This story appeared today in isgtw.
This image shows over a million luminous galaxies at redshifts indicating times when the universe was between seven and eleven billion years old, from which the sample in the current studies was selected. Image by David Kirkby of the University of California at Irvine and the SDSS collaboration.
Since 2000, the three Sloan Digital Sky Surveys (SDSS I, II, and III) have surveyed well over a quarter of the night sky, producing the biggest 3-D color map of the universe ever made. Now, scientists have used this visual information for the most accurate computation yet of how matter clumped together – from a time when the universe was only half its present age until now.
“The way galaxies cluster together over vast expanses of the sky tells us how both ordinary visible matter and underlying invisible dark matter are distributed, across space and back in time,” said Shirley Ho, an astrophysicist at Lawrence Berkeley National Laboratory and Carnegie Mellon University who led the work. “The distribution gives us cosmic rulers to measure how the universe has expanded, and a basis for calculating what’s in it: how much dark matter, how much dark energy, even the mass of the hard-to-see neutrinos it contains. What’s left over is the ordinary matter and energy we’re familiar with.”
For the present study, Ho and her colleagues first selected 900,000 luminous galaxies from among over 1.5 million such galaxies gathered by the Baryon Oscillation Spectrographic Survey, or BOSS, the largest component of the still-ongoing SDSS III. Most of these are ancient red galaxies, which contain only red stars because all their faster-burning stars are long gone, and which are exceptionally bright and visible at great distances. The galaxies chosen for this study populate the largest volume of space ever used for galaxy clustering measurements. Their brightness was measured in five different colors, allowing the redshift of each to be estimated.
“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, giving us unprecedented constraints on the expansion history, contents, and evolution of the universe,” said Berkeley Lab's Martin White, chair of the BOSS science survey teams. “The clustering we’re now measuring on the largest scales also contains vital information about the origin of the structure we see in our maps, all the way back to the epoch of inflation, and it helps us to constrain – or rule out – models of the very early universe.”
After augmenting their study with information from other data sets, the team derived a number of such cosmological constraints (measurements of the universe’s contents based on different cosmological models). Among the results: in the most widely accepted model, the researchers found – to less than two percent uncertainty – that dark energy accounts for 73 % of the density of the universe.
The team’s results are presented 11 January 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 at http://arxiv.org/abs/1201.2137.
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