"The volume probed here is the same as a cube 5.3 billion light years on a side," said principal author Nikhil Padmanabhan of Princeton University. "It reaches one-third of the way to the edge of the observable Universe, and we measure structures that extend over a significant fraction of that distance."

Up to now, Padmanabhan said, the most detailed sky map on such a scale was used only by surveys of the cosmic microwave background radiation, the cooled residual glow from the Big Bang that dimly illuminates the sky in all directions.

"By comparing the new measurements to the microwave background data, astronomers can test whether these enormous cosmic structures have grown at the expected rate - between the time the cosmic microwaves were emitted and the time that the light of the new structures was emitted," Padmanabhan explained.

"These measurements give much better determinations of the amount of dark matter in the universe," he said, adding that the sky map will help probe the nature of dark energy, the mysterious substance that seems to be accelerating the expansion of the universe.

"With the new measurements, our emerging picture of a universe dominated by dark matter and dark energy had a chance to fall on its face," said research co-author Uros Seljak. "Instead, it passed a new test with flying colors."

Among the SDSS-II data, the team found galactic structures imprinted by cosmic sound waves from the early universe. The waves, which measure some 450 million light-years, were first detected in early 2005 by independent teams using SDSS-II and the Two Degree Field Galaxy Redshift Survey.

"Astronomers are falling over themselves to measure the precise length of these sound waves," said co-author David Schlegel of Lawrence Berkeley National Laboratory in California. "Measuring this standard ruler at different epochs is one of the best tools we have for studying dark energy, the component of modern cosmology we understand least."

The researchers used a novel and efficient technique for mapping structure over a very large volume. Traditionally, astronomers determine distances to galaxies by the changes in the color of their light caused by the expansion of the universe - a property known as red shift. These changes are subtle, so the method usually requires a time-consuming, individual analysis for each galaxy - as astronomers typically must spread its light into a spectrum to reveal sharp features that can be measured precisely.

The SDSS-II team instead focused on a special class of bodies called luminous red galaxies, whose true colors are regular and well understood. They also are some of the brightest and reddest galaxies in the universe.

The highly accurate color measurements in the SDSS-II data are sufficient on their own to yield approximate distances for these galaxies and precise enough for large-scale clustering studies.

The new measurements are the first to show how the technique can reveal structure on the largest scales, with enough detail to detect sound waves and probe dark energy.

The team calibrated the relation between color and distance using spectral measurements for a small subset of the sample, obtained by an international collaboration of astronomers from the SDSS-II and the Australian-UK Two Degree Field team.

"This hybrid technique allows us to use all of our data to maximal effect," said co-author Daniel Eisenstein of the University of Arizona in Tucson. "We leveraged precise observations of 10,000 galaxies to gain fuzzy distances to nearly a million galaxies. The loss in accuracy is more than made up for by the sheer numbers we can now use."

The research appears in the May 15 issue of Monthly Notices of the Royal Astronomy Society.

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