Black Holes from Fermi Ball Collapse

TL;DR

This paper explores how Fermi balls, formed in a dark sector with scalar interactions, can rapidly grow and collapse into black holes, offering a natural scenario for primordial black hole formation.

Contribution

It provides a detailed calculation of Fermi ball properties with scalar quartic coupling and demonstrates their potential to form primordial black holes through collapse.

Findings

Fermi balls reach saturation quickly regardless of size.

They can grow via mergers or accretion in the dark sector.

Collapse into black holes occurs when density exceeds Schwarzschild limit.

Abstract

Fermi balls are non-topological solitons that can naturally form in an early universe containing a dark sector with heavy fermions and an attractive interaction mediated by a light scalar field. We compute the Fermi ball mass and radius scaling relations when the potential of the scalar field $\varphi$ has a non-negligible quartic coupling $\lambda\varphi^4$. The resulting Fermi balls reach `saturation' very rapidly, even when their radius is much smaller than the effective Yukawa force range. These objects can therefore grow by mergers or by accretion of ambient dark fermions, until they become so dense that they fall within their Schwarzschild radius and collapse to black holes. This setup, therefore, provides an example of a rather natural and economical dark sector scenario for the formation of primordial black holes.