Crowdsourcing Gravitational Waves from Superradiance: Black Holes as Extreme Axion Laboratories

Black hole (BH) superradiance is a powerful probe of ultralight axions. If nature contains a boson with a mass of order 1e-12 eV, vacuum fluctuations will lead to its efficient production around spinning stellar mass BHs, forming a gravitational atom that both drains the BH spin and decays to produce near-monochromatic gravitational waves. Existing superradiance constraints derive primarily from spin measurements of a handful of identified BHs, but in this talk I will present a detailed study of the understudied population level effect, highlighting gravitational waves arising from both the 100 million BHs in the Milky Way and the stochastic signal from axion clouds throughout the universe. We study the impact of a broad range of systematic uncertainties on the BH properties and compute the projected axion sensitivity for LIGO, as well as future instruments including high-frequency detectors, which, in the most optimistic cases, could reach the lowest masses available to the projected sensitivity of modern axion dark matter searches. I will then discuss related recent work illustrating how superradiance of BH populations can continue to offer us multiple pathways for further probing axions.