Seeing the unseen: A cosmic lens on dark matter

Observing dark matter, the unseen stuff that makes up most of the universe, takes some ingenuity. Michael Gladders has a few creative suggestions.

This mysterious matter is called “dark” because it doesn’t interact with light. But astronomers can observe its giant fingerprints across the cosmos in the distortions it causes, as gravity from huge clusters of galaxies and dark matter bend nearby light on its way toward Earth.

Gladders will take a new tack by seeking answers among the most massive objects in the universe—galaxy clusters. The huge tug of gravity from any immense distant object can become a sort of lens, altering light waves like a pair of reading glasses. Gladders’ research focuses on that effect, called gravitational lensing. If he knows what to expect without the distortion, he can calculate whatever was making it happen.

“Gravity almost acts like an optic, or a lens, would,” said Gladders, Assistant Professor in Astronomy. “The gravity in this case is caused by the mass of huge clusters of galaxies, which are mostly dark matter.”

Dark matter has baffled physicists and astronomers since the 1930s, when its presence was inferred by calculations to explain the motions of the galaxies within galaxy clusters. While no one yet understands dark matter, there are plenty of competing theories, and Gladders plans to help sort them out.

“We will ask how the lens acts to distort the background images in the sky,” he said. “Then we can say, ‘This is what you’d expect given this amount of dark matter.’ At a minimum you eliminate some of the competing models, and perhaps one will come to the fore.”

Gladders tests the strength of gravitational lenses, much like an optometrist can calculate the prescription for a pair of eyeglasses. The goal is to find just the right kind of clusters. “Clusters are rare,” Gladders said. “And those that exhibit lensing are even rarer, and really hard to find.”

To search for galaxy clusters that act as gravitational lenses, Gladders combs through large galaxy catalogs for the sometimes subtle high densities in the galaxy distribution that mark the clusters. The largest such source catalog is the Sloan Digital Sky Survey, which contains information on some 250 million objects.

The second-largest catalog is Gladders’ own Red-Sequence Cluster Survey, comprising roughly 150 million objects, some of them more than a million times fainter than the faintest objects visible by naked eye.

Gladders will reduce this list of suspects to the approximately 50,000 most-promising galaxy clusters. The task then is to generate a list of the few hundred gravitational lenses lurking in this vast data set. “These are visually spectacular objects,” Gladders said. “They have a real discovery aspect to them.”

Gladders is a regular at the world’s top observatories. He has visited the Magellan telescopes at Las Campanas Observatory, in the remote Andean foothills of Chile, more than 30 times. Getting there is a privilege and a feat, taking months of preparation. So when his last visit overlapped his teaching of the “Astronomy and Astrophysics of Stars” class at Chicago, Gladders simply lectured from Chile via the Internet.

He was able to share with students the joy of doing astronomy 18 hours a day, on three or four hours of sleep and lots of coffee, running the telescope, taking data, modifying the program. “When it’s cloudy one night, you have to figure how to get two nights of work done the next night,” he said.

“I still enjoy it, the fascinating moments on a mountaintop, in a control room, with banks of computers. There’s a mystical quality to the experience, staring at the night sky, figuring out deep questions you’ve been asking for years. As I prioritize things, that ranks above almost anything else.”

Born in Southampton, England, Gladders studied briefly at the University of Victoria in Canada, before being expelled for poor grades. “This can teach students that sometimes you can fall down and get back up,” he said.

He worked at geophysics for the oil industry in Calgary until he was readmitted to college. He moved on to a Ph.D. at the University of Toronto, and became a fellow at the Carnegie Observatories in Pasadena, Calif., before joining the University of Chicago faculty.

Gladders now is helping to acquire the big samples that astronomers need for robust tests of their models. “A few years ago the total known sample of lenses was just a few handfuls of objects,” he said. “We are now finding hundreds of these objects.”

As to direct measurement, Gladders said, some astronomers have high hopes for the Large Hadron Collider, the new accelerator under the Swiss-French border. “The LHC may reveal the ‘dark matter particle.’

“If not,” he added, “maybe astronomical tests are the only way.”

–Ford Burkhart

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