How light from black holes is narrowing the search for axions

The search for a hypothetical subatomic particle that could signal new physics just narrowed a bit — thanks to the light swirling around a gargantuan black hole in another galaxy.

The lightweight particle — dubbed the axion — has been proposed as a solution to the mystery of why the universe has so little antimatter and as a candidate for the elusive dark matter that fills the cosmos (SN: 3/24/20; SN: 3/6/20). The twisting and chaotic environs of galaxy M87’s central black hole, the first black hole to have its picture taken, are thought to encode information about such particles.

Now, the particulars of how light around M87’s black hole is oriented could rule out the likelihood of axion particles in a specific mass range, researchers report March 17 in Nature Astronomy. This study also shows that scientists could use a similar method in upcoming astrophysical observations to search for these particles in an assortment of masses.

“It’s a very exciting idea,” says physicist Benjamin Safdi of the University of California, Berkeley, who was not involved with this study. “They’ve come up with a new method, and they’ve shown that this method could in principle work.”

First proposed in the late 1970s, axions have yet to be found in experiments. Theoretical work since that initial proposal has shown an extended family of axions could exist, each variety having a different mass but all interacting weakly with ordinary matter. In 2020, physicist Yifan Chen of the Chinese Academy of Sciences in Beijing and colleagues described a way to look for axions using observations of the light surrounding black holes.

According to theory, a rapidly spinning black hole can build up a dense clumping of axion particles in the immediate surrounding area. Precisely which types of axions get built up depends on the width of the black hole. And the supermassive black hole in M87 has the right size for brewing a stew of ultralightweight axionlike particles. If this black hole indeed kicked up such a cloud, that would change the orientation, or polarization, of the light coming from that region. In particular, the polarization would wobble over time.

Unfortunately, no one had any images of polarized light from a black hole to examine — until last year. That’s when the Event Horizon Telescope, or EHT, an Earth-spanning network of radio telescopes, revealed its image of the polarized light around the supermassive black hole at the center of M87 (SN: 3/24/21).

This “is precisely the information that we need in order to carry out this theoretical proposal,” says particle physicist Yue Zhao of the University of Utah in Salt Lake City. “We have a very extreme condition that can produce a huge amount of axions, and we have the right tool to study the signature of the axion.”

So Zhao, Chen and colleagues examined the EHT data for a time-varying change in the polarization’s direction. While a cloud of axions would alter the direction, so too will the active and turbulent region around the black hole. This is a “kind of unavoidable background that we have to deal with,” Zhao says. Once they removed that from the total signal, they found that there is not enough of an additional wobble to say any signal could have come from the axion cloud. They ruled out the existence of ultralightweight axions with a mass of about 10 billionths of a billionth of a billionth of an electron’s mass.

But the same technique could be used to hunt for other axionlike particles. “The larger black hole you have, the lighter is your mass,” Zhao says. The physicists hope to use the EHT’s future observations of other black holes to look for axions of different masses. One black hole on EHT’s radar is the behemoth at the center of our own galaxy, Zhao notes, which is about one-thousandth the mass of M87’s (SN: 6/5/19). If our galaxy’s monster black hole has a cloud of axions, those would be heavier particles.

“This idea of looking for these axionlike particles, is, in my opinion, the most exciting thing happening in particle physics at the moment,” says Safdi.