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A 3-D map of galaxies from the SDSS
This image shows the positions of the 900,000 luminous galaxies used in these studies. Each green dot represents one galaxy. The image covers a redshift range from 0.25 to 0.75, reaching back to six billion years ago.

Image Credit: David Kirkby (University of California, Irvine) and the SDSS-III Collaboration

Since 2000, scientists with the Sloan Digital Sky Survey have been meticulously scanning the sky to create a map of our universe. The SDSS's map is in full color, covers more than one quarter of the entire sky, and consists of more than one trillion pixels, full of so much detail that to view it all, you would need five hundred thousand high-definition TVs. The final version of this map was made available online last year, with tools to look at and search through it.

Over the past year, the SDSS's map has been viewed more than a million times, by astronomers, students, and citizen scientists from all over the world—and has been studied in depth by teams of scientists from the SDSS collaboration, based all over the world. Today, SDSS scientists announced results published in four papers on the arXiv preprint server that, taken together, interpret this map to retrace the history of the universe over the last six billion years.

Results were presented this afternoon at the annual meeting of the American Astronomical Society in Austin, Texas, and have been submitted to the Astrophysical Journal. They are currently available online on the arXiv preprint server.

Today's results come from studying the clustering of galaxies all over the sky. "The galaxies we see today give us clues to the history of our universe," says Shirley Ho, an astrophysicist at Lawrence Berkeley National Laboratory (LBL) and the Bruce and Astrid McWilliams Center for Cosmology at Carnegie Mellon University, who was the lead author of one of the papers posted today. "The way galaxies cluster together today can tell us two things. First, galaxy clustering can provide a measuring stick to see how the universe has expanded over time. Second, we can use that information to calculate exactly how much matter the universe contains, and what fraction consists of ordinary matter, dark matter, dark energy, and neutrinos."

Other papers explore various pieces of the universe in more detail. A team led by Hee-Jong Seo of the Berkeley Center for Cosmological Physics at LBL and the University of California Berkeley compared the observed clustering to that in the early universe to get a detailed picture of the universe's expansion, while a team led by Roland de Putter of the University of Barcelona used the clustering data to help pin down the mass of the neutrino, a subatomic particle that was only recently proven to have any mass at all. And none of these results would have been possible without the work of a team led by Ashley Ross of the University of Portsmouth (UK), who carefully studied how other effects, such as the presence of stars in our galaxy, affect these conclusions.Together, these four papers help us understand the structure of our universe in more detail than ever before.

The first step in the research was to identify 900,000 "luminous galaxies" seen by the SDSS—so-called because they shine much brighter than typical galaxies, meaning that they can be seen at great distances across the universe. The luminous galaxies chosen for this study cover the largest volume of space ever used for galaxy clustering measurements. Each galaxy's brightness was measured in five colors, and these colors give astronomers a rough estimate of the distance to each galaxy. "By covering such a large area of sky and working at such large distances, these measurements are able to probe the clustering of galaxies on incredibly vast scales," says Martin White, a member of the research team based at Lawrence Berkeley National Laboratory and the University of California Berkeley.

The luminous galaxy measurements were then used by Ross's team, which studied it carefully to see what additional factors needed to be taken into account. "Because we are looking out at the universe from one place—the Earth—we don't always get a clear picture of what the universe as a whole looks like," says Ross. "We have to carefully consider what that means, to make sure that we don't mistake an accident of our Earthbound view for the true structure of the Universe." In particular, the team found that stars in our galaxy block our view of distant galaxies, and that failing to account for the effects of the foreground stars results in overestimating galaxy clustering.

Armed with the proper estimates of how luminous galaxies cluster, the researchers compared the estimates for the clustering of nearby galaxies with those much farther away. "This analysis is one of the most trustworthy ways to measure dark energy," Seo says. "The imprint of sound waves in the early Universe leaves a clear pattern on the clustering of galaxies known as "baryon acoustic oscillation." By comparing the known size of this feature from 300,000 years after the Big Bang to what we found for galaxies seven to eleven billion years later, we can measure how the Universe has expanded over that time and learn about the nature of dark energy."

"These studies allow us to look back six billion years, to a time when the universe was almost half as old as it is now," said Antonio Cuesta of Yale University, a key member of all four research teams. Among the results: assuming the most widely accepted and likeliest cosmological model, the researchers found that dark energy accounts for 73 percent of the universe, with a margin of error of only two percent.

The SDSS's map covers the entire universe, at scales almost unimaginably huge—but amazingly, it can also offer insight into the almost unimaginably small. The universe is full of tiny particles called neutrinos, the by-products of the nuclear reactions that make stars shine. Many trillions of the tiny particles pass harmlessly through the Earth every second. We know that neutrinos have mass, but how much mass?

Particle physicists are using studies of atoms to investigating the masses of neutrinos, but astronomy offers another way. A team led by Roland de Putter of the University of Valencia in Spain examined the SDSS's map to estimate the largest neutrino mass consistent with the universe we see. "One of the greatest laboratories for particle physics is the universe itself," de Putter says. The team's study pinpointed the largest possible neutrino mass at 0.3 electron-volts (5 × 10-34 grams)—a better constraint on the neutrino mass than can be offered by traditional particle physics methods, by a factor of 10.

The four papers published today fit together to help us understand the history of our universe in unprecedented detail. But even more detail is still to come. Later this year, the SDSS will come out with Data Release 9, which will include highly accurate distance measurements to many galaxies, substituting these accurate measurements for the estimates used in today's results.

"For each and every one of our million galaxies," Cuesta says, "we will replace its estimated distance with a very precise measure. Our upcoming map will bring the universe into sharp focus." Seeing the universe in sharp focus will almost certainly help advance our understanding of the whole universe—from the very large to the very small.

Video Animation

If you cannot see this video, visit this link.
This YouTube video shows the positions of the 900,000 luminous galaxies used in these studies. Each green dot represents one galaxy. The image covers a redshift range from 0.25 to 0.75, reaching to six billion years ago. The rotation of the image provides a view that shows what the distribution would look like from all sides. Click on the movie to start or stop playing the movie.

Animation credit: David Kirkby (University of California, Irvine) and the SDSS-III Collaboration

High resolution versions of this video are available in 1280×720 (HD720p) H.264 mp4 (or at a lower bitrate for iOS devices) or Theora Ogg.

Additional Resources

The four papers involved in this work are:



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. The SDSS-III web site is

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, 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, 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.