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Cold Dark Matter

January 27, 2015

To catch the whisper of a passing particle of dark matter, two experiments in northern Minnesota get cold. One of them, in fact, is chilled to a few thousandths of a degree above absolute zero.

Dark matter appears to make up about five-sixths of all the matter in the universe. Astronomers see it on large scales because it exerts a strong gravitational pull on the normal matter around it.

Yet no one has ever seen an individual bit of dark matter. It rarely interacts with normal matter in any way — and it may not interact at all.

The experiments in Minnesota use detectors made of germanium. If a particle of dark matter rams into the nucleus of a germanium atom, the nucleus should “wiggle” a bit, producing a tiny amount of heat and a tiny change in electric charge.

But the detectors need an enormous amount of shielding. They’re in a laboratory that’s a half-mile below ground; the layers of rock screen out cosmic rays, which can zap the germanium atoms.

At normal temperatures, the atoms vibrate so much that they would overwhelm any signal from a collision with a particle of dark matter. So the detectors in the Cryogenic Dark Matter Survey are chilled to almost absolute zero by a complicated refrigeration system.

Even so, the experiment hasn’t turned up any evidence of dark matter. But scientists are planning to build a bigger version of the experiment in a deeper mine — changes that could help catch the faint signal of dark matter.


Script by Damond Benningfield, Copyright 2014

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