Probing the existence of ultralight bosons with a single gravitational-wave measurement

We demonstrate that gravitational waves from binary systems can provide smoking gun evidence for ultralight bosons (such as ultralight axions). If ultralight bosons exist, they will form "clouds" by extracting rotational energy from astrophysical black holes of size comparable to the boson Compton wavelength through superradiant instabilities. The properties of the cloud are intimately related with those of the black hole, and they are encoded in the gravitational waves emitted by compact objects orbiting the black hole/cloud system. We show that a single measurement of these waves yields at least three independent ways to estimate the mass of the boson from the cloud. Gravitational wave observations by the Laser Interferometric Space Antenna (LISA) could either confirm the existence of ultralight bosons and measure their mass via "consistency tests" similar to the general relativity tests routinely performed with binary pulsars, or rule out the cloud's existence