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Register a new userConstraints on ultralight scalar bosons within black hole spin measurements from LIGO-Virgos GWTC-2
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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.
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Ultralight bosons can form clouds around rotating black holes if their Compton wavelength is comparable to the black hole size. The boson cloud spins down the black hole through a process called superradiance, lowering the black hole spin to a characteristic value. It has been suggested that spin measurements of the black holes detected by ground-based gravitational-wave detectors can be used to constrain the mass of ultralight bosons. Unfortunately, a measurement of the individual black hole spins is often uncertain, resulting in inconclusive results. Instead, we use hierarchical Bayesian inference to combine information from multiple gravitational-wave sources and obtain stronger constraints. We show that hundreds of high signal-to-noise ratio gravitational-wave detections are enough to exclude (confirm) the existence of non-interacting bosons in the mass range $left[10^{-13},3times 10^{-12}right]~rm{eV}$ $left([10^{-13},10^{-12}]~rm{eV}right)$. The precise number depends on the distribution of black hole spins at formation and the mass of the boson. From the few uninformative spin measurements of binary black hole mergers detected by LIGO and Virgo in their first two observing runs, we cannot draw statistically significant conclusions.
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.
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$.
We have examined gravitational wave echo signals for nine binary black hole merger events observed by Advanced LIGO and Virgo during the first and second observation runs. To construct an echo template, we consider Kerr spacetime, where the event horizon is replaced by a reflective membrane. We use frequency-dependent reflection rate at the angular potential barrier, which is fitted to the numerical data obtained by solving Teukolsky equations. This reflection rate gives a frequency-dependent transmission rate that is suppressed at lower frequencies in the template. We also take into account the overall phase shift of the waveform as a parameter, which arises when the wave is reflected at the membrane and potential barrier. Using this template based on black hole perturbation, we find no significant echo signals in the binary black hole merger events.
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.
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