Skip to main content

Seeking dark matter, a mile underground

By Meg Urry, Special to CNN
October 30, 2013 -- Updated 1855 GMT (0255 HKT)
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
October 20, 2014 -- Updated 1300 GMT (2100 HKT)
The world's response to Ebola is its own sort of tragedy, writes John Sutter
October 17, 2014 -- Updated 2033 GMT (0433 HKT)
Hidden away in Russian orphanages are thousands of children with disabilities who aren't orphans, whose harmful treatment has long been hidden from public view, writes Andrea Mazzarino
October 18, 2014 -- Updated 1722 GMT (0122 HKT)
When you hear "trick or treat" this year, think "nudge," writes John Bare
October 18, 2014 -- Updated 0442 GMT (1242 HKT)
The more than 200 kidnapped Nigerian schoolgirls have become pawns in a larger drama, writes Richard Joseph.
October 17, 2014 -- Updated 1345 GMT (2145 HKT)
Peggy Drexler said Amal Alamuddin was accused of buying into the patriarchy when she changed her name to Clooney. But that was her choice.
October 16, 2014 -- Updated 2043 GMT (0443 HKT)
Ford Vox says the CDC's Thomas Frieden is a good man with a stellar resume who has shown he lacks the unique talents and vision needed to confront the Ebola crisis
October 18, 2014 -- Updated 0858 GMT (1658 HKT)
How can such a numerically small force as ISIS take control of vast swathes of Syria and Iraq?
October 17, 2014 -- Updated 1342 GMT (2142 HKT)
How big a threat do foreign fighters in Syria and Iraq pose to the West? It's a question that has been much on the mind of policymakers and commentators.
October 17, 2014 -- Updated 1221 GMT (2021 HKT)
More than a quarter-million American women served honorably in the Iraq and Afghanistan wars. Now they are home, we have an obligation to help them transition back to civilian life.
October 16, 2014 -- Updated 2027 GMT (0427 HKT)
Paul Begala says Rick Scott's deeply weird refusal to begin a debate because rival Charlie Crist had a fan under his podium spells disaster for the Florida governor--delighting Crist
October 16, 2014 -- Updated 0407 GMT (1207 HKT)
The longer we wait to engage on Ebola, the more limited our options will become, says Marco Rubio.
October 15, 2014 -- Updated 1153 GMT (1953 HKT)
Democratic candidates who run from President Obama in red states where he is unpopular are making a big mistake, says Donna Brazile
October 16, 2014 -- Updated 0429 GMT (1229 HKT)
At some 7 billion people, the world can sometimes seem like a crowded place. But if the latest estimates are to be believed, then in less than a century it is going to feel even more so -- about 50% more crowded, says Evan Fraser
October 20, 2014 -- Updated 1653 GMT (0053 HKT)
Paul Callan says the Ebola situation is pointing up the need for better leadership
October 15, 2014 -- Updated 2245 GMT (0645 HKT)
Nurses are the unsung heroes of the Ebola outbreak. Yet, there are troubling signs we're failing them, says John Sutter
October 15, 2014 -- Updated 1700 GMT (0100 HKT)
Dean Obeidallah says it's a mistake to give up a business name you've invested energy in, just because of a new terrorist group
October 15, 2014 -- Updated 2301 GMT (0701 HKT)
Fear of Ebola is contagious, writes Mel Robbins; but it's time to put the disease in perspective
October 14, 2014 -- Updated 1744 GMT (0144 HKT)
Oliver Kershaw says that if Big Tobacco is given monopoly of e-cigarette products, public health will suffer.
October 18, 2014 -- Updated 1335 GMT (2135 HKT)
Stop thinking your job will make you happy.
October 15, 2014 -- Updated 0208 GMT (1008 HKT)
Ruben Navarrette says it's time to deal with another scandal involving the Secret Service — one that leads directly into the White House.
October 14, 2014 -- Updated 1125 GMT (1925 HKT)
Americans who choose to fight for militant groups or support them are young and likely to be active in jihadist social media, says Peter Bergen
October 13, 2014 -- Updated 1303 GMT (2103 HKT)
Stephanie Coontz says 11 years ago only one state allowed same sex marriage. Soon, some 60% of Americans will live where gays can marry. How did attitudes change so quickly?
October 14, 2014 -- Updated 2004 GMT (0404 HKT)
Legalizing assisted suicide seems acceptable when focusing on individuals. But such laws would put many at risk of immense harm, writes Marilyn Golden.
October 13, 2014 -- Updated 1307 GMT (2107 HKT)
Julian Zelizer says the issues are huge, but both parties are wrestling with problems that alienate voters
October 13, 2014 -- Updated 2250 GMT (0650 HKT)
Mel Robbins says the town's school chief was right to cancel the season, but that's just the beginning of what needs to be done
October 11, 2014 -- Updated 1543 GMT (2343 HKT)
He didn't discover that the world was round, David Perry writes. So what did he do?
ADVERTISEMENT