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Ultralight bosons can induce superradiant instabilities in spinning black holes, tapping their rotational energy to trigger the growth of a bosonic condensate. Possible observational imprints of these boson clouds include (i) direct detection of the nearly monochromatic (resolvable or stochastic) gravitational waves emitted by the condensate, and (ii) statistically significant evidence for the formation of holes at large spins in the spin versus mass plane (sometimes also referred to as Regge plane) of astrophysical black holes. In this work, we focus on the prospects of LISA and LIGO detecting or constraining scalars with mass in the range $m_sin [10^{-19},,10^{-15}]$ eV and $m_sin [10^{-14},,10^{-11}]$ eV, respectively. Using astrophysical models of black-hole populations calibrated to observations and black-hole perturbation theory calculations of the gravitational emission, we find that, in optimistic scenarios, LIGO could observe a stochastic background of gravitational radiation in the range $m_sin [2times 10^{-13}, 10^{-12}]$ eV, and up to $10^4$ resolvable events in a $4$-year search if $m_ssim 3times 10^{-13},{rm eV}$. LISA could observe a stochastic background for boson masses in the range $m_sin [5times 10^{-19}, 5times 10^{-16}]$, and up to $sim 10^3$ resolvable events in a $4$-year search if $m_ssim 10^{-17},{rm eV}$. LISA could further measure spins for black-hole binaries with component masses in the range $[10^3, 10^7]~M_odot$, which is not probed by traditional spin-measurement techniques. A statistical analysis of the spin distribution of these binaries could either rule out scalar fields in the mass range $sim [4 times 10^{-18}, 10^{-14}]$ eV, or measure $m_s$ with ten percent accuracy if light scalars in the mass range $sim [10^{-17}, 10^{-13}]$ eV exist.
Gravitational waves may be one of the few direct observables produced by ultralight bosons, conjectured dark matter candidates that could be the key to several problems in particle theory, high-energy physics and cosmology. These axionlike particles
Ultralight bosons can be abundantly produced through superradiance process by a spinning black hole and form a bound state with hydrogen-like spectrum. We show that such a gravitational atom typically possesses anomalously large mass quadrupole and l
Clouds of ultralight bosons - such as axions - can form around a rapidly spinning black hole, if the black hole radius is comparable to the bosons wavelength. The cloud rapidly extracts angular momentum from the black hole, and reduces it to a charac
Assuming that, for a given source of gravitational waves (GWs), we know its sky position, as is the case of GW events with an electromagnetic counterpart such as GW170817, we discuss a null stream method to probe GW polarizations including spin-0 (sc
The ultralight boson is a promising candidate for dark matter. These bosons may form long-lived bosonic clouds surrounding rotating black holes via superradiance, acting as sources of gravity and affecting the propagation of gravitational waves aroun