Searching for Ultralight Dark Matter with a Supermassive Black Hole

They say a picture’s worth a thousand words, but very few are worth 5 million gigabytes. In April 2019 the Event Horizon Telescope collaboration, an international team of over 200 scientists, unveiled the first-ever picture of a black hole (or more specifically–the event horizon around it). Capturing an image of M87* was a supermassive accomplishment in astrophysics, but research in Physical Review Letters shows how it could change our perceptions of dark matter.

The force of gravity around a black hole is so large that even light is warped, creating this orange halo of light. Credit: NSF/EHT

Seeing the (w)Hole Picture

Dark matter comprises nearly 27% of our universe, but we can’t see or observe any of it–it lies in a realm beyond our perception. Its existence has been theorized since the 1930s, but all efforts to directly detect it have been unsuccessful. In spite of its abundance, dark matter’s existence has only been proven indirectly, making its presence known through its gravitational effects on celestial bodies.

Numerous candidates for dark matter have been proposed throughout the years, of varying masses, properties, and degrees of name catchy-ness. There’s MACHOs, WIMPs, axions, Kaluza-Klein particles, gravitinos, and many more. In a manner quite similar to the 2019 Democratic Primaries, there are some frontrunners, but the field remains incredibly packed.

Like dark matter, blacks holes are invisible, but for an entirely different reason. Their masses are so large that the force of gravity can even warp particles of light around them, creating the shadowy halo image captured by the EHT. About 55 million light-years away, this supermassive black hole at the center of the Messier 87 galaxy is one of the largest black holes in the universe. To give you an idea of this insane scale, M87* is about 6.5 billion times the mass of our sun.

Like most of the world, particle physicists Hooman Davoudiasl and Peter Denton at Brookhaven National Laboratory were fascinated by the image, and what it represents for science itself.

For them though, the image also sparked an idea.

“When this image was released, it was quite a fascinating achievement–that a group of people could image a black hole and make it much more of a real concept” said Davoudiasl, “Its size is quite substantial and there was some indication of its spin, but the size of [M87*] is in a region of parameter space where certain dark matter particles can be constrained.”

Black holes, as a one-way exit from our universe, collectively capture our imagination. Possibly, as the paper states, “Black holes are at the same time simple and mysterious. They are characterized by only a few parameters – mass, spin, and charge”. The spin refers to a black hole’s angular momentum, a parameter that considers both an object’s mass and angular velocity. A supermassive spinning black hole, for example, would have a high angular momentum. Picture a spinning top. Now imagine that top has the mass of six and a half billion suns, and spins nearly at the speed of light…that’s M87*.

Diagram of a spinning black hole: Credit: nrumiano

Black Holes, Particles, and Superradiance

So how does one use something as unimaginably large as a black hole to look at something as incredibly tiny as a particle? It all has to do with a concept called superradiance. As the black hole spins, it can transfer its energy and momentum to a cloud of bosons surrounding it. As the particles absorb the energy, they can significantly reduce the speed of rotation of the black hole.

“All particles have a characteristic wavelength, according to the particle-wave duality.” said Denton, “Usually, we don’t think about this principle, but we start to think about it when the particles are really light, because the wavelengths are really long.”

For superradiance to occur, the wavelength of the particle must be close in size to the radius of the black hole. Scientists have used this concept to study dark matter around black holes before, but since M87* is the largest black hole measured, it can be used to look at the smallest possible masses of dark matter. Because M87*’s angular momentum is so large, any significant depletion in spin would be apparent in their observations.

The most unproblematic picture of dark matter we could find. 

One class of dark matter in the range of M87* is called fuzzy dark matter. Its name does not refer to its texture (to my disappointment), but to its behavior. This type of matter is almost unimaginably light, far smaller than even a neutrino. Their small mass means that rather than behaving like individual particles, they behave more like one murky mass of fuzz. For astrophysicists, this candidate is so appealing because unlike cold dark matter, a fuzzy dark matter model can produce galaxies with a distribution more consistent with what we actually observe.

If fuzzy dark matter particles were hanging around M87* (a rather poor place for any particle to chill), they would be depleting its spin by a lot. The large spin of M87* indicates that superradiance isn’t occurring, meaning that a certain mass range of fuzzy dark matter particles cannot exist around the event horizon, or anywhere. This doesn’t disprove the existence of all fuzzy dark matter, but it strongly questions the presence of the heaviest fuzzy particles.

The mass parameter space that black holes have measured. Pink: The mass range corresponding to M87*. Orange: the mass range of fuzzy dark matter (FDM). Credit: APS/Alan Stonebreaker (via Davoudiasl and Denton, 2019)

“It is quite remarkable that one can use some of the most massive objects in the universe to probe the lightest particles that one can think of.” wrote Dr. Richard Brito in an email, a Marie Curie Fellow researching superradiance and dark matter at the Sapienza University of Rome.

What makes it even more interesting, is dark matter isn’t the only particle that this study constrains. By their reasoning, any particle within this mass range; whether its dark matter or not, could not exist without harnessing the energy of the black hole. This means that as particle physicists continue to search the universe for its particle building blocks, they’re unlikely to find anything in this mass range.

“With the EHT, we now can look at black holes in a whole different light” wrote Sophia Nasr in an email, a PhD student studying cosmology and dark matter at UC Irvine, “so finding that one of these–namely M87*–is a good candidate to study ultralight bosonic dark matter, is really exciting.”

Sometimes it feels like physics covers an infinitely broad span of topics, but studies like this underscore how interconnected everything is, from the heaviest objects to the lightest particles. As more and more black holes are investigated by the EHT, LIGO, and future observatories, more black holes will be investigated, bringing us closer and closer to finding these ghostly particles.

–Lissie Connors

Lissie Connors (@LissieOfficial) covers social media and writes about science for the American Physical Society and Physics Central. If she were a dark matter particle, she would totally be a WIMP.

*The supermassive black hole at the center of the Messier 87 galaxy is indicated as M87*. So, M87* refers to the black hole, M87 refers to the galaxy.

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