The Most Precise Measurement Yet of the Expanding Universe Is Achieved by Astronomers of the Sloan Digital Sky Survey

This drawing illustrates how astronomers of the third Sloan Digital Sky Survey (SDSS-III) used quasar light to trace the expansion of the universe. The expansion is shown by the circular disks of increasing area from left to right. From the Big Bang, the expansion occurs rapidly, then slows down, then speeds up again as dark energy pushes apart walls and filaments of galaxies at different distances (purple). As light travels to us from very distant quasars (white dots on the left), it passes through the expanding universe, carrying with it the story of its journey through this expanding cosmic web. Astronomers have measured the expansion of the universe by tracing how quasar light has passed through these structures.  Credit: Paul Hooper at Spirit Design, with Mat Pieri and Gongbo Zhao, ICG

Astronomers at Penn State University and other institutions participating in the Sloan Digital Sky Survey have used 140,000 distant quasars to measure the expansion rate of the universe when it was only one-quarter of its present age. This measurement is the best yet of the expansion rate at any epoch in the last 13 billion years during the history of the universe. Measuring the expansion rate of the universe over its entire history is key to determining the nature of the dark energy that is responsible for causing this expansion rate to increase during the most recent six billion years.

"This observation represents an impressive advance in our attempts to determine the expansion history of the universe," remarked Donald Schneider, Distinguished Professor of Astronomy and Astrophysics at Penn State and coauthor of the investigation. "Forty years ago attempts to measure the expansion rate at the present time were plagued with uncertainties of a factor of two. Now, with these new data, we know to high precision the expansion rate over five billion years before the Sun began to shine." Schneider is the Survey Coordinator of the Sloan Digital Sky Survey.

The technique of measuring the structure of the young universe by using quasars to map the distribution of intergalactic hydrogen gas was pioneered by the largest component of the third Sloan Digital Sky Survey (SDSS-III), the Baryon Oscillation Spectroscopic Survey (BOSS). New BOSS observations of this structure are being presented on 7 April 2014 at the April 2014 meeting of the American Physical Society in Savannah, GA.

An artist's conception of how the Baryon Oscillation Spectroscopic Survey (BOSS) -- the largest component of the third Sloan Digital Sky Survey (SDSS-III) -- uses quasars to measure the distant universe. Light from distant quasars is partly absorbed by intervening gas, which is imprinted with a subtle ring-like pattern. Astronomers now have measured the scale of this ring-like pattern with an accuracy of two percent -- a precise measurement of how fast the universe was expanding when it was just 3 billion years old.  Image credit: Zosia Rostomian (Lawrence Berkeley National Laboratory) and Andreu Font-Ribera (BOSS Lyman-alpha team, Berkeley Lab)

These latest results combine two different methods of using quasars and intergalactic gas to measure the rate of expansion of the universe. The first analysis, by Andreu Font-Ribera of Lawrence Berkeley National Laboratory and his collaborators, compares the distribution of quasars to the distribution of hydrogen gas as a way of measuring distances in the universe. A second analysis team led by Timothee Delubac of the Centre de Saclay, France, focused on the patterns in the hydrogen gas, itself, to measure the distribution of mass in the young universe. Together, the two BOSS analyses establish that, 10.8 billion years ago, the universe was expanding by one percent every 44 million years.

"We have measured the expansion rate in the young universe with an unprecedented precision of 2 percent" Delubac said. "By probing the universe when it was only a quarter of its present age, BOSS has placed a key anchor to compare to more recent expansion measurements as dark energy has taken hold."

"The past two decades have seen a dramatic revision of our view of the universe, as we now realize that not only is the universe expanding but that the expansion is accelerating," Schneider remarked. "Measurements such as the BOSS result, which allow us to track the past behavior of the expansion, are crucial in our efforts to understand the nature of the cosmos."

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's Office of Science. This research used resources of the National Energy Research Scientific Computing Center (NERSC), which is supported by the Office of Science.

CONTACTS

• Donald P. Schneider, Penn State, dps7@psu.edu, 1-814-863-9554

• Michael Wood-Vasey, SDSS-III Spokesperson, University of Pittsburgh, wmwv@pitt.edu, 1-412-624-2751

• Barbara Kennedy, Penn State Science Public Information Officer, science@psu.edu, 1-814-863-4682

• Jordan Raddick, SDSS-III Public Information Officer, Johns Hopkins University, raddick@jhu.edu, 1-410-516-8889

