Crowdsourcing Gravitational Waves from Superradiant Axions

Black hole superradiance is a powerful probe of ultralight axions. If nature contains a boson with a mass of order $10^{-12}\,$eV, $\textit{mere vacuum fluctuations}$ will lead to its efficient production around spinning stellar mass black holes, forming a gravitational atom that both drains the black hole spin and decays to produce near-monochromatic gravitational waves. Existing superradiance constraints derive primarily from spin measurements of a handful of identified black holes. Here we instead present a detailed study of the population level effect: gravitational waves arising from both the 100 million black holes 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 black hole properties and compute the projected axion sensitivity for LIGO, as well as the future instruments Einstein Telescope, Cosmic Explorer, and a high-frequency Magnetic Weber Bar. We demonstrate that LIGO can robustly probe axion masses from roughly $10^{-13}\,$eV to $4 \times 10^{-12}\,$eV. If the black hole population extends to masses slightly below $5\,M_{\odot}$ - as hinted for by LIGO inspiral observations - LIGO would approach $10^{-11}\,$eV. Under that same assumption we show that a future high-frequency detector could push considerably higher, potentially beyond $10^{-10}\,$eV in the most optimistic scenarios, reaching towards the lowest masses within the projected sensitivity of axion dark matter searches.