In an otherwise unassuming facility in northern Seattle, a supercooled tangle of tubes and wires is poised to remake the world. Coursing with liquid helium, the device's interior hovers less than a tenth of a degree above absolute zero, the coldest possible temperature. Inside the frigid cavity, carefully shielded from noise, microwave radiation can resonate like sound waves in a bell, hunting for hints of particles whizzing through that, in all other contexts, would be invisible.
Meet the Axion Dark Matter eXperiment, or ADMX: the most sensitive scientific instrument of its kind ever built. If ADMX confirms the existence of its prey, a theoretical particle called the axion, it could finally explain the massive cosmic mystery of dark matter.
Scientists have been searching for this strange substance for decades, when observations of the universe revealed that all the visible matter we see is actually outnumbered six to one by mysterious, inert stuff we can only detect via its gravitational tug. Unlike normal matter, we don't know what dark matter is made of yet. So like Ghostbusters tracking an irksome specter, scientists are using the best theories about dark matter to build elaborate detectors, trying to catch the phantom by whatever glimmer it leaves behind.
ADMX has been more than 30 years in the making; it took that long for this kind of detector to reach the sensitivity that scientists think they need to nab the axion.
“The ingenuity and cleverness it takes to design and build these experiments is just fantastic to me, and that gets so little credit,” says Stanford University physicist Helen Quinn, whose theories laid the groundwork for the axion in the 1970s. “This is an incredible piece of experimental work.”
Searching in the dark
For decades, the leading candidates for dark matter were antisocial things called weakly interacting massive particles, or WIMPs for short. Not only did some theories predict these extra particles, but they also had just the right properties you'd expect dark matter to have—a coincidence called the “WIMP miracle.” What's more, WIMPs could be plausibly tested with technology we already understood, such as particle colliders and neutrino detectors.
Despite WIMPs' allure, searches for them have so far come up empty. The silence is unnerving, to the point that some researchers are calling for a new era in the search for dark matter.
“I don't want to knock [WIMPs] at all: We need to continue to investigate that paradigm, because we haven't exhausted it yet,” says Jodi Cooley, a physicist at Southern Methodist University and a principal investigator at the SuperCDMS dark matter detector. “But while we're still investigating, we should be looking at new, fresh ideas.”
Enter the axion, the particle ADMX is hunting.
Axion's role as a dark matter candidate came about almost by accident, while scientists were seeking to explain a curious asymmetry baked into the cosmos. The early universe is theorized to have spawned the exact same amounts of matter and antimatter, two kinds of particles alike in every way except for their electric charge. The two substances annihilate each other on contact—but we're all here, which means normal matter must play by slightly different rules, letting a fraction of it outlast its bizarro twin.
We owe our existence to this imbalance, which is called CP violation. In 1977, Quinn and Roberto Peccei proposed a theory about CP violation that gave rise to an extra surprise: In follow-up studies published within a week of each other, physicists Frank Wilczek and Steven Weinberg showed that the theory would also spit out a new kind of elementary particle. They called this theoretical nugget the axion.
“The idea comes out of the Peccei-Quinn theory, but Peccei and Quinn didn't notice it,” jokes Quinn. “If I'm the mother of the axion, the axion is a foundling.”
And if the axion exists, it might be the long-sought dark matter particle. Because of the way axions theoretically would have formed in the early universe, they would have been very cold, which means that denser regions of axions wouldn't just diffuse away like smoke. This would have bought gravity enough time to work its attractive magic. As gravity made clouds of axions even denser, they could have acted as gravitational scaffolds for normal matter, despite the tininess of each single axion. In this way, axions would have seeded the first galaxies—eventually yielding stars, planets, and people.
“The reason why [axions are] so attractive is that they weren't actually postulated to solve the dark matter problem,” says Renée Hložek, a cosmologist at the University of Toronto who studies axion dark matter. “We like it when it's buy-one, get-one-free.”
Seeing the invisible
Though axions and WIMPs both have elegant theories supporting them, axions were long held as a dark-matter underdog. This difference was, in part, a practical one. If the axion really exists, physicists weren't sure at first whether it could ever be detected.
