Primordial Black Holes from Sound Speed Resonance during Inflation

TL;DR

This paper introduces a resonance mechanism involving oscillatory sound speed during inflation that can generate primordial black holes, potentially accounting for dark matter, through significant peaks in the primordial power spectrum.

Contribution

It presents a novel resonance effect caused by oscillatory sound speed during inflation, leading to primordial black hole formation with specific mass functions.

Findings

Resonance effect amplifies primordial density fluctuations.

Primordial black holes can form with sufficient abundance.

Mechanism can produce black holes as dark matter candidates.

Abstract

We report on a novel phenomenon of the resonance effect of primordial density perturbations arisen from a sound speed parameter with an oscillatory behavior, which can generically lead to the formation of primordial black holes in the early Universe. For a general inflaton field, it can seed primordial density fluctuations and their propagation is governed by a parameter of sound speed square. Once if this parameter achieves an oscillatory feature for a while during inflation, a significant non-perturbative resonance effect on the inflaton field fluctuations takes place around a critical length scale, which results in significant peaks in the primordial power spectrum. By virtue of this robust mechanism, primordial black holes with specific mass function can be produced with a sufficient abundance for dark matter in sizable parameter ranges.