The search for a hypothetical subatomic particle that could signal new physics has narrowed somewhat, thanks to light swirling around a gigantic black hole in another galaxy.
The light particle, called the axion, has been proposed as a solution to the mystery of Why does the universe have so little antimatter? and as a candidate for elusive dark matter that fills the cosmosSerial number: 03/24/20; Serial number: 3/6/20). The twisted and chaotic surroundings of the central black hole of the galaxy M87, the first black hole to be photographed, are thought to encode information about such particles.
Now the details of how light is oriented around M87’s black hole could rule out the probability of axion particles in a specific mass rangeresearchers 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 a variety of masses.
“It’s a very exciting idea,” says physicist Benjamin Safdi of the University of California, Berkeley, who was not involved in this study. “They came up with a new method and showed that, in principle, this method could work.”
First proposed in the late 1970s, axions have yet to be found in experiments. Theoretical work since that initial proposal has shown that an extended family of axions could exist, each variety with a different mass but weakly interacting with ordinary matter. In 2020, physicist Yifan Chen of the Chinese Academy of Sciences in Beijing and his colleagues described a way to search for axions. using observations of light surrounding black holes.
According to theory, a rapidly spinning black hole can accumulate a dense accumulation of axion particles in the immediate surrounding area. Precisely which types of axions accumulate depends on the width of the black hole. And the supermassive black hole in M87 is just the right size to make a stew of ultralight axion-like particles. If this black hole did indeed kick up such a cloud, it 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 polarized light images of 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 polarized light image around the supermassive black hole at the center of M87 (Serial number: 03/24/21).
This “is precisely the information we need 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 large number of axions, and we have the right tool to study the signature of the axion.”
So Zhao, Chen and their colleagues examined the EHT data for a time-varying change in polarization direction. While an axion cloud would alter the direction, so would the active, turbulent region around the black hole. This is a “kind of unavoidable context that we have to deal with,” says Zhao. Once they removed that from the total signal, they found that there isn’t enough additional oscillation to say that any signal could have come from the axion cloud. They ruled out the existence of ultralight axions with a mass of about 10 trillionths of a trillionth of a trillionth of the mass of an electron.
But the same technique could be used to search for other axion-like particles. “The bigger the black hole you have, the lighter your mass is,” says Zhao. Physicists hope to use future EHT observations of other black holes to search for axions of different masses. A black hole on the EHT radar is the giant at the center of our own galaxyZhao notes, which is about a thousandth of the mass of M87 (Serial number: 5/6/19). If our galaxy’s monster black hole has a cloud of axions, those would be heavier particles.
“This idea of looking for these axion-like particles is, in my opinion, the most exciting thing going on in particle physics right now,” says Safdi.