It's still unclear what dark matter is, but now we know what it isn't

April 14, 2021
Physicists have concluded that some masses of boson particles don't actually exist. (Unsplash/John Paul Summers)

Physicists have concluded that some masses of boson particles don't actually exist. (Unsplash/John Paul Summers)

Physicists have concluded that some masses of boson particles — members of the things-that-could-explain-dark-matter club — don't actually exist, meaning the parameters for locating the presumably vast but hypothetical material just became more refined.

Working in collaboration with the Laser Interferometer Gravitational-Wave Observatory (LIGO), which is known for its detection of gravitational waves and consequent evidence for the existence of black holes in 2015, the scientists published their findings April 14 in Physical Review Letters after noticing something peculiar: records of two black holes that are spinning way too quickly. 

Led by co-author Kwan Yeung (Ken) Ng, a graduate student at the Massachusetts Institute of Technology's Kavli Institute for Astrophysics and Space Research, the team concluded that the extremely fast spin is due to a lack of nearby bosons. According to the researchers, these particles, which have less than a billionth of the mass of an electron, should surround black holes and also slow them down.

They're also believed to contribute to the elusive identity of dark matter. Even though dark matter and energy collectively account for nearly 95% of the mass of the universe, humans have never located a single particle of it.

"[Bosons] could be dark matter particles, or they could be something that people call axions, which are proposed particles that could solve problems with the magnetic dipoles of particles," Salvatore Vitale, an assistant professor of physics at MIT, told The Academic Times. "Because they can be any of these things, that means they could also have an incredibly broad range of masses." 

Those masses are specific to each black hole. Though it ultimately depends on a case-by-case basis, Vitale explained that lighter black holes tend to have heavier bosons, and vice versa. These particles slow the mysterious object's spin through a process similar to the standard principle called conservation of angular momentum

Vitale painted a picture of the law by conjuring an analogy: Suppose a carousel is spinning, and a child steps onto it briefly. If the child then jumps off the carousel, that jump will have been energized by the spin of the ride. Perhaps counterintuitively, the ride technically slowed down as the child made the leap. That's because energy devoted to the jump was taken away from the energy of the carousel's spin.

"Energy is conserved in the universe, so if you gain energy, that energy must come from somewhere," Vitale explained. "You have converted angular momentum of the carousel to kinetic energy of the thing that fell off. That's Newton — there's nothing crazy there."

Conservation of angular momentum isn't a new theory; it's something routinely taught in high school physics. However, when the notion is applied to bosons and black holes, it becomes a little more complex.

"It turns out that black holes can do exactly the same thing," Vitale continued. "If you throw your garbage bin close enough to the black hole, the garbage will fall inside a black hole, [but] the bin will bounce back from the black hole. If you do the math, the bin comes back at you with more energy."

In this analogy, the garbage bin represents a boson. These particles get caught in the spin of a black hole but presumably exit it periodically. However, because of the intense gravitational force of the hole itself, the bosons are pulled back in. Imagine billions and billions of the particles taking part in this process and stealing the black hole's spin-energy over and over again — that would slow down the phenomenon, and quite significantly. 

Because the data regarding the two black holes pinpointed in the new study doesn't indicate any boson drag for the several millions of years of their existence, it can be inferred that there weren't any bosons — or, rather, any bosons of the particular range of masses that those black holes would require.

"You can kind of reverse engineer that and say, 'Look, I just found two black holes that actually have a very large spin, so if the bosons existed, they should have spun them down,'" Vitale said. "You can use that as a proof that the bosons of those mass ranges did not exist."

The team measured spin rates of the black holes in terms of percentage, with 100%, or 1, being the maximum. The limit stems from the event horizon found around the black hole, the separation between the observable universe and the black hole's singularity. The latter isn't describable by human physics. 

If the black hole spins above the maximum, it starts to lose the horizon, which would violate all kinds of physical laws. For instance, it would impact causality, or classical rules about linearity, such as time only moving forward in one dimension. But these black holes don't move faster than the maximum; they just approach it.

"For pretty much the overwhelming majority of black holes that [have been] detected so far, the spins … were consistent with being zero or 0.1 — so, 10%, 20% of the maximum," Vitale said. "By contrast, these two objects that we take interest in, one of them is consistent with spinning at this maximum limit … and the other one is around 40% of the maximum allowed," in terms of the central value and in spite of natural noise in the measurement.

As the team's findings indicate, LIGO's concrete information about gravitational waves is invaluable. It allows researchers to use them as a metric, and according to Vitale, those measurements shed light on specific properties — in this case, spin — of the black holes. That's because the phenomena are known to give rise to such waves in the first place. It's like a reverse calculation, and one that's very clean and precise, the researcher says.

"If you are sitting on the shore of a lake, and someone throws a stone at some distance from you, at some point you will receive the waves on the water that were generated by the stone at the surface of the lake — and that tells us something," Vitale said. "The rocks are these big black holes colliding with one another, somewhere else, far away."

The paper, "Constraints on ultralight scalar bosons within black hole spin measurements from the LIGO-Virgo GWTC-2," published April 14 in Physical Review Letters, was authored by Ken K. Y. Ng and Salvatore Vitale, Massachusetts Institute of Technology; Otto A. Hannuksela, Nikhef National Institute for Subatomic Physics; and Tjonnie G. F. Li, Chinese University of Hong Kong. ​​​​

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