Penn State astronomers participate in new search for dark energy

October 01, 2009

University Park, Pa. -- The most ambitious attempt yet to trace the history of the universe has seen "first light." Two Penn State scientists, Professors of Astronomy Niel Brandt and Donald Schneider, are members of the Baryon Oscillation Spectroscopic Survey (BOSS), a part of the Sloan Digital Sky Survey III (SDSS-III), which has begun a quest to collect electromagnetic-radiation spectra for 1.4 million galaxies and 160,000 quasars by 2014.

"These observations should provide quite accurate measurements of the expansion history of the universe, and thus should reveal the relative importance of ordinary matter, dark matter and dark energy over a wide range of cosmic time," Brandt said. 

To achieve the survey's first-light milestone after years of preparations, the SDSS-III team used new, specially built spectrograph instruments installed on the Sloan Foundation 2.5-meter telescope at Apache Point Observatory in New Mexico. On the night of Sept. 14-15, the team used the new instruments to measure the spectra of a thousand galaxies and quasars. The first data from the new spectrographs came from a region of sky in the constellation Aquarius, causing team member Nic Ross to joke that BOSS first light marked the "dawning of the Age of Aquarius." Ross, who for the past year has been leading the BOSS quasar-target-selection effort as a postdoctoral scholar at Penn State, moved to the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) in early September.

Schneider, who is the SDSS-III survey coordinator, and Brandt plan to involve Penn State undergraduate students, graduate students and postdoctoral research associates in the analysis of BOSS data.

"In addition to the primary goal of measuring the properties of dark energy, the BOSS data will provide a wealth of data on a wide range of other scientific questions," said Brandt. Brandt is leading the BOSS effort in two areas: the identification of sources of X-ray emission and a study of the variability of winds from quasars.

"The data from BOSS will be the best data ever obtained on the large-scale structure of the universe," said David Schlegel of Berkeley Lab, the BOSS principal investigator. BOSS uses the same telescope as the original Sloan Digital Sky Survey, but it uses new, specially built spectrographs to measure the spectra.

"The new spectrographs are much more efficient in infrared light," explained Natalie Roe of Berkeley Lab, the instrument scientist for BOSS. "The light emitted by distant galaxies arrives at Earth as infrared light, so these improved spectrographs are able to look much further back in time."

The ability to look further back in time is important in allowing BOSS to take advantage of a feature in the universe called "baryon oscillations," which began when pressure waves traveled through the early universe.

"Like earthquake waves passing through rock, these pressure waves pushed some of the matter in the universe closer together as they traveled, and left some of the matter further apart," said Nikhil Padmanabhan, a BOSS researcher who recently moved from Berkeley Lab to Yale University. "When the universe was only a few hundred thousand years old, the universe cooled enough to halt the waves, leaving imprints 500 million light years long that have lasted until the present day."

"We can see this frozen wave in the distribution of galaxies today," said Daniel Eisenstein of the University of Arizona, director of SDSS-III. "By measuring the length of the baryon oscillations, we can determine how dark energy has affected the expansion history of the universe. That, in turn, helps us figure out what dark energy could be."

"Studying baryon oscillations is an exciting method for measuring dark energy in a way that's complementary to techniques in supernova cosmology," said Schlegel.

"BOSS's galaxy measurements will be a revolutionary data set that will provide rich insights into the universe," added Martin White of Berkeley Lab, BOSS's survey scientist.

BOSS's spectrographs work with more than 2,000 metal plates that fit over the telescope; all these plates will mark the precise locations of nearly two million objects across the northern sky. Optical fibers plugged into 1,000 tiny holes in each of these "plug plates" carry the light from each observed galaxy or quasar to BOSS's new spectrographs.

Using these plug plates for the first light image was fast and easy, but it didn't quite turn out the way astronomers planned.

"In our first test images, it looked like we'd just taken random spectra from all over," Schlegel said. After some hair-pulling, the problem turned out to be simple. "After we flipped the plus and minus signs in the program, everything worked perfectly."

The first public data release from SDSS-III is planned for December 2010. Making high-quality astronomical data available to all on the Web, with no need to spend nights awake at a mountaintop telescope, promises a revolution in astronomical science and education. "This continues the legacy of the SDSS, one of the most productive astronomical surveys ever undertaken," said Jim Gunn of Princeton University, who recently was awarded the Presidential Medal for Science for his pioneering work with the original SDSS. "The leadership of this next generation of the SDSS has passed to the young scientists who did most of the hard work in SDSS I and II, and they have done a wonderful job, quickly and well. Bravo!"

Contact information, more detailed information about the project and high-resolution images related to this story are on the Web at

Last Updated November 01, 2010