Black Holes are neither Particle Accelerators nor Dark Matter Probes

TL;DR

Maximally spinning black holes cannot serve as effective probes of high-energy collisions or dark matter annihilation for distant observers, as relativistic effects limit the observable energy and the most energetic photons originate far from the black hole.

Contribution

This paper demonstrates that black holes cannot be used as probes of high-energy collisions or dark matter annihilation, countering previous suggestions and clarifying observational limitations.

Findings

Maximally spinning black holes do not enhance observable collision energies.

High-energy photons from dark matter annihilation originate far from black holes.

Relativistic effects limit the energy carried to distant observers.

Abstract

It has been suggested that maximally spinning black holes can serve as particle accelerators, reaching arbitrarily high center-of-mass energies. Despite several objections regarding the practical achievability of such high energies, and demonstrations past and present that such large energies could never reach a distant observer, interest in this problem has remained substantial. We show that, unfortunately, a maximally spinning black hole can never serve as a probe of high energy collisions, even in principle and despite the correctness of the original diverging energy calculation. Black holes can indeed facilitate dark matter annihilation, but the most energetic photons can carry little more than the rest energy of the dark matter particles to a distant observer, and those photons are actually generated relatively far from the black hole where relativistic effects are negligible. Therefore, any strong gravitational potential could probe dark matter equally well, and an appeal to black holes for facilitating such collisions is unnecessary.