Photo Credit: Peter Rejcek

Construction workers on a lift assemble the metal superstructure for the ground shield on the South Pole Telescope during the 2011-12 field season. The shield will eliminate ground reflection inteference as the telescope begins a new experiment on cosmic inflation.

Its five-year mission: To survey the early universe for massive galaxy clusters, a search designed to understand more about one of cosmology’s greatest mysteries — dark energy .

Mission complete. Now it’s time for something new.

The South Pole Telescope (SPT) , the largest such instrument ever installed at the U.S. Antarctic Program’s research station at the bottom of the world, completed its scan of 2,500-square degrees of night sky at the end of the 2011 winter. The 10-meter telescope has spied hundreds of previously unseen galaxy clusters, including the most massive ever detected, since seeing its first light at the beginning of the 2007 South Pole winter.

“We’re trying to understand what dark energy could be, and our way of looking at it is to see the structures involved,” explained John Carlstrom , principal investigator for the experiment, which includes dozens of collaborators. “The project as a whole has been more successful than any of us thought.”

Galaxy clusters, thanks to the pull of gravity, are the largest structures to have evolved in the cosmos. The SPT hunts for them using the Sunyaev-Zel’dovich (SZ) effect — a small distortion in the cosmic microwave background (CMB) , a “glow” left over from the Big Bang some 14 billion years ago. Such distortions are created as background radiation passes through a large galaxy cluster — effectively creating a shadow from the cluster in the microwave background.

In 2008, the telescope detected its first galaxy clusters using the SZ effect. Two years later, astronomers announced the discovery of the most massive galaxy cluster yet, tipping the scales at the equivalent of 800 trillion suns, and holding hundreds of galaxies. [See previous article — Heavyweight discovery: South Pole Telescope finds most massive galaxy cluster to date.]

Mapping the number of such clusters over the history of the universe can tell researchers how much influence dark energy had on their growth, according to Bradford Benson , a postdoctoral research fellow at the University of Chicago . He is also the lead author of a recent paper that puts additional constraints, based on SPT data, on the models cosmologists use to understand a universe increasingly dominated by dark energy.

It sounds like a disembodied entity that Captain Kirk and company might battle in an episode of Star Trek. Dark energy is the prevalent explanation for the accelerating expansion of the universe. Dark energy appears to counteract the gravitational attraction between galaxies.

In a younger, smaller universe billions of years ago, gravity had a greater influence, allowing galaxy clusters to grow and clump together like dust bunnies on a wood floor. In a study last year using SPT data, lead author Christian L Reichardt , with the University of California, Berkeley , and colleagues found that dark energy accounted for no more than 1.8 percent of the total energy-density of the universe at a time when it was only 400,000 years old. Today, dark energy accounts for more than 70 percent of all the matter and energy in the universe.

“Everything is getting diluted. Dark energy is just sitting there,” said Carlstrom, director of the Kavli Institute of Cosmological Physics at the University of Chicago. “In the future, it dominates. Right now it is two-thirds of the energy-density of the universe.”

Now, Carlstrom, Benson and their team want to peer back to near the beginning of time, a fraction of a moment after the Big Bang, when the universe expanded exponentially, a theory known as inflation.1 2   Next