ALPs) during inflation. When coupled to a non-Abelian gauge sector via a Chern-Simons term, ALPs support an intriguing, testable, phenomenology with very distinctive features including chiral primordial gravitational waves. For sufficiently small values of the gauge vev and coupling, scalar perturbations in the gauge sector exhibit a known instability. We harness the power of such instability for primordial black hole (PBH) generation. In the case of an axion-inflaton, one is dynamically driven into a strong-backreaction regime that crosses the instability band thereby sourcing a peaked scalar spectrum leading to PBH production and the related scalar-induced gravitational waves. Remarkably, this dynamics is largely insensitive to the initial conditions and the shape of the potential, highlighting the universal nature of the sourcing mechanism. In the case of spectator ALPs one can identify the parameter space that sets off the strong backreaction regime and the ensuing features. We show that spectator ALP models may also access the scalar instability region without triggering strong backreaction.