Probing ultralight bosons with binary black holes

Superradiant instabilities of rotating black holes can give rise to extended bosonic clouds surrounding them — gravitational atoms. These long-lived structures serve as natural laboratories for probing putative ultralight bosons in nature.

The presence of a companion can significantly modify both the cloud's evolution and the orbital dynamics, leaving a trail of feedback effects that require detailed modeling. Within the worldline effective field theory framework, we develop a systematic treatment of binaries on generic (eccentric and inclined) orbits.

Interestingly, for both dense and dilute clouds, we find different types of smoking-gun phenomena — the destabilization of equatorial orbits and the growth of eccentricity in the former, and negative Love numbers in the latter. Our framework allows for a precise characterization of the imprints of ultralight particles in gravitational-wave signals from binary black holes — signatures that are within reach of upcoming observatories such as LISA, Cosmic Explorer and the Einstein Telescope.