Dark Matter Density Spikes around Primordial Black Holes

TL;DR

This paper demonstrates that dark matter density spikes form immediately around primordial black holes due to low-velocity particles, leading to bright gamma-ray sources that impose strong constraints on black hole abundance.

Contribution

It introduces a new mechanism for early formation of dark matter spikes around primordial black holes and derives stringent observational constraints from gamma-ray data.

Findings

Density spikes form immediately after black hole formation.

Spikes are denser than those formed by secondary accretion.

Gamma-ray observations constrain primordial black hole abundance.

Abstract

We show that density spikes begin to form from dark matter particles around primordial black holes immediately after their formation at the radiation-dominated cosmological stage. This follows from the fact that in the thermal velocity distribution of particles there are particles with low velocities that remain in finite orbits around black holes and are not involved in the cosmological expansion. The accumulation of such particles near black holes gives rise to density spikes. These spikes are considerably denser than those that are formed later by the mechanism of secondary accretion. The density spikes must be bright gamma-ray sources. Comparison of the calculated signal from particle annihilation with the Fermi-LAT data constrains the present-day cosmological density parameter for primordial black holes with masses $M_{\rm BH}\geq10^{-8}M_\odot$ from above by values from $\Omega_{\rm BH}\leq1$ to $\Omega_{\rm BH}\leq10^{-8}$, depending on $M_{\rm BH}$. These constraints are several orders of magnitude more stringent than other known constraints.