Primordial black holes from cusp collapse on cosmic strings

TL;DR

This paper introduces a new mechanism for primordial black hole formation from cosmic string cusps, suggesting a higher production rate and unique observational signatures, impacting constraints on cosmic string properties.

Contribution

The study reveals that cusp collapse on cosmic strings can produce PBHs more frequently than previously thought, with distinctive high-spin, ultrarelativistic characteristics.

Findings

Cusp collapse leads to PBHs with masses smaller than the loop by (Gμ)^2.

Cusp-induced PBHs are highly spinning and ultrarelativistic.

New constraints on Gμ are derived from PBH evaporation and gravitational-wave data.

Abstract

Primordial black holes (PBHs) are of fundamental interest in cosmology and astrophysics, and have received much attention as a dark matter candidate and as a potential source of gravitational waves. One possible PBH formation mechanism is the gravitational collapse of cosmic strings. Thus far, the entirety of the literature on PBH production from cosmic strings has focused on the collapse of (quasi)circular cosmic string loops, which make up only a tiny fraction of the cosmic loop population. We demonstrate here a novel PBH formation mechanism: the collapse of a small segment of cosmic string in the neighbourhood of a cusp. Using the hoop conjecture, we show that collapse is inevitable whenever a cusp appears on a macroscopically-large loop, forming a PBH whose rest mass is smaller than the mass of the loop by a factor of the dimensionless string tension squared, $(G\mu)^2$. Since cusps are generic features of cosmic string loops, and do not rely on finely-tuned loop configurations like circular collapse, this implies that cosmic strings produce PBHs in far greater numbers than has previously been recognised. The resulting PBHs are highly spinning and boosted to ultrarelativistic velocities; they populate a unique region of the BH mass-spin parameter space, and are therefore a "smoking gun" observational signature of cosmic strings. We derive new constraints on $G\mu$ from the evaporation of cusp-collapse PBHs, and update existing constraints on $G\mu$ from gravitational-wave searches.