Skip to main content

Seeking dark matter, a mile underground

By Meg Urry, Special to CNN
updated 2:55 PM EDT, Wed October 30, 2013
Galaxies act as a lens to stretch images of more distant galaxies, the blue arcs that are evidence of dark matter.
Galaxies act as a lens to stretch images of more distant galaxies, the blue arcs that are evidence of dark matter.
STORY HIGHLIGHTS
  • Meg Urry: A mile down in an old gold mine, physicists search for "dark matter" particles
  • Urry: Putting the experiment deep underground screens out particles other than dark matter
  • We know dark matter only by gravitational effect, but it is most of the universe's mass
  • Urry: Nobody has seen it, but experiment shows earlier weak detections were not real

Editor's note: Meg Urry is the Israel Munson professor of physics and astronomy at Yale University and director of the Yale Center for Astronomy and Astrophysics.

(CNN) -- A mile down in an abandoned gold mine in South Dakota, physicists in a state-of-the-art scientific laboratory are searching for elusive "dark matter" particles, which make up most of the mass in our universe.

So far, no one has ever seen dark matter directly. You can't see it, touch it, smell it, throw a net over it, or tag it in the ways particle physicists deal with ordinary particles. We only know it by its gravitational effects on galaxies.

On Wednesday, team members from LUX -- for Large Underground Xenon experiment -- announced the first results from their operation in the Sanford Underground Research Facility, deep in the former Homestake Mine in Lead, South Dakota -- where for three months they have been taking an 11-minute, 4,850 foot elevator ride down a mine shaft to work in their lab.

Previous dark matter experiments suggested we were very close to the first direct detection. They predicted that with just slightly more sensitive detectors, like those in the LUX experiment, scientists would have definite evidence.

Meg Urry
Meg Urry

Now LUX has shown those predictions were wrong. Were they correct, more than 1,600 particles should have been seen in the LUX data -- about one particle an hour. No such signals were seen. What the experiment succeeded in doing was ruling out earlier weak detections, showing they were not the real deal.

Weak detections can be real or just a random fluctuation. Taking a photograph of something faint and far away might, in the shortest exposure, suggest a hint of something -- maybe the shape of an alien spaceship or the Loch Ness monster. But in better exposures that collect more light or have sharper resolution, those hints should turn into obvious images -- unless it was just a fluctuation, that is, in which case it would not become clearer no matter how long the exposure.

If you can't see it, how did scientists find evidence of dark matter? One clear sign appeared in astronomical images taken in the 1980s, with photographic plates and new more sensitive detectors, that showed a really odd phenomenon: strange, long arcs of faint blue light behind groups of redder, rounder galaxies. At first, the arcs were dismissed as anomalies. They were like the Bigfoot of the sky: too odd to explain based on current knowledge, but not clear enough to claim a new species.

As detectors improved even more and telescopes got bigger, more arcs were seen, typically behind clusters of galaxies. That meant an arc had to be the stretched image of a background galaxy -- stretched by the gravity of the foreground cluster.

Albert Einstein predicted this phenomenon, called "gravitational lensing." According to his theory, gravity bends light much as the lenses in my glasses bend incoming light rays. An image of a very distant galaxy would be undistorted and true if no other galaxies were along the path between it and the Earth. But when light passes near or through a large mass -- like a cluster of galaxies, which can weigh as much as 1,000 times our Milky Way galaxy -- it bends because of the gravitational force of the cluster. The more massive the cluster, the more the light path curves.

The more the light path bends, the more distorted the image of the distant background galaxy. So, a distant galaxy that just happens to line up behind a nearby, massive cluster of galaxies looks strangely elongated. And most important, the distortion of the image tells astronomers how much the nearby cluster weighs.

Imagine having that on your resume: I searched for dark matter in an abandoned gold mine.
Meg Urry

But the arcs implied far too much mass for the number of stars we could see. Other mass estimates, based on the motions of galaxies in a cluster and of stars in the outskirts of a galaxy, also suggested the presence of a lot of unseen mass.

This had to be some kind of matter that, unlike ordinary protons and electrons, didn't emit light. In fact, this dark matter, as it was called, didn't appear to follow any of the laws of physics, except gravity.

By now a mountain of data points to the existence of dark matter. We even have data that cannot be explained away by alternate theories of gravity.

So, what is dark matter? One candidate is a class of particles named WIMPs, an acronym standing for "weakly interacting massive particles." Some types of WIMPs have been ruled out by the LUX data.

LUX and similar experiments try to detect the recoil of the nucleus of an atom -- for example, the nucleus of the noble gas xenon -- when it is hit spot-on by a dark matter particle. The dark matter escapes but the atom emits a flash of light that can be detected.

The chances of this happening are exceedingly small -- about 1 in 10 trillion. It's like trying to see someone's nose twitch in a stadium full of crazy football fans. Putting a dark matter experiment deep underground quiets the noise by screening out lots of particles other than dark matter.

