What if Planet 9 is a Primordial Black Hole? (2019)
This is a very &quot;dense&quot; paper. So: if dark matter is something that annihilates itself, that&#x27;s a two-particle interaction, so its macroscopic rate scales as density squared ρ². If there&#x27;s a black hole in the solar system, the dark matter halo immediately surrounding it would be compressed far denser than around a normal object, so it&#x27;d have a much brighter annihilation signal. Nothing&#x27;s known about dark matter annihilation. They speculate, if its cross section is large enough, this annihilation could be observable by gamma-ray telescopes like Fermi or the upcoming CTA. And that would distinguish a black hole from something else. That&#x27;s all I could understand. It&#x27;d be a discovery of a solar system black hole, and new dark matter physics, two-for-one.<p>I&#x27;m curious what math led to that funny exponent in eq.(5): ρ(r) ~ r^{-9&#x2F;4}.