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Register a new userMultiband gravitational-wave searches for ultralight bosons
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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 could spontaneously form clouds around astrophysical black holes, leading to potent emission of continuous gravitational waves that could be detected by instruments on the ground and in space. Although this scenario has been thoroughly studied, it has not been yet appreciated that both types of detector may be used in tandem (a practice known as multibanding). In this paper, we show that future gravitational-wave detectors on the ground and in space will be able to work together to detect ultralight bosons with masses $25 lesssim mu/left(10^{-15}, mathrm{eV}right)lesssim 500$. In detecting binary-black-hole inspirals, the LISA space mission will provide crucial information enabling future ground-based detectors, like Cosmic Explorer or Einstein Telescope, to search for signals from boson clouds around the individual black holes in the observed binaries. We lay out the detection strategy and, focusing on scalar bosons, chart the suitable parameter space. We study the impact of ignorance about the systems history, including cloud age and black hole spin. We also consider the tidal resonances that may destroy the boson cloud before its gravitational signal becomes detectable by a ground-based follow-up. Finally, we show how to take all of these factors into account, together with uncertainties in the LISA measurement, to obtain boson mass constraints from the ground-based observation facilitated by LISA.
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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.
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 leads to significant orbital precession when it forms an eccentric binary with a second compact object. Dynamically formed black hole binaries or pulsar-black hole binaries are typically eccentric during their early inspirals. We show that the large orbital precession can generate distinct and observable signature in their gravitational wave or pulsar timing signals.
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 around the host black hole. If the mass ratio of a compact merger is sufficiently small, the bosonic cloud will survive the inspiral phase of a binary merger and can affect the quasinormal-mode frequencies of the perturbed black hole and bosonic cloud system. In this work, we compute the shift of gravitational QNMFs of a rotating black hole due to the presence of a surrounding bosonic cloud. We then perform a mock analysis on simulated LISA observational data containing injected ringdown signals from supermassive black holes with and without a bosonic cloud. We find that with less than an hour of observational data of the ringdown phase of nearby supermassive black holes such as Sagittarius A* and M32, we can rule out or confirm the existence of cloud-forming ultralight bosons of mass $ sim 10^{-17} rm eV$.
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 characteristic value that depends on the bosons mass as well as on the black hole mass and spin. Therefore, a measurement of a black hole mass and spin can be used to reveal or exclude the existence of such bosons. Using the black holes released by LIGO and Virgo in their GWTC-2, we perform a simultaneous measurement of the black hole spin distribution at formation and the mass of the scalar boson. We find that the data strongly disfavors the existence of scalar bosons in the mass range between $1.3times10^{-13}~mathrm{eV}$ and $2.7times10^{-13}~mathrm{eV}$ for a decay constant $f_agtrsim 10^{14}~mathrm{GeV}$. The statistical evidence is mostly driven by the two {binary black holes} systems GW190412 and GW190517, which host rapidly spinning black holes. The region where bosons are excluded narrows down if these two systems merged shortly ($sim 10^5$ years) after the black holes formed.
The direct measurement of gravitational waves is a powerful tool for surveying the population of black holes across the universe. The first gravitational wave catalog from LIGO has detected black holes as heavy as $sim50~M_odot$, colliding when our Universe was about half its current age. However, there is yet no unambiguous evidence of black holes in the intermediate-mass range of $10^{2-5}~M_odot$. Recent electromagnetic observations have hinted at the existence of IMBHs in the local universe; however, their masses are poorly constrained. The likely formation mechanisms of IMBHs are also not understood. Here we make the case that multiband gravitational wave astronomy --specifically, joint observations by space- and ground-based gravitational wave detectors-- will be able to survey a broad population of IMBHs at cosmological distances. By utilizing general relativistic simulations of merging black holes and state-of-the-art gravitational waveform models, we classify three distinct population of binaries with IMBHs in the multiband era and discuss what can be observed about each. Our studies show that multiband observations involving the upgraded LIGO detector and the proposed space-mission LISA would detect the inspiral, merger and ringdown of IMBH binaries out to redshift ~2. Assuming that next-generation detectors, Einstein Telescope, and Cosmic Explorer, are operational during LISAs mission lifetime, we should have multiband detections of IMBH binaries out to redshift ~5. To facilitate studies on multiband IMBH sources, here we investigate the multiband detectability of IMBH binaries. We provide analytic relations for the maximum redshift of multiband detectability, as a function of black hole mass, for various detector combinations. Our study paves the way for future work on what can be learned from IMBH observations in the era of multiband gravitational wave astronomy.
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