Friday, May 9, 2014

The Hunt for the Invisible Axion

By Rachel Courtland
Dark matter, the most abundant form of matter in the universe, is invisible and intangible. But that doesn’t keep Leslie Rosenberg from seeing it nearly everywhere he looks. Like most physicists, he finds ample evidence of it written on the sky. It’s there in the swirling of galaxies, the aftermath of cosmic collisions, and the vast, weblike scaffolding that the universe’s luminous matter seems to hang upon.
It’s also, he hopes, near at hand. Dark matter almost certainly sweeps through Earth like water through cheesecloth. But Rosenberg, a professor at the University of Washington, in Seattle, thinks he might have just the thing to coax it out of hiding.
Tucked into the concrete floor of a large warehouselike laboratory at the edge of campus, the Axion Dark Matter eXperiment (ADMX) contains the world’s most sensitive radio receiver in its frequency range. Its builders are fond of boasting that if the detector were placed on Mars, it could pick up a cellphone signal sent from Earth, assuming there were no interference.
It’ll need that kind of sensitivity to have any chance of detecting dark matter. Based on a wide range of observations going back to the 1930s, astronomers estimate that for every kilogram of ordinary matter in the universe—be it hydrogen, silicon, concrete, or feathers—there are some 5 kilograms of dark matter. But for all its ubiquity, no one knows what the dark stuff is made of. They only know that, with the exception of exerting a gravitational pull, it interacts very little with ordinary matter.

ADMX is designed to hunt for one of the leading dark matter candidates: the axion. And now, after some 25 years of development, Rosenberg and his colleagues may be on the verge of finally seeing the elusive particle—if it does in fact exist. Later this year, a new cryogenic cooling system will chill the ADMX detector down to just a tenth of a degree above absolute zero, bringing the experiment to its peak sensitivity. Over the coming months and years, the detector will scan radio bands in the hopes of literally tuning into dark matter’s wavelength, converting axions into electromagnetic radiation that can be picked up and amplified through sensitive quantum electronics. Any day now, finding dark matter could just be a matter of hitting the right frequency, says collaborator Gianpaolo Carosi, a research scientist at Lawrence Livermore National Laboratory, in Livermore, Calif. “We’re trying to look for that little tone above the hiss.”

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