Thursday, January 22, 2009

Probing dark energy

Scientists have studied the night sky for thousands of years searching for clues to help them understand the universe. The South Pole Telescope (SPT)team, including Case Western Reserve University professor John Ruhl and graduate student Zachary Staniszewski, achieved a major milestone toward using a new technique to probe the most mysterious component of the universe, dark energy.

Staniszewski is the lead author on the multi-institution collaboration's paper, “Galaxy clusters discovered with a Sunyaev-Zel'dovich effect survey” released Oct. 10 in a pre-publication posting on astro-ph, an electronic preprint archive. The paper chronicles the discovery of three galaxy clusters using a new survey technique.

The technique relies on an effect that galaxy clusters have on the Cosmic Microwave Background (CMB) light that passes through them. The SPT team surveyed a 40-square-degree patch of sky looking for galaxy clusters via this effect, called the Sunyaev-Zeldovich (SZ) effect. The survey found four galaxy clusters, one previously known and three new ones. It is the first time this technique has been used to discover new clusters.

The 10-meter South Pole Telescope’s millimeter-wave camera captures images of the CMB, radiation left over from 270,000 years after the Big Bang Galaxy clusters affect the brightness of the CMB after it has passed through them; this brightness change is the SZ effect, and is independent of distance to the galaxy cluster, unlike the optical or x-ray brightness of the clusters.

These galaxies, Staniszewski says, are likely billions of light years away and about seven billion years old.

Part of the data set was taken by Staniszewski during his winter stay at the South Pole manning the telescope. Bolstering the confidence that the newly discovered clusters are real, the paper includes analysis of data from follow-up optical observations with the Blanco Cosmology Survey instrument, led by University of Illinois astrophysicist and SPT collaborator Joe Mohr.

“The theory of how galaxy clusters imprint a signal on the CMB has been around for nearly 40 years, when two Russian physicists first proposed it in 1969,” Staniszewski said. “The SZ effect has been seen in clusters that were already identified by optical and x-ray observations, but no one had yet used it to discover a brand new galaxy cluster. These are the first galaxy clusters discovered using this method.”

The SPT will produce a larger catalog of new clusters with more data and analysis. “These three new clusters are just the tip of the iceberg," Ruhl said. “The full survey is what will help us achieve our ultimate goal, which is to understand dark energy.”

“The expansion of the universe is accelerating, and because of that we believe that dark energy dominates the energy density of the universe. Dark energy provides a negative pressure, or repulsive force, that accelerates the expansion, rather than slowing it down like the attractive force of gravity does,” Staniszewski said. “We're trying to figure out what dark energy actually is, and it turns out that a survey of galaxy clusters can be used to trace the expansion history of the universe, which can tell you about the dark energy."

Gravity works to pull pockets of galaxies together, into clusters, a process that is more effective at early times when matter is dense. Dark energy speeds up the expansion of the universe and spreads out the matter, which slows down clustering. Counting the number of galaxy clusters as a function of time throughout the history of the universe can therefore be used to understand dark energy.

The SPT science team, led by principal investigator John Carlstrom at the University of Chicago includes more than 40 scientists from Case Western Reserve, University of Chicago, University of California at Berkeley, Cardiff University, University of Colorado, Harvard Smithsonian Center for Astrophysics, University of Illinois, Lawrence Berkeley Laboratory, McGill University, and NASA Marshall Spaceflight Center.

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