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Abstract

We study the gravitational-wave (GW) signatures of clouds of ultralight
bosons around black holes (BHs) in binary inspirals. These clouds, which are
formed via superradiance instabilities for rapidly rotating BHs, produce
distinct effects in the population of BH masses and spins, and, for real
fields, a continuous monochromatic GW signal. We show that the presence of a
binary companion greatly enriches the dynamical evolution of the system, most
remarkably through the existence of resonant transitions between the growing
and decaying modes of the cloud (analogous to Rabi oscillations in atomic
physics). These resonances have rich phenomenological implications for current
and future GW detectors. Notably, the amplitude of the GW signal from the
clouds may be reduced, and in many cases terminated, much before the binary
merger. The presence of a boson cloud can also be revealed in the GW signal
from the binary through the imprint of finite-size effects, such as
spin-induced multipole moments and tidal Love numbers. The time dependence of
the cloud's energy density during the resonance leads to a sharp feature, or at
least attenuation, in the contribution from the finite-size terms to the
waveforms. The observation of these effects would constrain the properties of
putative ultralight bosons through precision GW data, offering new probes of
physics beyond the Standard Model.