Record-breaking map of 1.2-million galaxies ready to reveal dark matter secrets

July 14, 2016

Astronomers are announcing this week the sharpest view yet of the properties of dark energy -- the force that currently is driving the accelerated expansion of the universe. "These results are a milestone in the study of the large-scale structure of the universe," said Penn State professor Donald Schneider, who was the survey coordinator and scientific publications coordinator for the Sloan Digital Sky Survey III (SDSS-III) -- a collaboration of hundreds of scientists whose work produced the largest-ever, three-dimensional map of distant galaxies as well as one of the most precise measurements yet of dark energy.

"The SDSS-III investigation determined the locations of 1.2 million galaxies as a result of its observations, which began in 2009 and concluded in 2014," Schneider said. Jeremy Tinker of New York University, a co-leader of the team that performed the investigation, commented commented that "This map has allowed us to make the best measurements yet of the effects of dark energy in the expansion of the universe."

Shaped by a continuous tug-of-war between dark matter and dark energy, the SDSS-III map now has allowed astronomers to measure the expansion rate of the universe and thus to determine the amount of matter and dark energy that make up the present-day universe. A collection of papers describing these results was submitted this week to the Monthly Notices of the Royal Astronomical Society.

These new measurements were carried out by the Baryon Oscillation Spectroscopic Survey (BOSS) program of the SDSS-III collaboration. The scientists measured the expansion rate of the universe by determining the size of the baryonic acoustic oscillations (BAO) in the three-dimensional distribution of galaxies. Ariel Sanchez of the Max-Planck Institute of Extraterrestrial Physics, who led the effort to estimate the exact amount of dark matter and dark energy based on the BOSS data, explains: "Measuring the acoustic scale across cosmic history gives a direct ruler with which to measure the universe's expansion rate. With BOSS, we have traced the BAO's subtle imprint on the distribution of galaxies spanning a range of time from 2 to 7 billion years ago."

Scientists determine the original BAO size by pressure waves that travelled through the young universe up to when it was only 400,000 years old, at which point these pressure waves became frozen in the matter distributed throughout the universe. The end result is that galaxies are preferentially separated by a characteristic distance, which astronomers call the acoustic scale. The size of the acoustic scale at 13.8 billion years ago has been exquisitely determined from observations of the cosmic microwave background from the light emitted when the pressure waves became frozen. Measuring the distribution of galaxies since that time until now -- when the universe is 13.8 billion years old -- allows astronomers to measure how dark matter and dark energy have competed to govern the rate of expansion of the universe.

Rita Tojeiro of the University of St. Andrews is the other co-leader of the BOSS galaxy clustering working group along with Jeremy Tinker. "If we were to scale the volume of our survey to a cube 1 mile on each side, then the visible part of an individual galaxy would be about 1 millimeter across," she explains. "Our challenge in the analysis of this map was equivalent to measuring the distances between all the pairs of galaxies separated by 100 yards throughout a cubic mile of space."

To measure the size of these ancient giant waves to such sharp precision, BOSS had to make an unprecedented and ambitious galaxy map, many times larger than previous surveys. At the time the BOSS program was planned, dark energy previously had been determined to significantly influence the expansion of the universe starting about 5 billion years ago. BOSS was thus designed to measure the BAO feature from before this point (7 billion years ago) out to near the present day (2 billion years ago).

"We see a dramatic connection between the sound wave imprints seen in the cosmic microwave background 400,000 years after the Big Bang to the clustering of galaxies 7-12 billion years later," Tojeiro said. "The ability to observe a single well-modeled physical effect from recombination until today is a great boon for cosmology."

The map also reveals the distinctive signature of the coherent movement of galaxies toward regions of the universe with more matter, due to the attractive force of gravity. Crucially, this amount of movement is explained well by the predictions of general relativity. So the SDSS-III BOSS measurements support the idea that the acceleration of the expansion rate of the universe is driven by a phenomenon at the largest cosmic scales, such as dark energy, rather than by a breakdown of the theory of gravitation.

"In the near future, astronomers plan to push this high-precision study of BAO features to periods that are even earlier in the universe's history," Schneider said. "We hope that investigations of these distant realms will, when combined with the SDSS-III BOSS measurements, reveal the nature of dark energy, which -- although it appears to comprise the majority of the material in the universe -- remains a profound mystery."

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 website is

The SDSS-III press release concerning this research achievement is online at 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, 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.

  • SDSS Sloan Digital Sky Survey

    This image shows a section of the three-dimensional map constructed by the Sloan Digital Sky Survey's BOSS project. The rectangle on the far left shows a cutout of 1000 sq. degrees in the sky containing nearly 120,000 galaxies, or roughly 10% of the total survey. The spectroscopic measurements of each galaxy -- every dot in that cutout -- transform the two-dimensional picture into a three-... Read more ›

    IMAGE: Jeremy Tinker and SDSS-III

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Last Updated July 15, 2016