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

Far from light and plunged into months-long darkness, Antarctica's South Pole Telescope is one of the best places on Earth for observing the universe. (Keith Vanderlinde / National Science Foundation)
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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.

The implication of that discovery was momentous: it meant that the dominant force in the evolution of the universe isn’t gravity. It is...something else. Both teams announced their findings in 1998. Turner gave the “something” a nickname: dark energy. It stuck. Since then, astronomers have pursued the mystery of dark energy to the ends of the Earth—literally.

“The South Pole has the harshest environment on Earth, but also the most benign,” says William Holzapfel, a University of California at Berkeley astrophysicist who was the on-site lead researcher at the South Pole Telescope (SPT) when I visited.

He wasn’t referring to the weather, though in the week between Christmas and New Year’s Day—early summer in the Southern Hemisphere—the Sun shone around the clock, the temperatures were barely in the minus single digits (and one day even broke zero), and the wind was mostly calm. Holzapfel made the walk from the National Science Foundation’s Amundsen-Scott South Pole Station (a snowball’s throw from the traditional site of the pole itself, which is marked with, yes, a pole) to the telescope wearing jeans and running shoes. One afternoon the telescope’s laboratory building got so warm the crew propped open a door.

But from an astronomer’s perspective, not until the Sun goes down and stays down—March through September— does the South Pole get “benign.”

“It’s six months of uninterrupted data,” says Holzapfel. During the 24-hour darkness of the austral autumn and winter, the telescope operates nonstop under impeccable conditions for astronomy. The atmosphere is thin (the pole is more than 9,300 feet above sea level, 9,000 of which are ice). The atmosphere is also stable, due to the absence of the heating and cooling effects of a rising and setting Sun; the pole has some of the calmest winds on Earth, and they almost always blow from the same direction.

Perhaps most important for the telescope, the air is exceptionally dry; technically, Antarctica is a desert. (Chapped hands can take weeks to heal, and perspiration isn’t really a hygiene issue, so the restriction to two showers a week to conserve water isn’t much of a problem. As one pole veteran told me, “The moment you go back through customs at Christchurch [New Zealand], that’s when you’ll need a shower.”) The SPT detects microwaves, a part of the electromagnetic spectrum that is particularly sensitive to water vapor. Humid air can absorb microwaves and prevent them from reaching the telescope, and moisture emits its own radiation, which could be misread as cosmic signals.


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