How Big is Big? Probing the Conditions of the Universe on the Largest Scales
The SDSS telescope at Apache Point, NM has been used to create a map of regular galaxies (black points) and luminous red galaxies (red points) out to 40% of the distance to the edge of the visible universe. Light from the most distant red galaxies has taken 5.6 billion years to reach us, while the edge of the visible universe is 13.7 billion light years away. This map of the universe allowed astronomers to detect galactic structures more than a billion light years across. Credit: Hogg, SDSS-II collaboration
A team of scientists, which includes a Penn State astronomer, is announcing the construction of a new map of the sky containing more than 600,000 galaxies that covers distances of well over a billion light-years. “The map was created with data from several years of observations by the Sloan Digital Sky Survey,” said Penn State Professor of Astronomy and Astrophysics Donald Schneider, a coauthor of the investigation. “The extremely large luminosities of the galaxies allow us to detect them at great distances, and the large-scale distribution of the sources carries information about the conditions present in the very early universe.” Schneider is the Chairman of the Sloan Digital Sky Survey (SDSS) Quasar Science Group and the SDSS Scientific Publications Coordinator.
Since the 1970s, astronomers have discovered structures in the three-dimensional distribution of galaxies on ever larger scales, up to hundreds of millions of light years. Researchers from the Sloan Digital Sky Survey (SDSS-II) announced the first measurements of galactic structures more than a billion light years across.
The three-dimensional map of more than 600,000 galaxies covers over one-tenth of the sky. “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.”
“Structure on these scales were previously measured only in the cosmic microwave background radiation,” the cooled glow of the Big Bang that bathes the Earth from all directions, added co-author Uros Seljak, also of Princeton. 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,” said Padmanabhan.
“These measurements give much better determinations of the amount of dark matter in the Universe, and they also probe the nature of dark energy, the mysterious substance 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. Instead, it passed a new test with flying colors,” Seljak stated.
One especially exciting feature of the new measurements is the detection of galactic structure imprinted by cosmic sound waves in the early Universe. These sound waves have a length scale of 450 million light years, and they were first detected in early 2005 by independent teams from the SDSS-II and the Two Degree Field Galaxy Redshift Survey (2dFGRS).
The new study is the first to detect these sound waves at large distances from Earth that probe earlier cosmic epochs.
“Astronomers are falling over themselves to measure the precise length of these sound waves,” said co-author David Schlegel of Lawrence Berkeley National Laboratory. “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 new study uses a novel and efficient technique for mapping structure over a very large volume. Traditionally, astronomers determine distances to galaxies by measuring the changes in their colors caused by the expansion of the Universe. These changes are subtle, so this method usually requires a time-consuming, individual analysis for each galaxy. Researchers typically have to spread its light into a spectrum to reveal sharp features that can be precisely measured.
The SDSS-II team instead focused on a special class of galaxies, luminous red galaxies, whose true colors are very regular and well understood. As their name suggests, these are some of the brightest and reddest galaxies in the Universe. These highly accurate color measurements in the SDSS-II data are sufficient on their own to yield approximate distances for these galaxies. And they are precise enough for large scale clustering studies.
The new measurements are the first to show that this technique can reveal structure on the largest scales, with enough detail to detect sound waves and probe dark energy.
The relation between color and distance was calibrated 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 (2dF) 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. “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 findings are presented in “The Clustering of Luminous Red Galaxies in the Sloan Digital Sky Survey Imaging Data,” a paper submitted to the Monthly Notices of the Royal Astronomical Society, and available on the astrophysics preprint server, arxiv.org/astro-ph/0605302 on Monday, May 15th.
A simultaneous and independent analysis by a team of astronomers from Canada and the UK, led by University of British Columbia researcher Chris Blake, reached similar conclusions, analyzing a similar but not identical data set. Their research paper was also released today on the astrophysics preprint server.
A complete list of authors is available at <http://www.sdss.org/news/releases/20060515.structure.html>.
ABOUT THE SLOAN DIGITAL SKY SURVEY (www.sdss.org)
The Sloan Digital Sky Survey (SDSS-II) addresses fascinating, fundamental questions about the universe. With the survey, astronomers will be able to see the large-scale patterns of galactic sheets and voids in the universe. Scientists have varying ideas about the evolution of the universe, and different patterns of large-scale structure point to different theories of how the universe evolved. The Sloan Digital Sky Survey will tell us which theories are right -- or whether we have to come up with entirely new ideas.
Funding for the SDSS and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the U.S. Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society, and the Higher Education Funding Council for England. The SDSS Web Site is http://www.sdss.org/.
The SDSS is managed by the Astrophysical Research Consortium for the Participating Institutions: American Museum of Natural History, Astrophysical Institute Potsdam, University of Basel, Cambridge University, Case Western Reserve University, University of Chicago, Drexel University, The Fermi National Accelerator Laboratory, the Institute for Advanced Study, the Japan Participation Group, Johns Hopkins University, the Joint Institute for Nuclear Astrophysics, the Kavli Institute for Particle Astrophysics and Cosmology, the Korean Scientist Group, the Chinese Academy of Sciences (LAMOST), Los Alamos National Laboratory, the Max-Planck-Institute for Astronomy (MPA), the Max-Planck-Institute for Astrophysics (MPIA), New Mexico State University, Ohio State University, University of Pittsburgh, University of Portsmouth, Princeton University, the United States Naval Observatory, and the University of Washington.
CONTACTS AT SDSS:
Nikhil Padmanabhan, Princeton University, npadmana@princeton.edu, 609-258-4355
David Schlegel, Lawrence Berkeley National Laboratories, djschlegel@lbl.gov, 510-495-2595
Uros Seljak, Princeton University, useljak@princeton.edu, 609-258-4413
David Weinberg, Scientific Spokesperson, Sloan Digital Sky Survey, dhw@astronomy.ohio-state.edu, 614-292-6543
Gary S. Ruderman, Public Information Officer, Sloan Digital Sky Survey, sdsspio@aol.com, 312-320-4794
CONTACTS AT PENN STATE:
Donald Schneider, 814-863-9554, dps@astro.psu.edu
Barbara Kennedy (PIO), 814-863-4682, science@psu.edu