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When Dark Energy Turned On

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30 March 2012
Beam of light shooting out from one plane and  crossing through 2 more planes.

These schematic images show the universe at three different times in its history. The false-color image on the far left shows the "cosmic microwave background," a record of what the universe looked like when it was just 300,000 years old. The density variations there grew into the walls and filaments of galaxies we see today. The white circle shows the size of the "baryon acoustic oscillations" imprinted in the structure of the universe. The center image shows the results released today by SDSS-III, which shows galaxies at a later time; the right image shows previous SDSS results, which show galaxies even later. Over time, as the universe expanded, galaxies grew farther apart. The size of the original variations thus grew larger, leading to the peak separation between galaxies observed by SDSS-III today (represented by the ever-larger white circles).

Credit: E.M. Huff, the SDSS-III team, and the South Pole Telescope team; graphic by Zosia Rostomian

The Sloan Digital Sky Survey (SDSS-III) today announced the most accurate measurements yet of the distances to galaxies in the faraway universe, giving an unprecedented look at the time when the universe first began to expand at an ever-increasing rate.

The results, announced today at a press conference in Manchester, England, are the culmination of more than two years of work by the team of scientists and engineers, including a Penn State astronomer, behind the Baryon Oscillation Spectroscopic Survey (BOSS), one of the SDSS-III's four component surveys.

"These studies provide us with new insights about the evolutionary history of the universe," said Donald Schneider, Distinguished Professor of Astronomy and Astrophysics at Penn State, who also serves as the SDSS-III survey coordinator and scientific publications manager.

"There's been a lot of talk about using galaxy maps to find out what's causing accelerating expansion," says David Schlegel of the U.S. Department of Energy's Lawrence Berkeley National Laboratory, the principal investigator of BOSS. "We've been making a map, and now we're using it -- starting to push our knowledge out to the distances when dark energy turned on."

"The result is phenomenal," says Will Percival, a professor at the University of Portsmouth in the United Kingdom, and one of the leaders of the analysis team. "We have only one-third of the data that BOSS will deliver, and that has already allowed us to measure how fast the Universe was expanding six billion years ago -- to an accuracy of two percent."

One of the most amazing discoveries of the last two decades in astronomy, recognized with the 2011 Nobel Prize in Physics, was that not only is our universe expanding, but that this expansion is accelerating -- not only are galaxies are becoming farther apart from each other, they are becoming farther apart faster and faster.

What could be the cause of this accelerating expansion? The leading contender is a strange property of space dubbed "dark energy."

Another explanation, considered possible but less likely, is that at very large distances the force of gravity deviates from Einstein's General Theory of Relativity and becomes repulsive.

Whether the answer to the puzzle of the accelerating universe is dark energy or modified gravity, the first step to finding that answer is to measure accurate distances to as many galaxies as possible. From those measurements, astronomers can trace out the history of the universe's expansion.

BOSS is producing the most detailed map of the universe ever made, using a new custom-designed spectrograph of the SDSS 2.5-meter telescope at Apache Point Observatory in New Mexico. With this telescope and its new spectrograph, BOSS will measure spectra of more than a million galaxies over six years. The maps analyzed in today's papers are based on data from the first year and a half of observations, and contain more than 250,000 galaxies. Some of these galaxies are so distant that their light has traveled more than six billion years to reach the earth -- nearly half the age of the universe.

Maps of the universe like BOSS's show that galaxies and clusters of galaxies are clumped together into walls and filaments, with giant voids between. These structures grew out of subtle variations in density in the early universe, which bore the imprint of "baryon acoustic oscillations" -- pressure-driven (acoustic) waves that passed through the early universe.

Billions of years later, the record of these waves can still be read in our universe. "Because of the regularity of those ancient waves, there's a slightly increased probability that any two galaxies today will be separated by about 500 million light-years, rather than 400 million or 600 million," says Daniel Eisenstein of the Harvard-Smithsonian Center for Astrophysics, director of SDSS-III and a pioneer in baryon oscillation surveys for nearly a decade. In a graph of the number of galaxy pairs by separation distance, that magic number of 500 million light years shows up as a peak, so astronomers often speak of the "peak separation" between galaxies. The distance that corresponds to this peak depends on the amount of dark energy in the Universe. But measuring the peak separation between galaxies depends critically on having the right distances to the galaxies in the first place.

That's where BOSS comes in. "We've detected the peak separation more clearly than ever before," says Nikhil Padmanabhan of Yale University, who along with Percival co-chairs the BOSS team's galaxy clustering group. "These measurements allow us to determine the contents of the Universe with unprecedented accuracy."

In addition to providing highly accurate distance measurements, the BOSS data also enable a stringent new test of General Relativity, explains Beth Reid, a NASA Hubble Fellow at Lawrence Berkeley National Laboratory. "Since gravity attracts, galaxies at the edges of galaxy clusters fall in toward the centers of the clusters," Reid says. "General Relativity predicts just how fast they should be falling. If our understanding of General Relativity is incomplete, we should be able to tell from the shapes we see in BOSS's maps near known galaxy clusters."

Reid led the analysis of these "redshift space distortions" in BOSS. After accounting for the effects of dark energy, Reid's team found that the rate at which galaxies fall into clusters is consistent with Einstein's predictions. "We already knew that the predictions of General Relativity are extremely accurate for distances within the solar system," says Reid, "and now we can say that they are accurate for distances of 100 million light-years. We're looking a billion times further away than Einstein looked when he tested his theory, but it still seems to work."

What's amazing about these results -- six papers covering the measurements of cosmic distance and the role of gravity in galaxy clustering -- is that they all come together to tell the same story. "All the different lines of evidence point to the same explanation," says Ariel Sanchez, a research scientist at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, and lead author on one of the papers. "Ordinary matter is only a few percent of the universe. The largest component of the universe is dark energy -- an irreducible energy associated with space itself that is causing the expansion of the universe to accelerate."

But this is just the beginning, says BOSS principal investigator Schlegel. "For the past 13 years, we've had a simple model of how dark energy works. But the truth is, we only have a little bit of data, and we're just beginning to explore the times when dark energy turned on. If there are surprises lurking out there, we expect to find them."

[ Jordan Raddick ]




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 Office of Science. The SDSS-III web site is 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.

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