Dark Energy: The Biggest Mystery in the Universe

      At the South Pole, astronomers try to unravel a force greater than gravity that will determine the fate of the cosmos

Twice a day, seven days a week, from February to November for the past four years, two researchers have layered themselves with thermal underwear and outerwear, with fleece, flannel, double gloves, double socks, padded overalls and puffy red parkas, mummifying themselves until they look like twin Michelin Men. Then they step outside, trading the warmth and modern conveniences of a science station (foosball, fitness center, 24-hour cafeteria) for a minus-100-degree Fahrenheit featureless landscape, flatter than Kansas and one of the coldest places on the planet. They trudge in darkness nearly a mile, across a plateau of snow and ice, until they discern, against the backdrop of more stars than any hands-in-pocket backyard observer has ever seen, the silhouette of the giant disk of the South Pole Telescope, where they join a global effort to solve possibly the greatest riddle in the universe: what most of it is made of.

For thousands of years our species has studied the night sky and wondered if anything else is out there. Last year we celebrated the 400th anniversary of Galileo’s answer: Yes. Galileo trained a new instrument, the telescope, on the heavens and saw objects that no other person had ever seen: hundreds of stars, mountains on the Moon, satellites of Jupiter. Since then we have found more than 400 planets around other stars, 100 billion stars in our galaxy, hundreds of billions of galaxies beyond our own, even the faint radiation that is the echo of the Big Bang.

Now scientists think that even this extravagant census of the universe might be as out-of-date as the five-planet cosmos that Galileo inherited from the ancients. Astronomers have compiled evidence that what we’ve always thought of as the actual universe—me, you, this magazine, planets, stars, galaxies, all the matter in space—represents a mere 4 percent of what’s actually out there. The rest they call, for want of a better word, dark: 23 percent is something they call dark matter, and 73 percent is something even more mysterious, which they call dark energy.

“We have a complete inventory of the universe,” Sean Carroll, a California Institute of Technology cosmologist, has said, “and it makes no sense.”

Scientists have some ideas about what dark matter might be—exotic and still hypothetical particles—but they have hardly a clue about dark energy. In 2003, the National Research Council listed “What Is the Nature of Dark Energy?” as one of the most pressing scientific problems of the coming decades. The head of the committee that wrote the report, University of Chicago cosmologist Michael S. Turner, goes further and ranks dark energy as “the most profound mystery in all of science.”

The effort to solve it has mobilized a generation of astronomers in a rethinking of physics and cosmology to rival and perhaps surpass the revolution Galileo inaugurated on an autumn evening in Padua. They are coming to terms with a deep irony: it is sight itself that has blinded us to nearly the entire universe. And the recognition of this blindness, in turn, has inspired us to ask, as if for the first time: What is this cosmos we call home?

Scientists reached a consensus in the 1970s that there was more to the universe than meets the eye. In computer simulations of our galaxy, the Milky Way, theorists found that the center would not hold—based on what we can see of it, our galaxy doesn’t have enough mass to keep everything in place. As it rotates, it should disintegrate, shedding stars and gas in every direction. Either a spiral galaxy such as the Milky Way violates the laws of gravity, or the light emanating from it—from the vast glowing clouds of gas and the myriad stars—is an inaccurate indication of the galaxy’s mass.

But what if some portion of a galaxy’s mass didn’t radiate light? If spiral galaxies contained enough of such mystery mass, then they might well be obeying the laws of gravity. Astronomers dubbed the invisible mass “dark matter.”

“Nobody ever told us that all matter radiated,”Vera Rubin, an astronomer whose observations of galaxy rotations provided evidence for dark matter, has said. “We just assumed that it did.”

The effort to understand dark matter defined much of astronomy for the next two decades. Astronomers may not know what dark matter is, but inferring its presence allowed them to pursue in a new way an eternal question: What is the fate of the universe?

They already knew that the universe is expanding. In 1929, the astronomer Edwin Hubble had discovered that distant galaxies were moving away from us and that the farther away they got, the faster they seemed to be receding.

This was a radical idea. Instead of the stately, eternally unchanging still life that the universe once appeared to be, it was actually alive in time, like a movie. Rewind the film of the expansion and the universe would eventually reach a state of infinite density and energy—what astronomers call the Big Bang. But what if you hit fast-forward? How would the story end?

The universe is full of matter, and matter attracts other matter through gravity. Astronomers reasoned that the mutual attraction among all that matter must be slowing down the expansion of the universe. But they didn’t know what the ultimate outcome would be. Would the gravitational effect be so forceful that the universe would ultimately stretch a certain distance, stop and reverse itself, like a ball tossed into the air? Or would it be so slight that the universe would escape its grasp and never stop expanding, like a rocket leaving Earth’s atmosphere? Or did we live in an exquisitely balanced universe, in which gravity ensures a Goldilocks rate of expansion neither too fast nor too slow—so the universe would eventually come to a virtual standstill?

Assuming the existence of dark matter and that the law of gravitation is universal, two teams of astrophysicists—one led by Saul Perlmutter, at the Lawrence Berkeley National Laboratory, the other by Brian Schmidt, at Australian National University—set out to determine the future of the universe. Throughout the 1990s the rival teams closely analyzed a number of exploding stars, or supernovas, using those unusually bright, short-lived distant objects to gauge the universe’s growth. They knew how bright the supernovas should appear at different points across the universe if the rate of expansion were uniform. By comparing how much brighter the supernovas actually did appear, astronomers figured they could determine how much the expansion of the universe was slowing down. But to the astronomers’ surprise, when they looked as far as halfway across the universe, six or seven billion light-years away, they found that the supernovas weren’t brighter—and therefore nearer—than expected. They were dimmer—that is, more distant. The two teams both concluded that the expansion of the universe isn’t slowing down. It’s speeding up.

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