Impact of the cosmic neutrino background on black hole superradiance

We assess the effect of the cosmic neutrino background (C nu B) on superradiant instabilities caused by an ultralight scalar field around spinning black holes (BHs). When the scalar couples to neutrinos via a Yukawa interaction, thermal corrections from the C nu B induce a quartic self-interaction and an effective mass term for the scalar. We show that, for Yukawa couplings as small as y phi nu similar to 10-16 (for astrophysical BHs) or 10-20 (for supermassive BHs), the quartic term can quench the instability and set observable bounds, even if the scalar does not constitute dark matter. We assess the robustness of these constraints against several sources of uncertainty, including gravitational focusing of relic neutrinos, galactic clustering, and nonlinear backreaction. An enhanced local neutrino density weakens the bounds by up to an order of magnitude compared to a uniform background, yet the induced self-interaction remains strong enough to significantly affect the superradiant dynamics. Our results open a new observational window on neutrino-coupled scalars via BH superradiance.