For one, theory says that axions will be absurdly light. If you had as many axions as there are grains of sand on Earth, their combined mass might equal that of a millionth of a billionth of a single sand grain. What's more, outside of gravity, axions are predicted to hardly interact with normal matter.
Leslie Rosenberg, lead scientist of the lab and professor at University of Washington.
So how could we ever find the axion, let alone understand it? In 1983, University of Florida physicist Pierre Sikivie laid out a groundbreaking gameplan to find them on Earth, assuming that axions make up the dark matter halo theorized to fringe our galaxy.
In a strong magnetic field, these axions should convert into microwave radiation, with a frequency that depends on the axion's mass. To spot this radiation, Sikivie suggested building a supercooled chamber within a strong magnetic field, where these microwaves could resonate. Researchers could then tweak this resonant frequency like turning the dial on a radio.
In theory, if physicists chose the right frequency, the microwaves spawned by axions flying through the chamber would resonate—making a tiny, detectable flash. “It's very difficult to detect, but it's not impossible,” says Sikivie. “[ADMX] probably will, in fact, detect it, in my opinion—if people can have the endurance, the perseverance.”
'I'm like a racehorse'
By 1987, Brookhaven National Laboratory turned on the first “axion haloscope,” and soon, Sikivie helped build the second. But these early detectors struggled to cancel out background noise. The tech just wasn't there yet.
Now, after more than two decades, ADMX is going where no other detector has gone before. In an April 2018 paper in Physical Review Letters, its builders announced that the device was sensitive enough to directly probe the axion's best-guess masses—the first machine ever to do so. Placed on another planet, ADMX would be so sensitive that it could pick up cellular service from Earth.
“We could easily get four bars on Mars with your cell phone—easy, no problem,” says University of Washington physicist Leslie Rosenberg, ADMX's lead scientist. Rosenberg has devoted his whole life to ADMX, living nearby just in case it malfunctions and needs repair. He hasn't taken a week-long vacation in 28 years.
“I'm like a racehorse who sees the finish line,” he says. “It has been hard, it has worn me down, [but] on the other hand, I can almost see them—I can almost see these axions. That's what keeps me going.”
Day to day, Rosenberg's quest plays out on a computer screen. Rotating crews monitor the instrument remotely, as ADMX's central cavity automatically shifts its resonance frequency every hundred seconds and listens for axions' faint blip. The instrument hums along 24-7 for nine months at a time. At this rate, it'll take ADMX about five years to scan the full range it's designed to probe.
If ADMX happens upon the right frequency, researchers will quickly know. This is one of the neat quirks of axion hunting: It's obscenely hard to find the signal, but once you do, enough data to confirm the find piles up within days.
“Once you find a needle in a haystack, it's quite clear that it's a needle as opposed to a piece of straw,” says Sikivie.
If and when they see the signal, the researchers will be well-prepared; they constantly run drills. To keep the rest of the team on its toes, some ADMX researchers are allowed to secretly send artificial signals into the detector, only to reveal them later.
“We've gone from having some tiny chance that someone was wrong and we're looking in the right place, to looking in the right place and [a detection] could be any day,” says University of Washington physicist Gray Rybka, ADMX's co-spokesperson. “Man, we'd better take this very, very seriously.”
Race to the finish
The good word about the axion seems to be spreading: A research team at Yale is building its own axion haloscope to rival ADMX, and teams in South Korea and Australia are also on the move.
Even if ADMX and these other detectors come up empty, that doesn't necessarily ring the particle's death knell. Quinn notes that axion theories are flexible enough, and some version of the particle may well elude the detector. But as the great detection rush continues, the odds may be improving that someone somewhere will finally reveal the true nature of this elusive component of the cosmos.
“I hope that in 15 years' time, I'm going to give a lecture on what dark matter actually is,” says Hložek. “The idea that we will know in a generation is just incredible.”
In the meantime, Sikivie has found something else within ADMX: satisfaction that decades on, his and others' theories are being seriously put to the test.
“[When] I go sit next to the ADMX experiment, it's kind of a very strong, emotional experience,” he says. “I try to avoid it, because I feel so happy there, there must be something wrong with me. I shouldn't get too spoiled by the feeling.”
