Ultralight bosons are hypothetical particles estimated to have mass less than a billionth of the mass of an electron. They interact relatively little with their environment and have so far escaped research to confirm their existence. If they exist, ultralight bosons such as axions would likely be some form of dark matter, the mysterious, invisible substance that makes up 85% of matter in the universe.
Now, physicists at MIT’s LIGO lab have searched for ultralight bosons using black holes – objects that are mind-blowing orders of magnitude more massive than the particles themselves. According to predictions from quantum theory, a black hole of a certain mass should attract clouds of ultralight bosons, which in turn should collectively slow down the rotation of a black hole. If the particles exist, then all black holes of a particular mass should have relatively low spins.
But physicists have found that two previously detected black holes spin too fast to be affected by ultralight bosons. Due to their large spins, the existence of black holes precludes the existence of ultralight bosons with masses between 1.3×10-13 electronvolts and 2.7×10-13 electronvolts – about one quintillionth of the mass of an electron.
The team’s results, published today in Physical examination letters, further refine the search for axions and other ultralight bosons. The study is also the first to use black hole spins detected by LIGO and Virgo, along with gravitational wave data, to search for dark matter.
“There are different types of bosons, and we probed one,” says co-author Salvatore Vitale, assistant professor of physics at MIT. “There may be more, and we can apply this analysis to the growing dataset that LIGO and Virgo will provide over the next few years.”
The co-authors of Vitale are lead author Kwan Yeung (Ken) Ng, a graduate student from the Kavli Institute for Astrophysics and Space Research at MIT, as well as researchers from Utrecht University in the Countries -Bas and Chinese University of Hong Kong.
The energy of a carousel
Ultralight bosons are sought after in a wide range of ultralight masses, from 1×10-33 electronvolts to 1×10-6 electronvolts. Scientists have so far used tabletop experiments and astrophysical observations to exclude shards from this vast space of possible masses. Since the early 2000s, physicists have proposed that black holes could be another way to detect ultralight bosons, due to an effect known as superradiance.
If ultralight bosons exist, they could interact with a black hole under the right circumstances. Quantum theory postulates that at very small scales, particles cannot be described by classical physics, nor even as individual objects. This scale, known as the Compton wavelength, is inversely proportional to the mass of the particles.
Since ultralight bosons are exceptionally light, their wavelength should be exceptionally long. For a certain mass range of bosons, their wavelength can be comparable to the size of a black hole. When this happens, the superradiance should develop rapidly. Ultralight bosons are then created from the void around a black hole, in amounts large enough that the tiny particles collectively drag over the black hole and slow its rotation.
“If you jump on and then get off a carousel, you can steal the energy from the carousel,” says Vitale. “These bosons do the same to a black hole.”
Scientists believe this slowdown in the boson can occur over several thousand years – relatively quickly on astrophysical timescales.
“If the bosons exist, we would expect older black holes of the appropriate mass to not have large spins, since the boson clouds would have extracted most of them,” Ng says. “This implies that the discovery of a black hole with large spins may rule out the existence of bosons with certain masses.
Turn up, turn down
Ng and Vitale applied this reasoning to black hole measurements performed by LIGO, the Laser Interferometer Gravitational-wave Observatory, and its companion detector Virgo. The detectors “listen” to gravitational waves or the reverberations of distant cataclysms, such as the fusion of black holes, called binary.
In their study, the team examined the 45 black hole binaries reported by LIGO and Virgo to date. The masses of these black holes – between 10 and 70 times the mass of the sun – indicate that if they had interacted with ultralight bosons, the particles would have been between 1×10-13 electronvolts and 2×10-11 electronvolts in mass.
For each black hole, the team calculated the rotation it would have to have if the black hole was driven by ultralight bosons in the corresponding mass range. From their analysis, two black holes stood out: GW190412 and GW190517. Just as there is a maximum speed for physical objects – the speed of light – there is a maximum rotation that black holes can spin. GW190517 is running near this maximum. The researchers calculated that if ultralight bosons existed, they would have reduced its rotation by a factor of two.
“If they existed, these things would have absorbed a lot of angular momentum,” says Vitale. “They really are vampires.
The researchers also took into account other possible scenarios to generate the large spins of black holes, while still allowing the existence of ultralight bosons. For example, a black hole could have been spun by bosons, but then accelerated again by interactions with the surrounding accretion disk – a disk of matter from which the black hole could suck energy and momentum.
“If you do the math, you find that it takes too long to create a black hole at the level we see here,” says Ng. “So we can safely ignore this spin-up effect.”
In other words, the high spins of black holes are unlikely to be due to an alternate scenario in which ultralight bosons also exist. Given the high masses and spins of the two black holes, the researchers were able to rule out the existence of ultralight bosons with masses between 1.3×10-13 electronvolts and 2.7×10-13 electronvolts.
“We basically excluded certain types of bosons in this mass range,” says Vitale. “This work also shows how gravitational wave detections can contribute to the search for elementary particles.”
This research was funded in part by the National Science Foundation.