Monochromatic Mass Spectrum of Primordial Black Holes

TL;DR

This paper investigates how certain inflationary bubble transitions can produce a nearly monochromatic spectrum of primordial black holes, potentially accounting for all dark matter in a specific mass range.

Contribution

It develops new techniques to estimate transition rates during slow-roll inflation and shows how these lead to a monochromatic black hole mass spectrum under broad model conditions.

Findings

Black hole mass spectrum can be nearly monochromatic due to short-lived bubble formation.

Primordial black holes in the asteroid mass range could account for all dark matter.

Transition timing during inflation critically influences black hole mass distribution.

Abstract

During slow-roll inflation, non-perturbative transitions can produce bubbles of metastable vacuum. These bubbles expand exponentially during inflation to super-horizon size, and later collapse into black holes when the expansion of the universe is decelerating. Estimating the rate for these transitions during a time-dependent slow-roll phase requires the development of new techniques. Our results show that in a broad class of models, the inflationary fine-tuning that gives rise to small density fluctuations causes these bubbles to appear only during a time interval that is short compared to the inflationary Hubble time. As a result, despite the fact that the final mass of the black hole is exponentially sensitive to the moment bubbles form during inflation, the resulting primordial black hole mass spectrum can be nearly monochromatic. If the transition occurs near the middle of inflation, the mass can fall in the "asteroid" range $10^{17}-10^{22}$ g in which all known observations are compatible with black holes comprising 100% of dark matter.