Self-organized criticality in boson clouds around black holes
  Boson clouds around black holes exhibit interesting physical phenomena
through the Penrose process of superradiance, leading to black hole spin-down.
Axionic clouds are of particular interest, since the axion Compton wavelength
could be comparable to the Schwarzschild radius, leading to the formation of
"gravitational atoms" with a black hole nucleus. These clouds collapse under
certain conditions, leading to a "Bosenova". We model the dynamics of such
unstable boson clouds by a simple cellular automaton and show that it exhibits
self-organized criticality. Our results suggest that the evolution through the
black hole Regge plane is due to self-organized criticality.
