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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To minimize these problems, astronomers who analyze microwaves and submillimeter waves have made the South Pole a second home. Their instruments reside in the Dark Sector, a tight cluster of buildings where light and other sources of electromagnetic radiation are kept to a minimum. (Nearby are the Quiet Sector, for seismology research, and the Clean Air Sector, for climate projects.)

Astronomers like to say that for more pristine observing conditions, they would have to go into outer space—an exponentially more expensive proposition, and one that NASA generally doesn’t like to pursue unless the science can’t easily be done on Earth. (A dark energy satellite has been on and off the drawing board since 1999, and last year went “back to square one,” according to one NASA adviser.) At least on Earth, if something goes wrong with an instrument, you don’t need to commandeer a space shuttle to fix it.

The United States has maintained a year-round presence at the pole since 1956, and by now the National Science Foundation’s U.S. Antarctic Program has gotten life there down to, well, a science. Until 2008, the station was housed in a geodesic dome whose crown is still visible above the snow. The new base station resembles a small cruise ship more than a remote outpost and sleeps more than 150, all in private quarters. Through the portholes that line the two floors, you can contemplate a horizon as hypnotically level as any ocean’s. The new station rests on lifts that, as snow accumulates, allow it to be jacked up two full stories.

The snowfall in this ultra-arid region may be minimal, but that which blows in from the continent’s edges can still make a mess, creating one of the more mundane tasks for the SPT’s winter-over crew. Once a week during the dark months, when the station population shrinks to around 50, the two on-site SPT researchers have to climb into the telescope’s 33-foot-wide microwave dish and sweep it clean. The telescope gathers data and sends it to the desktops of distant researchers. The two “winter-overs” spend their days working on the data, too, analyzing it as if they were back home. But when the telescope hits a glitch and an alarm on their laptops sounds, they have to figure out what the problem is—fast.

“An hour of down time is thousands of dollars of lost observing time,” says Keith Vanderlinde, one of 2008’s two winter-overs. “There are always little things. A fan will break because it’s so dry down there, all the lubrication goes away. And then the computer will overheat and turn itself off, and suddenly we’re down and we have no idea why.” At that point, the environment might not seem so “benign” after all. No flights go to or from the South Pole from March to October (a plane’s engine oil would gelatinize), so if the winter-overs can’t fix whatever is broken, it stays broken—which hasn’t yet happened.

More than most sciences, astronomy depends on the sense of sight; before astronomers can reimagine the universe as a whole, they first have to figure out how to perceive the dark parts. Knowing what dark matter is would help scientists think about how the structure of the universe forms. Knowing what dark energy does would help scientists think about how that structure has evolved over time—and how it will continue to evolve.

Scientists have a couple of candidates for the composition of dark matter—hypothetical particles called neutralinos and axions. For dark energy, however, the challenge is to figure out not what it is but what it’s like. In particular, astronomers want to know if dark energy changes over space and time, or whether it’s constant. One way to study it is to measure so-called baryon acoustic oscillations. When the universe was still in its infancy, a mere 379,000 years old, it cooled sufficiently for baryons (particles made from protons and neutrons) to separate from photons (packets of light). This separation left behind an imprint—called the cosmic microwave background—that can still be detected today. It includes sound waves (“acoustic oscillations”) that coursed through the infant universe. The peaks of those oscillations represent regions that were slightly denser than the rest of the universe. And because matter attracts matter through gravity, those regions grew even denser as the universe aged, coalescing first into galaxies and then into clusters of galaxies. If astronomers compare the original cosmic microwave background oscillations with the distribution of galaxies at different stages of the universe’s history, they can measure the rate of the universe’s expansion.

Another approach to defining dark energy involves a method called gravitational lensing. According to Albert Einstein’s theory of general relativity, a beam of light traveling through space appears to bend because of the gravitational pull of matter. (Actually, it’s space itself that bends, and light just goes along for the ride.) If two clusters of galaxies lie along a single line of sight, the foreground cluster will act as a lens that distorts light coming from the background cluster. This distortion can tell astronomers the mass of the foreground cluster. By sampling millions of galaxies in different parts of the universe, astronomers should be able to estimate the rate at which galaxies have clumped into clusters over time, and that rate in turn will tell them how fast the universe expanded at different points in its history.

The South Pole Telescope uses a third technique, called the Sunyaev-Zel’dovich effect, named for two Soviet physicists, which draws on the cosmic microwave background. If a photon from the latter interacts with hot gas in a cluster, it experiences a slight increase in energy. Detecting this energy allows astronomers to map those clusters and measure the influence of dark energy on their growth throughout the history of the universe. That, at least, is the hope. “A lot of people in the community have developed what I think is a healthy skepticism. They say, ‘That’s great, but show us the money,’” says Holzapfel. “And I think within a year or two, we’ll be in a position to be able to do that.”

The SPT team focuses on galaxy clusters because they are the largest structures in the universe, often consisting of hundreds of galaxies—they are one million billion times the mass of the Sun. As dark energy pushes the universe to expand, galaxy clusters will have a harder time growing. They will become more distant from one another, and the universe will become colder and lonelier.


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