MORE INFORMATION ABOUT THE BOSS PROJECT'S INNOVATIVE METHODS

Astronomers on the BOSS team determine the expansion rate at a given time in the universe by measuring the size of baryon acoustic oscillations (BAO), a signature imprinted in the way matter is distributed, resulting from sound waves in the early universe. This imprint is visible in the distribution of galaxies, quasars, and intergalactic hydrogen throughout the cosmos. "Three years ago, BOSS used 14,000 quasars to demonstrate we could make the biggest 3-D maps of the universe," says David Schlegel of Lawrence Berkeley National Laboratory, principal investigator of BOSS. "Two years ago, with 48,000 quasars, we first detected baryon acoustic oscillations in these maps. Now, with more than 140,000 quasars, we've made extremely precise measures of these BAO signatures." The astronomers explain that, as the light from a distant quasar passes through intervening hydrogen gas distributed throughout the universe, patches of greater density absorb more light. Each absorbing patch absorbs light from the spectrum of the quasar at a characteristic wavelength of neutral hydrogen. As the universe expands, the quasar spectrum is stretched out, and each subsequent patch leaves its absorption mark at a different relative wavelength. The BOSS project obtains a spectrum of the quasar spectrum, which contains the signatures of all the patches encountered by the quasar light. Astronomers then measure from the quasar spectrum how much the universe has expanded since the light passed through each patch of hydrogen. With these high-quality quasar spectra, the position of the gas clouds can be mapped in three dimensions. The BOSS team determines the expansion rate by using these maps to measure the size of the BAO pattern at different epochs of cosmic time. These new measurements provide key data for astronomers seeking the nature of the dark energy postulated to be driving the increase in the expansion rate of the universe.

REFERENCES

1. "Quasar-Lyman-alpha Forest Cross-Correlation from BOSS DR11: Baryon Acoustic Oscillations," by Andreu Font-Ribera, David Kirkby, Nicolas Busca, Jordi Miralda-Escude, Nicholas P. Ross, Anze Slosar, Eric Aubourg, Stephen Bailey, Vaishali Bhardwaj, Julian Bautista, Florian Beutler, Dmitry Bizyaev, Michael Blomqvist, Howard Brewington, Jon Brinkmann, Joel R. Brownstein, Bill Carithers, Kyle S. Dawson, Timothee Delubac, Garrett Ebelke, Daniel J. Eisenstein, Jian Ge, Karen Kinemuchi, Khee-Gan Lee, Viktor Malanushenko, Elena Malanushenko, Moses Marchante, Daniel Margala, Demitri Muna, Adam D. Myers, Pasquier Noterdaeme, Daniel Oravetz, Nathalie Palanque-Delabrouille, Isabelle Paris, Patrick Petitjean, Matthew M. Pieri, Graziano Rossi, Donald P. Schneider, Audrey Simmons, Matteo Viel, Christophe Yeche, and Donald G. York, has been submitted to the Journal of Cosmology and Astroparticle Physics and is available online at http://arxiv.org/abs/1311.1767.

2. "Baryon Acoustic Oscillations in the Ly-alpha forest of BOSS DR11 quasars," by Timothee Delubac, Julian E. Bautista, Nicolas G. Busca, James Rich, David Kirkby, Stephen Bailey, Andreu Font-Ribera, Anze Slosar, Khee-Gan Lee, Matthew M. Pieri, Jean-Christophe Hamilton, Michael Blomqvist, Jo Bovy, William Carithers, Kyle S. Dawson, Daniel J. Eisenstein, J.-M. Le Goff, Daniel Margala, Jordi Miralda-Escude, Adam Myers, Robert C. Nichol, Pasquier Noterdaeme, Ross O'Connell, Nathalie Palanque-Delabrouille, Isabelle Paris, Patrick Petitjean, Nicholas P. Ross, Graziano Rossi, David J. Schlegel, Donald P. Schneider, David H. Weinberg, and Christophe Yeche, has been submitted to Astronomy & Astrophysics.

ABOUT THE SLOAN DIGITAL SKY SURVEY

More information about SDSS-III is online at www.sdss3.org/. 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, Brookhaven National Laboratory, University of Cambridge, Carnegie Mellon University, University of Florida, the French Participation Group, the German Participation Group, Harvard University, the Instituto de Astrofisica de Canarias, the Michigan State/Notre Dame/JINA Participation Group, Johns Hopkins University, Lawrence Berkeley National Laboratory, Max Planck Institute for Astrophysics, Max Planck Institute for Extraterrestrial Physics, New Mexico State University, New York University, Ohio State University, Pennsylvania State University, University of Portsmouth, Princeton University, the Spanish Participation Group, University of Tokyo, University of Utah, Vanderbilt University, University of Virginia, University of Washington, and Yale University.

The main SDSS-III press release is available online at www.sdss3.org/press/precise.php.