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 2:49 PM EDT, Tue April 15, 2014
Kathleen Blee says the KKK and white power or neo-Nazi groups give haters the purpose and urgency to use violence.
updated 7:46 AM EDT, Tue April 15, 2014
Sen. Sheldon Whitehouse and Rep. Henry Waxman say read deep, and you'll see the federal Keystone pipeline report spells out the pipeline is bad news
updated 1:43 PM EDT, Tue April 15, 2014
Frida Ghitis says President Obama needs to stop making empty threats against Russia and consider other options
updated 5:29 PM EDT, Tue April 15, 2014
Peter Bergen and David Sterman say the Kansas Jewish Center killings are part of a string of lethal violence in the U.S. that outstrips al Qaeda-influenced attacks. Why don't we pay more attention?
updated 12:41 PM EDT, Tue April 15, 2014
Danny Cevallos says families of the passengers on Malaysian Airlines Flight 370 need legal counsel
updated 11:23 AM EDT, Mon April 14, 2014
David Frum says Russia is on a rampage of mischief while Western leaders and Western alliances charged with keeping the peace hem and haw
updated 7:56 AM EDT, Mon April 14, 2014
Most adults make the mistakes of hitting the snooze button and of checking emails first thing in the morning, writes Mel Robbins
updated 1:54 PM EDT, Mon April 14, 2014
David Wheeler says as middle-class careers continue to disappear, we need a monthly cash payment to everyone
updated 7:55 AM EDT, Mon April 14, 2014
Democrats need to show more political spine when it comes to the issue of taxes.
updated 11:55 AM EDT, Mon April 14, 2014
Donna Brazile recalls the 50th Anniversary of the Civil Rights Act as four presidents honored the heroes of the movement and Lyndon Johnson, who signed the law
updated 9:17 AM EDT, Mon April 14, 2014
Elmer Smith remembers Chuck Stone, the legendary journalist from Philadelphia who was known as a thorn in the side of police and an advocate for the little guy
updated 2:56 PM EDT, Sun April 13, 2014
Al Franken says Comcast, the nation's largest cable provider, wants to acquire Time Warner Cable, the nation's second-largest cable provider. Should we be concerned?
updated 11:22 AM EDT, Fri April 11, 2014
Philip Cook and Kristin Goss says the Pennsylvania stabbing attack, which caused grave injury -- but not death, carries a lesson on guns for policymakers
updated 3:06 PM EDT, Fri April 11, 2014
Wikipedia lists 105 football movies, but all too many of them are forgettable, writes Mike Downey
updated 10:32 AM EDT, Fri April 11, 2014
John Sutter and hundreds of iReporters set out to run marathons after the bombings -- and learned a lot about the culture of running
updated 12:49 PM EDT, Fri April 11, 2014
Timothy Stanley says it was cowardly to withdraw the offer of an honorary degree to Ayaan Hirsi Ali. The university should have done its homework on her narrow views and not made the offer
updated 10:16 AM EDT, Fri April 11, 2014
Al Awlaki
Almost three years after his death in a 2011 CIA drone strike in Yemen, Anwar al-Awlaki continues to inspire violent jihadist extremists in the U.S, writes Peter Bergen
updated 9:21 PM EDT, Fri April 11, 2014
David Bianculli says Colbert is a smart, funny interviewer, but ditching his blowhard persona to take over the mainstream late-night role may cost him fans
updated 1:31 PM EDT, Thu April 10, 2014
Rep. Paul Ryan says the Republican budget places its trust in the people, not in Washington
updated 5:28 PM EDT, Thu April 10, 2014
Aaron David Miller says Obama isn't to blame for Kerry's lack of progress in resolving Mideast talks
updated 11:22 AM EDT, Mon April 14, 2014
David Weinberger says beyond focusing on the horrors of the attack a year ago, it's worth remembering the lessons it taught about strength, the dangers of idle speculation and Boston's solidarity
updated 12:32 PM EDT, Thu April 10, 2014
Katherine Newman says the motive for the school stabbing attack in Pennsylvania is not yet known, but research on such rampages turns up similarities in suspects and circumstances
updated 7:03 AM EDT, Fri April 11, 2014
Simon Tisdall: Has John Kerry's recent track record left Russia's wily leader ever more convinced of U.S. weakness?
updated 12:40 PM EDT, Thu April 10, 2014
Mel Robbins says Nate Scimio deserves credit for acting bravely in a frightening attack and shouldn't be criticized for posting a selfie afterward
updated 2:39 PM EDT, Wed April 9, 2014
Wendy Townsend says the Rattlesnake Roundup -- where thousands of pounds of snakes are killed and tormented -- is barbaric
updated 9:45 AM EDT, Thu April 10, 2014
Dr. Mary Mulcahy says doctors who tell their patients the truth risk getting bad ratings from them
updated 9:28 AM EDT, Wed April 9, 2014
Peggy Drexler says the married Rep. McAllister, caught on video making out with a staffer, won't get a pass from voters who elected him as a Christian conservative with family values
updated 7:43 AM EDT, Wed April 9, 2014
David Frum says the president has failed to react strongly to crises in Iran, Syria, Ukraine and Venezuela, encouraging others to act out
updated 4:57 PM EDT, Wed April 9, 2014
Eric Liu says Paul Ryan gets it very wrong: The U.S.'s problem is not a culture of poverty, it is a culture of wealth that is destroying the American value linking work and reward
updated 7:51 AM EDT, Wed April 9, 2014
Frida Ghitis writes: "We are still seeing the world mostly through men's eyes. We are still hearing it explained to us mostly by men."
updated 10:08 AM EDT, Thu April 10, 2014
Chester Wisniewski says the Heartbleed bug shows how we're all tangled together, relying on each other for Internet security
updated 3:26 PM EDT, Wed April 9, 2014
Danny Cevallos says an Ohio school that suspended a little kid for pointing his finger at another kid and pretending to shoot shows the growth in "zero tolerance" policies at school run amok
ADVERTISEMENT
ADVERTISEMENT
ADVERTISEMENT
ADVERTISEMENT