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 12:11 PM EDT, Fri August 29, 2014
Leon Aron says the U.S. and Europe can help get Russia out of Ukraine by helping Ukraine win its just war, sharing defense technologies and intelligence
updated 1:24 PM EDT, Fri August 29, 2014
Timothy Stanley the report on widespread child abuse in a British town reveals an institutional betrayal by police, social services and politicians. Negligent officials must face justice
updated 9:06 PM EDT, Fri August 29, 2014
Peter Bergen and David Sterman say a new video of an American suicide bomber shows how Turkey's militant networks are key to jihadists' movement into Syria and Iraq. Turkey must stem the flow
updated 11:16 AM EDT, Thu August 28, 2014
Whitney Barkley says many for-profit colleges deceive students, charge exorbitant tuitions and make false promises
updated 10:34 AM EDT, Fri August 29, 2014
Mark O'Mara says the time has come to decide whether we really want police empowered to shoot those they believe are 'fleeing felons'
updated 10:32 AM EDT, Thu August 28, 2014
Bill Frelick says a tool of rights workers is 'naming and shaming,' ensuring accountability for human rights crimes in conflicts. But what if wrongdoers know no shame?
updated 10:43 PM EDT, Thu August 28, 2014
Jay Parini says, no, a little girl shouldn't fire an Uzi, but none of should have easy access to guns: The Second Amendment was not written to give us such a 'right,' no matter what the NRA says
updated 1:22 PM EDT, Sat August 30, 2014
Terra Ziporyn Snider says many adolescents suffer chronic sleep deprivation, which can indeed lead to safety problems. Would starting school an hour later be so wrong?
updated 9:30 AM EDT, Fri August 29, 2014
Peggy Drexler says after all the celebrity divorces, it's tempting to ask the question. But there are still considerable benefits to getting hitched
updated 2:49 PM EDT, Fri August 29, 2014
The death of Douglas McAuthur McCain, the first American killed fighting for ISIS, highlights the pull of Syria's war for Western jihadists, writes Peter Bergen.
updated 6:42 PM EDT, Tue August 26, 2014
Former ambassador to Syria Robert Ford says the West should be helping moderates in the Syrian armed opposition end the al-Assad regime and form a government to focus on driving ISIS out
updated 9:21 AM EDT, Wed August 27, 2014
Ruben Navarrette says a great country does not deport thousands of vulnerable, unaccompanied minors who fled in fear for their lives
updated 9:19 AM EDT, Wed August 27, 2014
Robert McIntyre says Congress is the culprit for letting Burger King pay lower taxes after merging with Tim Hortons.
updated 7:35 PM EDT, Tue August 26, 2014
Wesley Clark says the U.S. can offer support to its Islamic friends in the region most threatened by ISIS, but it can't fight their war
updated 7:26 AM EDT, Wed August 27, 2014
Jeff Yang says the tech sector's diversity numbers are embarrassing and the big players need to do more.
updated 4:53 PM EDT, Tue August 26, 2014
America's painful struggle with racism has often brought great satisfaction to the country's rivals, critics, and foes. The killing of Michael Brown and its tumultuous aftermath has been a bonanza.
updated 4:19 PM EDT, Tue August 26, 2014
Ed Bark says in this Emmy year, broadcasters CBS, ABC and PBS can all say they matched or exceeded HBO. These days that's no small feat
updated 3:19 PM EDT, Tue August 26, 2014
Rick Martin says the death of Robin Williams brought back memories of his own battle facing down depression as a young man
updated 11:58 AM EDT, Tue August 26, 2014
David Perry asks: What's the best way for police officers to handle people with psychiatric disabilities?
updated 3:50 PM EDT, Mon August 25, 2014
Julian Zelizer says it's not crazy to think Mitt Romney would be able to end up at the top of the GOP ticket in 2016
updated 4:52 PM EDT, Mon August 25, 2014
Roxanne Jones and her girlfriends would cheer from the sidelines for the boys playing Little League. But they really wanted to play. Now Mo'ne Davis shows the world that girls really can throw.
updated 12:29 PM EDT, Mon August 25, 2014
Peter Bergen and Emily Schneider say a YouTube video apparently posted by ISIS seems to show that the group has a surveillance drone, highlighting a new reality: Terrorist groups have technology once only used by states
updated 5:04 PM EDT, Mon August 25, 2014
Kimberly Norwood is a black mom who lives in an affluent neighborhood not far from Ferguson, but she has the same fears for her children as people in that troubled town do
updated 5:45 PM EDT, Fri August 22, 2014
It apparently has worked for France, say Peter Bergen and Emily Schneider, but carries uncomfortable risks. When it comes to kidnappings, nations face grim options.
updated 1:27 PM EDT, Tue August 26, 2014
John Bare says the Ice Bucket Challenge signals a new kind of activism and peer-to-peer fund-raising.
updated 8:31 AM EDT, Fri August 22, 2014
James Dawes says calling ISIS evil over and over again could very well make it harder to stop them.
updated 9:05 PM EDT, Sat August 23, 2014
As the inquiry into the shooting of Michael Brown continues, critics question the prosecutor's impartiality.
updated 6:47 PM EDT, Fri August 22, 2014
Newt Gingrich says it's troubling that a vicious group like ISIS can recruit so many young men from Britain.
updated 10:50 AM EDT, Thu August 21, 2014
David Weinberger says Twitter and other social networks have been vested with a responsibility, and a trust, they did not ask for.
updated 7:03 AM EDT, Fri August 22, 2014
John Inazu says the slogan "We are Ferguson" is meant to express empathy and solidarity. It's not true: Not all of us live in those circumstances. But we all made them.
updated 8:23 AM EDT, Fri August 22, 2014
Retired Lt. Gen. Mark Hertling says he learned that the territory ISIS wants to control is amazingly complex.
updated 3:51 PM EDT, Wed August 20, 2014
Cerue Garlo says Liberia is desperate for help amid a Ebola outbreak that has touched every aspect of life.
updated 1:42 PM EDT, Thu August 21, 2014
Eric Liu says Republicans who want to restrict voting may win now, but the party will suffer in the long term.
updated 11:38 AM EDT, Thu August 21, 2014
Jay Parini: Jesus, Pope and now researchers agree: Wealth decreases our ability to sympathize with the poor.
updated 8:00 AM EDT, Thu August 21, 2014
Judy Melinek offers a medical examiner's perspective on what happens when police kill people like Michael Brown.
updated 6:03 PM EDT, Tue August 19, 2014
It used to be billy clubs, fire hoses and snarling German shepherds. Now it's armored personnel carriers and flash-bang grenades, writes Kara Dansky.
updated 1:27 PM EDT, Wed August 20, 2014
Maria Haberfeld: People who are unfamiliar with police work can reasonably ask, why was an unarmed man shot so many times, and why was deadly force used at all?
ADVERTISEMENT
ADVERTISEMENT
ADVERTISEMENT
ADVERTISEMENT