That's what brought the National Science Foundation and the Department of Energy to the Homestake gold mine, more than 130 years after gold-feverish dreamers flooded into South Dakota. After the Homestake Mine closed in 2003, several underground rooms were retrofitted as a dedicated science facility.

More than 100 hardworking physicists and engineers on the LUX team made this new measurement experiment possible, including many students and postdoctoral scholars searching for their version of gold. Whatever these young scientists end up doing in the future -- and with the skills they learn, they could do just about anything -- they made an important step toward direct detection of dark matter particles.

Imagine having that on your resume: I searched for dark matter in an abandoned gold mine.

"This is only the beginning for LUX," says one of the experiment leaders, Dan McKinsey, my colleague at Yale. "Now that we understand the instrument and its backgrounds, we will continue to take data, testing for more and more elusive candidates for dark matter."

As LUX and others continue to take data, we wait for a future announcement of, we hope, the first direct detection of dark matter.

Follow us on Twitter @CNNOpinion.

Join us on Facebook/CNNOpinion.

The opinions in this commentary are solely those of Meg Urry.

ADVERTISEMENT
Part of complete coverage on
updated 6:10 PM EST, Mon November 24, 2014
If Obama thinks pushing out Hagel will be seen as the housecleaning many have eyed for his national security process, he'll be disappointed, says David Rothkopf.
updated 8:11 AM EST, Tue November 25, 2014
The decision by the St. Louis County prosecuting attorney to announce the Ferguson grand jury decision at night was dangerous, says Jeff Toobin.
updated 3:57 AM EST, Tue November 25, 2014
China's influence in Latin America is nothing new. Beijing has a voracious appetite for natural resources and deep pockets, says Frida Ghitis.
updated 4:51 PM EST, Mon November 24, 2014
Iranian President Hassan Rouhani speaks during a press conference in the capital Tehran on June 14, 2014.
The decision to extend the deadline for talks over Iran's nuclear program doesn't change Tehran's dubious history on the issue, writes Michael Rubin.
updated 2:25 PM EST, Fri November 21, 2014
Maria Cardona says Republicans should appreciate President Obama's executive action on immigration.
updated 7:44 AM EST, Fri November 21, 2014
Van Jones says the Hunger Games is a more sweeping critique of wealth inequality than Elizabeth Warren's speech.
updated 6:29 PM EST, Thu November 20, 2014
obama immigration
David Gergen: It's deeply troubling to grant legal safe haven to unauthorized immigrants by executive order.
updated 8:34 PM EST, Thu November 20, 2014
Charles Kaiser recalls a four-hour lunch that offered insight into the famed director's genius.
updated 3:12 PM EST, Thu November 20, 2014
The plan by President Obama to provide legal status to millions of undocumented adults living in the U.S. leaves Republicans in a political quandary.
updated 10:13 PM EST, Thu November 20, 2014
Despite criticism from those on the right, Obama's expected immigration plans won't make much difference to deportation numbers, says Ruben Navarette.
updated 8:21 PM EST, Thu November 20, 2014
As new information and accusers against Bill Cosby are brought to light, we are reminded of an unshakable feature of American life: rape culture.
updated 5:56 PM EST, Thu November 20, 2014
When black people protest against police violence in Ferguson, Missouri, they're thought of as a "mob."
updated 3:11 PM EST, Wed November 19, 2014
Lost in much of the coverage of ISIS brutality is how successful the group has been at attracting other groups, says Peter Bergen.
updated 8:45 AM EST, Wed November 19, 2014
Do recent developments mean that full legalization of pot is inevitable? Not necessarily, but one would hope so, says Jeffrey Miron.
updated 8:19 AM EST, Wed November 19, 2014
We don't know what Bill Cosby did or did not do, but these allegations should not be easily dismissed, says Leslie Morgan Steiner.
updated 10:19 AM EST, Wed November 19, 2014
Does Palestinian leader Mahmoud Abbas have the influence to bring stability to Jerusalem?
updated 12:59 PM EST, Wed November 19, 2014
Even though there are far fewer people being stopped, does continued use of "broken windows" strategy mean minorities are still the target of undue police enforcement?
updated 9:58 PM EST, Mon November 17, 2014
The truth is, we ran away from the best progressive persuasion voice in our times because the ghost of our country's original sin still haunts us, writes Cornell Belcher.
updated 4:41 PM EST, Tue November 18, 2014
Children living in the Syrian city of Aleppo watch the sky. Not for signs of winter's approach, although the cold winds are already blowing, but for barrel bombs.
updated 8:21 AM EST, Mon November 17, 2014
We're stuck in a kind of Middle East Bermuda Triangle where messy outcomes are more likely than neat solutions, says Aaron David Miller.
updated 7:16 AM EST, Mon November 17, 2014
In the midst of the fight against Islamist rebels seeking to turn the clock back, a Kurdish region in Syria has approved a law ordering equality for women. Take that, ISIS!
updated 11:07 PM EST, Sun November 16, 2014
Ruben Navarrette says President Obama would be justified in acting on his own to limit deportations
ADVERTISEMENT
ADVERTISEMENT
ADVERTISEMENT
ADVERTISEMENT