During the winter of 2007, I spent a day half a mile underground. It started with a scary yet exciting ride down the steep incline of a mine shaft, the claustrophobic darkness of the elevator cage aggravated by the din of its steel wheels grinding against the rails. The cage stopped at the 27th level of a disused iron mine in northern Minnesota. The mine, long since abandoned by miners, had been taken over by physicists to search for dark matter, the universe’s presumed missing mass. Inside that mine was the then gold standard of such efforts, the Cryogenic Dark Matter Search (CDMS) experiment. Deep below Earth’s surface, it was shielded from almost every possible source of noise, in the hope that it would “hear” a dark-matter particle hitting one of its detectors.
Even as I admired the physicists’ tenacity, the words of one kept coming to mind. Did such detectors, which were looking for so-called weakly interacting massive particles (WIMPs), even make sense, he asked rhetorically, despite his wholehearted devotion to the project. “Thirty years from now, when one picks up a book on cosmology, the whole WIMP thing could literally be a footnote,” Richard Gaitskell of Brown University in Providence, R.I., told me then. It could turn out that physicists had bet on the wrong horse in their attempts to explain astronomical observations.
Well, 10 of those 30 years have gone by. CDMS became SuperCDMS—larger, quieter, more sensitive. Experimentalists worldwide dug deeper to reduce the noise still further. The Large Underground Xenon experiment, for which Gaitskell is a co-spokesperson, has a mile of rock above it. But apart from the occasional heartbeat-skipping murmurs that turned out to be false alarms, as well as a few highly controversial claims, physicists hear only silence. Searches for other proposed types of dark matter have turned up nothing, too.
Couple this to the other major cosmological conundrum—the nature of the dark energy that is thought to be causing the universe’s expansion to accelerate—and it’s not unreasonable to question the standard model of cosmology, which is built on Einstein’s general relativity and requires both dark matter and dark energy to explain the observed universe. And it’s not unreasonable, either, to think that answers could come from efforts to unify physics. A unified theory will deepen our understanding of gravity, which is the only fundamental force of nature that theorists have not yet reconciled with quantum mechanics. In so doing, it might conceivably account for the galactic motions that we now ascribe to dark matter and dark energy.