Black Hole Pinball: How Spin Creates Dark Matter express
Light passing by a black hole falls into a gravitational trap: its trajectory curves and the particle bounces along a nearly closed orbit, like a pinball. The longer this frantic dash, the more likely it is that jolts from magnetic fields transform the photon into an axion — an invisible particle that likely makes up dark matter. This possibility was predicted by Frank Wilczek.
If the high-energy light from active black holes dims more than expected, then some of it has converted into axions. Thus, cosmic pinball becomes a cheap laboratory for hunting dark matter, replacing ground-based experiments.
Original abstract
as written by the authors arXiv:2605.22696
We investigate photon-axion conversion in the vicinity of rotating Kerr black holes where strong gravity traps photons on near-circular trajectories, effectively enhancing the path length. We explore the observable signatures of such a conversion near the photon region. The process, driven by ambient magnetic fields, is significantly more efficient around supermassive black holes such as M87*, since the luminosity of photons increases with the mass of the BH. By numerically evaluating photon path lengths (on which the conversion depends), we analyze how key parameters-photon frequency, axion mass, photon-axion coupling, magnetic field strength, plasma density, and black hole spin-affect the conversion probability and the resultant dimming of photon spectral luminosity. We find that the conversion is most efficient at high frequencies (X-rays and gamma rays), while the frequency window associated with efficient conversion widens with an increase in the photon-axion coupling and a decrease in the electron density and the axion mass. The magnitude of dimming of the photon spectral luminosity depends primarily on the magnetic field, the photon-axion coupling and the BH spin. Our study reveals that rotating black holes generally exhibit enhanced dimming compared to static ones. Thus, if future telescopes achieving a resolution $$\sim 10^{-5}$$ arcsec in the X-ray/gamma-ray band detect a dimming of the photon spectral luminosity, then they can provide interesting constraints on the axion mass and its coupling with photons.
In a pinball machine, the ball can bounce around for several seconds; near the black hole M87*, light lingers for just a millisecond, but in that time it covers tens of thousands of kilometers, and that instant is enough to spawn an axion, which then passes unimpeded through an entire planet.