With the whir of a giant digital camera, the biggest mystery in the universe is about to become a bit less mysterious.

Fifteen years ago, the world of science was rocked by the discovery that, contrary to our previous notions of gravity gradually slowing the expansion of the universe after the big bang, distant galaxies instead appeared to be flying apart at an ever-accelerating rate.

The observation implied that space itself was stretching apart faster and faster.

It was akin to watching a dropped ball reverse course, speed upward and disappear into the sky.

The discovery made many cosmologists — the scientists who probe the very nature of the universe itself — acutely uncomfortable. For either our understanding of gravity is somehow cockeyed, or some mysterious repulsive force permeates the universe.

The new force was quickly and glibly dubbed dark energy, as a sort of counterpart to dark matter, which seems to affect ordinary matter through gravity alone and whose nature is equally mysterious.

In 2011, the Nobel Committee blessed the improbable discovery as real, handing their prize in physics to the two teams that nearly simultaneously made the observation.

“As unhappy as it made some of us, the expansion of the universe is indeed accelerating,” said Marc Kamionkowski, a professor of physics and astronomy at Johns Hopkins University. “That’s how the universe works.”

Now, after years of planning and construction, four new projects at telescopes in Chile, Hawaii and the South Pole are getting a handle on what, exactly, is doing this unseemly cosmic pushing.

Leading the way is the world’s most powerful digital camera, constructed at Fermilab, the Energy Department facility in Illinois. The $50 million Dark Energy Camera took a decade to plan and build, and it sports a resolution of 570 megapixels — about a hundredfold more pixels than a smartphone camera.

Technicians installed it atop a telescope in Chile last year, and after initial jitters — the camera was so heavy it made the telescope jiggle — the camera has been “tested, tweaked and fine-tuned,” said Joshua Frieman, the Fermilab scientist leading the project, which has enlisted 120 scientists from 23 countries.

On Aug. 31, the big camera began snapping its way across a huge swath of the southern sky.

Each click captures light from nearly 100,000 distant galaxies. Over the next five years, the project, called the Dark Energy Survey, will catalog some 300 million galaxies and thousands of exploding stars flung across distant space and time, in what Frieman called “the biggest galactic survey yet.”

Every night, scientists will beam 400 gigabytes of camera data to a supercomputing center at the University of Illinois at Urbana-Champaign, where machines will build a giant time-lapse map of the universe going back some 8 billion years — or more than halfway to the big bang that started it all.

Frieman calls it “a movie of cosmic history.”

Scrutinizing this movie will narrow down the possibilities for what is causing cosmic acceleration. Because this acceleration cannot be measured directly, its nature can only be divined indirectly, by measuring, for instance, how clumps of galaxies coalesce across space and time.

One possibility is that our understanding of gravity, as explained by Einstein’s general theory of relativity, breaks down across huge distances. The theory marked the culmination of Einstein’s hardest thinking, and since its inception in 1916 it has withstood every test thrown at it. But general relativity may be incomplete.

Another possibility is that a mysterious repulsive force permeates every point in the universe. This dark energy, if revealed, would be instant Nobel Prize fodder, Kamionkowski said.

“This is the one a lot of people would bet on,” said David Spergel, an astrophysicist at Princeton University. “It’s where I’d put most of my money.”

Beyond these two possibilities, there are a “whole bunch of crazy ideas,” Kamionkowski said.

Spergel called most of these notions “intellectual aardvarks,” saying, “They look beautiful only to their mothers.”

One such idea posits that the visible universe — what we think of as everything — is only one part of a far larger cosmos.

Combined with the Dark Energy Survey, three other projects coming online will further explore cosmic acceleration.

Atop Mauna Kea in Hawaii, a camera at the Subaru Telescope is mapping galaxies in much of the northern sky. And at the South Pole and in Chile, two telescopes will take another tack, peering at the glow left over from the big bang.

Studying this “cosmic background radiation” should reveal how cosmic acceleration has sped up or slowed down over the 13.7-billion-year history of the universe.

Combining data from all four projects could lead to a big “eureka moment,” said Kamionkowski, a day the world’s puzzled cosmologists would welcome.

“We don’t know what makes up three-quarters of the universe,” Spergel said. “It’s a little embarrassing.”

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