Crowdsourcing Gravitational Waves from Superradiant Axions

TL;DR

This paper explores how gravitational wave observations from black holes can detect ultralight axions, analyzing population effects and systematic uncertainties to project sensitivities of current and future detectors.

Contribution

It introduces a population-level analysis of gravitational waves from black hole superradiance to improve axion detection prospects across multiple observatories.

Findings

LIGO can probe axion masses from ~10^{-13} eV to 4×10^{-12} eV.

Future high-frequency detectors could extend sensitivity beyond 10^{-10} eV.

Black hole populations influence the potential to detect or constrain axion properties.

Abstract

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.