until it was detectable. Just to give you an idea of how faint the signal would be—it would be more than 100,000 times weaker than the broadcasts from the Pioneer 10 space probe as it reached the edge of the solar system!

Rosenberg is optimistic that this special apparatus, upgraded from earlier detection devices, will prove the ultimate arbiter for whether or not axions form the lion’s share of dark matter. Even if it turns out that axions constitute but a minute component, he believes his group’s detector will reveal their presence and gauge their relative influence.

“The new upgrade will be sensitive enough to detect even the weakest signals,” says Rosenberg. “In other words, this upgraded search will likely be the definitive search … and no matter what we discover, it will be illuminating. Either we will find the axion, proving that it exists and is part of the cosmological evolution of the universe, or we won’t…. If there is no axion, there must be entirely new physics—some strange, new physics that we cannot as yet fathom. And that, too, would be very interesting indeed.”

BATTLE OF THE WIMPS

With MACHOs and neutrinos looking increasingly unsuitable to be major dark-matter components and axions still just a conjecture, some physicists have climbed even farther out on the limb of speculation and made a case for WIMPs. This is a broad category, covering a host of theoretical entities that share an intense dislike for interacting with ordinary matter. One major advantage of WIMPs over neutrinos and axions is that they’d be much heavier. So they could be somewhat less common but still massive enough to make a profound difference.

Typically, experimental searches for WIMPs have been conducted in places comparable to neutrino-hunting expeditions—deep underground. The reasons are similar. In both cases, ground-level



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