The Dark Halo of the Galaxy
The story of how physicists at a Florida university decided to find the inner music of space
When you look at the night sky, you’re seeing speckles of light emitted by faraway stars in our galaxy, nestled in a swath of darkness. The darkness, conveniently called “space,” seems empty — a vast room filled with stuff that barely occupies the realm. Indeed, the stuff that makes stars and planets, as well as the stuff that makes you — all of the ordinary matter we know, like protons, neutrons, and electrons, is only 5 percent of the total matter in the Universe. The rest of it (well, mostly), theoretical physicists say, is dark matter — the as-yet unidentified substance that holds the galaxy together and encloses its elliptical shape.
In the early 1980s, theoretical physicist Pierre Sikivie of the University of Florida pinpointed axions — extremely low-mass subatomic particles — as the possible constituents of dark matter. The challenge was in detecting them. Assuming that one could “hear” the frequency of such tiny things in a space devoid of ordinary matter, Sikivie invented a method of detecting the dark matter. This idea percolated for three decades among the robust and brilliant astrophysicist community. Finally, a detector named ADMX (the Axion Dark Matter Experiment) is sensitive enough to capture these infinitesimal entities and see if the dark halo of the galaxy is indeed axion-based.
ADMX, housed at the University of Washington and managed by the U.S. Department of Energy’s Fermi National Accelerator Laboratory, has finished the first run of the second-generation experiment. Initially designed by Neil Sullivan and David Tanner, UF professors of physics, along with then-student Chris Hagman in the early 1990s as a tabletop device, ADMX is now substantially larger and supercooled, allowing it to “tune in,” in fractions of a minute, to the electromagnetic waves emitted by axion decay within its one-meter-tall chamber.
“You’re out in the desert in the middle of the night trying to find a radio station to keep you awake. You tune the radio and you listen for a few seconds to see if there’s music or speech and it’s just noise. You tune the radio again. Finally, you find… some strange radio station out in Nevada!” says Tanner. “That’s what ADMX does, a million times, more or less, one right after the other. There is only a single radio station in the universe and we’re trying to find it.”
Laughs Sikivie, “And it just goes ‘bzzz.’”
The buzz would be from the photons created when axions convert to electromagnetic waves. “It will be music to our ears when we find it,” Sikivie says, a dreamy look slipping over his face.
The search for music began more than three decades ago. In 1983, Sikivie walked into Tanner’s and Sullivan’s offices to ask for guidance in designing an experiment that would test his theory that axions, rather than the popular and aptly named weakly interacting massive particles (WIMPs), composed dark matter. “Largely because I had just taught electromagnetism here at UF and knew all the methods of calculating hard-to-detect waves, I thought I’d apply it to axion dark matter,” he says. “I realized it would be a difficult job, but in principle, you could detect it here [on Earth], because it’s throughout the galaxy.” Theoretically, a cavity with absolutely nothing inside would have nothing but the dark matter remaining, and if the cavity were to be located inside a strong magnetic field, the axions would convert to photons if the cavity were “tuned” to the same frequency as the photons.
Tanner, whose expertise is in the interaction of electromagnetic radiation with matter in the optical spectrum, was happy to help, as was Neil Sullivan, who specializes in magnetic resonance — the absorption of radiation by atomic and subatomic particles.
“David is a very curious person,” Sikivie says with a friendly wink. “Any physics question, David will get interested to some extent.” Their collaboration has continued through the decades, as has their friendship, which is palpable in the room with them. “Pierre provides theoretical — and emotional — support for ADMX,” says Tanner.
Since the experiment’s inception, the team has expanded from six to 40 collaborators, as different challenges have been met. One problem was that, not unlike trying to catch an elusive radio station, the noise generated by the surrounding environment made it difficult to hear, especially given that the buzz would be at just 10–21 watt or less. ADMX is supercooled to reduce the hum of thermal radiation and the operations of the device’s electronics. Tanner and Sullivan engineered the cryogenics to achieve a near-constant temperature of just one tenth of one degree above absolute zero Kelvin — at absolute zero Kelvin, atoms stop moving. The UF pilot experiment created in the 1990s was only six inches wide and 18 inches tall. This experiment, and a similar one at the University of Rochester with the Brookhaven National Laboratory, demonstrated the practicality of the method, but neither pilot experiment was nearly sensitive enough. Making it larger and colder increased the detector’s sensitivity a thousand-fold. In the late 1990s, Karl van Bibber, then at Lawrence Livermore National Laboratory, joined by Leslie Rosenberg (now at the University of Washington but then at MIT), Sullivan, and Tanner, to construct a large-scale detector there. That initiated the apparatus that is the basis of the current ADMX.
ADMX will continue its radio-knob turning for five more years, after which it requires a new magnet. The current magnet weighs 20 tons. Tanner and Sikivie say they intend to change its proportions and utilize new technology to proceed with one that is four times stronger, effectively creating 16 times more signal and further accelerating the detection process.
“You have to be very patient in this game,” says Sikivie. “In physics, it’s more difficult to get big results, because the ‘easy pickings’ are gone. It’s harder but in some sense more important.” To detect the axions would unlock secrets of the galaxy; they carry “an extremely rich source of information,” says Sikivie.
“Potentially, from the signal, you could trace back the history of the dark matter flow in the galaxy,” says Tanner. “They flow like water coming off the face of a cliff. You could see all that in the signal we would detect.”
“It basically gives you the history of the galaxy and the galactic halo,” says Sikivie. “The halo has flows in its dark matter, and the flows affect the various ways the dark matter fell into the halo and flowed back and forth. It’s kind of amazing.”