The meeting is sponsored by the University of Chicago's Center for Cosmological Physics, along with the universities of California at Berkeley and Davis and the University of Michigan.

"You have masses attracted to each other through gravity. If they're being forced apart through the expansion of the universe, then you can see that you have forces working in opposite," said John Carlstrom, workshop co-organizer and the S. Chandrasekhar Distinguished Service Professor in Astronomy & Astrophysics at the University of Chicago. Which force will win this cosmic tug-of-war?

Workshop participants will focus on how a phenomenon called the Sunyaev-Zeldovich Effect can be used to determine when dark energy began interfering with galaxy-cluster formation and thus became an important force in the evolution of the universe. Among the workshop participants will be Rashid Sunyaev, one of the scientists who first described the effect in 1972.

Director of the Max-Planck Institute for Astrophysics in Germany, Sunyaev in July received the $150,000 Gruber Prize in Cosmology. At the workshop he will preview his latest work, an X-ray experiment to measure the internal motions and turbulence in galaxies and galaxy clusters.

The SZ effect describes the apparent cooling of radiation as it passes through the hot gas contained in intervening galaxy clusters. The effect is scientifically useful because it is related to the mass of a cluster rather than its distance from Earth.

"If your instrument is sensitive enough you will be able to detect a cluster no matter how far away it is," said workshop co-organizer Andrey Kravtsov, Assistant Professor in Astronomy & Astrophysics at the University of Chicago.

Cosmologists have used the technique to detect galaxy clusters and study their evolution. "Looking at the SZ effect is looking at large-scale structure as traced by galaxy clusters," Carlstrom said. "That's very sensitive to this tug-of-war between gravity and the dark energy."

A galaxy cluster consists of thousands of galaxies, but they make only a small contribution to a cluster's mass. Gas that gravity has compressed and heated to hundreds of millions of degrees Fahrenheit accounts for approximately 10 times more mass than the galaxies themselves, Carlstrom said. And dark matter, an invisible material of unknown composition, accounts for even more mass than the gas.

Scientists conducted several small-scale SZ experiments to detect galaxy clusters in the 1990s. These experiments covered a tiny area of sky and focused on galaxy clusters that had already been detected by X-ray and optical telescopes. Limited though they were, the experiments nevertheless yielded interesting results, Carlstrom said.

The next series of experiments, beginning this fall, will map larger areas of sky with greater sensitivity and detect previously unknown clusters. "It'll be the first really good look at how well SZ surveys can do cosmology," Carlstrom said.

The new experiments will include the SZ Array in California, led by Carlstrom. His goal is to survey 12 square degrees of sky a year. "We should detect anywhere from several tens to a few hundred clusters, depending on what's out there," Carlstrom said. "We don't really know."

The idea is to watch the formation of galaxy clusters throughout the history of the universe. "The further away the cluster is, the earlier in the evolution of the universe you see," Kravtsov said.

Other new SZ experiments of similar scale to the SZA include the University of Cambridge's Arcminute MicroKelvin Imager, and the California Institute of Technology's Bolocam (Bolometer Camera). Presenting an overview of the status and prospects of SZA will be workshop co-organizer Clem Pryke, Assistant Professor in Astronomy & Astrophysics. Speaking about AMI will be Ruediger Kneissl of Cambridge's Cavendish Lab. Representing the Bolocam collaboration will be the University of Colorado's Glenn Jason.

Succeeding these experiments in several years will be the more ambitious South Pole Telescope, another project led by Carlstrom, and the Atacama Cosmology Telescope led by Princeton University. Carlstrom expects the South Pole Telescope to survey thousands of square degrees of sky in a year, detecting approximately 10,000 galaxy clusters.

"With that number of clusters, we should really be able to learn about how structure formed in the universe, when it formed and when its formation slowed down," Carlstrom said.

Also participating in the workshop will be cosmology theorists such as Martin White of the University of California, Berkeley; Ue-Li Pen of the Canadian Institute for Theoretical Physics, and the University of Chicago's Kravtsov. Their work encompasses numerical simulations, which help estimate what mass of cluster the SZ experiments should be able to detect given their sensitivity.

Cluster mass becomes an important issue when telescopic limitations render merging clusters as a single blob. "In those situations, some of the assumptions that people routinely make when interpreting observations break down," Kravtsov said.

The SZ experiments have the cosmology world teetering on the brink of discovery. "It's just an amazing time," Carlstrom said.

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