Extremely light bosons are hypothetical particles whose mass is predicted to be less than a billionth of the mass of an electron. They communicate relatively little with the environment and have so far avoided searches to confirm their existence. If they existed, ultralight bosons like axions would probably be a form of dark matter, mysterious, invisible things that make up 85 percent of the matter in space.
Now, physicists from MIT’s LIGO lab were looking for ultralight bosons using black holes – objects that are astonishingly larger in size than the particles themselves. According to the predictions of quantum theory, a black hole of a certain mass should draw in clouds of ultralight bosons, which in turn should jointly slow down the rotation of the black hole. If particles exist, then all black holes of a certain mass should have relatively low turns.
But physicists have found that two previously discovered black holes rotate too fast to be affected by ultralight bosons. Due to its large turns, the existence of black holes precludes the existence of ultralight bosons with masses between 1.3×10-13 electron volts and 2.7×10-13 electron volts – about five millionths of an electron’s mass.
Team results, published today in Letters of physical examination, further narrow the search for axions and other ultralight bosons. The study is also the first to use the rotation of black holes discovered by LIGO and Virgo and gravitational wave data to search for dark matter.
“There are different types of bosons, and we’ve researched them,” says co-author Salvatore Vitale, an assistant professor of physics at MIT. “There may be others, and we can apply this analysis to the growing set of data that LIGO and Virgo will provide over the next few years.”
Vitale’s co-authors are lead author Kwan Yeung (Ken) Ng, a graduate student at MIT’s Kavli Institute of Astrophysics and Space Research, along with researchers at Utrecht University in the Netherlands and Chinese University in Hong Kong.
Ultralight bosons are sought in a wide range of superlight masses, from 1×10-33 electron volt at 1×10-6 electron volts. Scientists have so far used table experiments and astrophysical observations to rule out remnants of this vast space of possible masses. Since the early 2000s, physicists have suggested that black holes could be another means of detecting ultralight bosons because of an effect known as superradiation.
If there are ultralight bosons, they could communicate with a black hole under the right circumstances. Quantum theory claims that on a very small scale, particles cannot be described by classical physics or even as individual objects. This scale, known as the Compton wavelength, is inversely proportional to the mass of the particles.
As ultralight bosons are extremely light, it is predicted that their wavelength will be extremely large. For a certain range of boson masses, their wavelength can be comparable to the size of a black hole. When this happens, superradiation is expected to develop rapidly. Ultralight bosons are then formed from the vacuum around the black hole, in sufficient quantities to pull tiny particles collectively around the black hole and slow down its rotation.
“If you jump on a carousel and then lower it, you can steal energy from it,” says Vitale. “These bosons do the same thing with a black hole.”
Scientists believe that this slowdown of bosons can occur over thousands of years – relatively quickly in astrophysical time frames.
“If there are bosons, we would expect the old black holes of appropriate mass not to have large turns, because boson clouds would pull out the majority,” says Ng. “This implies that the discovery of a black hole with large screws can rule out the existence of bosons with certain masses.”
Turn around, turn down
Ng and Vitale applied this reasoning to black hole measurements made by LIGO, the laser interferometer gravitational wave observatory, and its accompanying Virgo detector. Detectors “listen” to gravitational waves or echoes of distant cataclysms, such as merging black holes, known as binary files.
In their study, the team reviewed all 45 black hole binaries reported to date by LIGO and Virgo. The masses of these black holes – between 10 and 70 times the mass of the sun – indicate that, if they acted with ultralight bosons, the particles would be between 1×10-13 electron volts and 2×10-11 electrovolts in mass.
For each black hole, the team calculated the rotation it should have if the black hole was rotated by ultralight bosons within the appropriate mass range. Two black holes emerged from their analysis: GW190412 and GW190517. Just as there is a maximum speed for physical objects – the speed of light – so there is an upper spin at which black holes can rotate. GW190517 spins close to that maximum. The researchers calculated that there would be ultralight bosons that would pull its rotation up to twice.
“If they existed, these things would suck a lot of momentum,” Vitale says. “They’re really vampires.”
The researchers also explained other possible scenarios for creating large turns of black holes, while allowing the existence of ultralight bosons. For example, a black hole could be rotated by bosons, but was then accelerated again by interactions with the surrounding accretion disk — a disk of matter from which a black hole could suck energy and momentum.
“If you’re doing math, you’ll find that it’s too long for a black hole to rotate to the level we see here,” Ng says. “So we can safely ignore this spin-up effect.”
In other words, large rotations of black holes are unlikely to be the result of an alternative scenario in which ultralight bosons also exist. Given the masses and large turns of both black holes, the researchers were able to rule out the existence of ultralight bosons with masses between 1.3×10-13 electron volts and 2.7×10-13 electron volts.
“We basically excluded some types of bosons in this mass range,” says Vitale. “This paper also shows how gravitational wave detections can contribute to the search for elementary particles.”
The researchers suggest that LISA should be able to see ultralight bosons near supermassive black holes
Ken KY Ng et al. Searching for ultralight bosons within the spin measurement population of binary black hole junctions, Physical examination D (2021). DOI: 10.1103 / PhysRevD.103.063010
Provided by the Massachusetts Institute of Technology
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