Searching for Ultralight Axions with Gravitational Waves

I will discuss how black holes can become nature's laboratories for new ultralight particles and ongoing observations of gravitational waves can inform models of particle physics. When a particle's Compton wavelength is comparable to the horizon size of a black hole, energy and angular momentum from the black hole are converted into exponentially growing clouds of bosons, creating a gravitational atom in the sky. Theories beyond the Standard Model often include new, light, feebly interacting particles -- including the QCD axion -- whose discovery requires novel observations and search strategies. I will show how previously open parameter space of axions which interact only gravitationally can be constrained by observations of rapidly spinning black holes. I will also show how such `gravitational atoms' may source up to thousands of monochromatic gravitational wave signals, with searches underway in current data, enabling LIGO to discover or exclude new particles.