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Astronomers Begin New Search for Dark Energy

Astronomers from the University of Arizona and 41 other institutions are beginning the most ambitious project yet to map the three-dimensional structure of the universe in a quest to understand dark energy.

"Making a three-dimensional map is essential to understanding why the universe is expanding at an ever-accelerating rate," said UA astronomy professor Daniel Eisenstein, director of the Sloan Digital Sky Survey III, known an SDSS-III, a collaboration of 350 scientists.

The new SDSS-III mapping project, called the Baryon Oscillation Spectroscopic Survey, or BOSS, collected its first astronomical data -- a milestone called achieving "first light" -- on a thousand galaxies and quasars on Sept. 14 - 15.

The BOSS team uses new, extremely sensitive optical-infrared spectrographs on the Sloan Foundation 2.5-meter telescope at Apache Point Observatory in New Mexico.

Their goal is to collect spectra for 1.4 million galaxies and 160,000 quasars by 2014.

Measuring the spectra, or colors, of galaxies and quasars allows astronomers to determine how far away and how far back in time these celestial objects are.

"The data from BOSS will be the best ever obtained on the large-scale structure of the universe," said BOSS principal investigator David Schlegel of the U.S. Department of Energy's Lawrence Berkeley National Laboratory.

In the early universe, cosmic matter -- the protons and neutrons, or "baryons" -- interacted with the light from the Big Bang to create pressure oscillations much like sound waves. Just as sound waves compress air molecules in our atmosphere, these "baryon acoustic oscillations" created density variations as they traveled through the early universe.

When the universe was around 400,000 years old, conditions were finally cool enough to halt the propagation of the sound waves, and this left a "frozen" sound wave signature, said UA astronomy professor Xiaohui Fan.

Fan is UA's representative to the SDSS-III collaboration council.

"We can see these frozen waves in the distribution of galaxies today,"
Eisenstein said. "The signature is that pairs of galaxies are somewhat more likely to be separated by 500 million light years, rather than 400 million or 600 million light years."

The sound wave signature today is expected to be about 500 million light years long because the universe has greatly expanded since those early times, Fan said.

"By measuring the length of the baryon oscillations, we can determine how dark energy has affected the expansion history of the universe,"

Eisenstein said. "That, in turn, helps us figure out what dark energy could be."

Astronomers study baryon oscillations as an exciting new method for measuring "dark energy," the name they give to the mysterious physical mechanism that is causing the universe to expand at an accelerating rate.

Astronomers have known since the 1920s that the universe is expanding, but they were stunned when they discovered in 1998 that the universe is expanding at an accelerating rate.

"We're trying to understand why that is. It's a very odd thing,"
Eisenstein said. "Gravity pulls things together, so you'd expect gravity would be pulling the universe back together so that it would expand at a decelerating rate.

"But something is causing the universe to expand at an accelerating rate. Either we misunderstand how gravity works on the largest scales, or there's some extra thing in the universe that actually causes gravity to repel structure," Eisenstein said.

The BOSS spectrographs have more than 2,000 large metal plates that are placed at the focal plane of the telescope. These plates are drilled with the precise locations of nearly two million objects across the northern sky. Optical fibers plugged into a thousand tiny holes in each of the "plug plates" carry the light from each observed galaxy or quasar to BOSS's new spectrographs.

The SDSS-III team plans to release its first data to the public in December 2010.


BOSS is the largest of four surveys in SDSS-III, which includes 350 scientists from 42 institutions. The BOSS design and implementation has been led from the U.S. Department of Energy's Lawrence Berkeley National Laboratory. The optical systems were designed and built at Johns Hopkins University, with new CCD cameras designed and built at Princeton University and the University of California at Santa Cruz/Lick Observatory. The University of Washington contributed new optical fiber systems, and Ohio State University designed and built an upgraded BOSS data-acquisition system. The "fully depleted" 16-megapixel CCDs for the red cameras evolved from Berkeley Lab research and were fabricated in Berkeley Lab's MicroSystems Laboratory.

Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the participating institutions, the National Science Foundation, and the U.S. Department of Energy.

SDSS-III is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS-III Collaboration, including the University of Arizona, the Brazilian Participation Group, University of Cambridge, University of Florida, the French Participation Group, the German Participation Group,the Michigan State/Notre Dame/JINA Participation Group, Johns Hopkins University, the U.S. Department of Energy's Lawrence Berkeley National Laboratory, Max Planck Institute for Astrophysics, New Mexico State University, New York University, the Ohio State University, University of Portsmouth, Princeton University, University of Tokyo, the University of Utah, Vanderbilt University, University of Virginia, University of Washington and Yale University.

Lori Stiles | University of Arizona
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