Stochastic Effects in Axion Inflation and Primordial Black Holes Formation

TL;DR

This paper explores how stochastic effects in axion inflation influence gauge field dynamics and lead to the formation of primordial black holes, relaxing previous constraints on model parameters.

Contribution

It introduces a stochastic inflation framework for axion inflation, showing how noise effects balance gauge field growth and impact primordial black hole production.

Findings

Large curvature perturbations induce copious PBH formation.

PBHs follow Gaussian statistics under stochastic effects.

Constraints on the instability parameter are relaxed by about fifty percent.

Abstract

We revisit the model of axion inflation in the context of stochastic inflation and investigate the effects of the stochastic noises associated to the electromagnetic fields. Because of the parity violating interaction, one polarization of the gauge field is amplified inducing large curvature perturbation power spectrum. Taking into account the stochastic kicks arising from the short modes at the time of horizon crossing we obtain the corresponding Langevin equations for the long modes of the electromagnetic and axion fields. It is shown that a mean-reverting process governs the dynamics of the electromagnetic fields such that the tachyonic growth of the gauge fields is balanced by the diffusion forces. As the instability parameter grows towards the end of inflation, the large curvature perturbations induced from gauge field perturbations lead to copious production of small mass primordial black holes (PBHs). It is shown that the produced PBHs follow a Gaussian statistics. Imposing the observational constraints on PBHs formation relaxes the previous bounds on the instability parameter by about fifty percents.