3D map of Universe bolsters case for dark energy and dark matter

Here are some resources on the new SDSS measurements of the galaxy power spectrum and cosmological parameters, including the press release and print-quality images. If you are a scientist, you'll want to click here for the papers, downloadable data and technical figures.


The SDSS is two separate surveys in one: galaxies are identified in 2D images (right), then have their distance determined from their spectrum to create a 2 billion lightyears deep 3D map (left) where each galaxy is shown as a single point, the color representing the luminosity - this shows only those 66,976 our of 205,443 galaxies in the map that lie near the plane of Earth's equator. (Click for high resolution jpg, version without lines.)

The new SDSS results (black dots) are the most accurate measurements to date of how the density of the Universe fluctuates from place to place on scales of millions of lightyears. These and other cosmological measurements agree with the theoretical prediction (blue curve) for a Universe composed of 5% atoms, 25% dark matter and 70% dark energy. The larger the scales we average over, the more uniform the Universe appears. (Click for high resolution jpg, no frills version.)

Press release

The following release was issued by the Sloan Digital sky Survey press office in Chicago, Illinois, on October 22, 2003

3D Map of Universe Bolsters Case for Dark Energy and Dark Matter



October 27, 2003 -- Astronomers from the Sloan Digital Sky Survey (SDSS) have made the most precise measurement to date of the cosmic clustering of galaxies and dark matter, refining our understanding of the structure and evolution of the Universe.

"From the outset of the project in the late 80's, one of our key goals has been a precision measurement of how galaxies cluster under the influence of gravity", explained Richard Kron, SDSS's director and a professor at The University of Chicago.

SDSS Project spokesperson Michael Strauss from Princeton University and one of the lead authors on the new study elaborated that: "This clustering pattern encodes information about both invisible matter pulling on the galaxies and about the seed fluctuations that emerged from the Big Bang."

The findings are described in two papers submitted to the Astrophysical Journal and to the Physical Review D; they can be found on the physics preprint Web site, www.arXiv.org, on October 28.


The leading cosmological model invokes a rapid expansion of space known as inflation that stretched microscopic quantum fluctuations in the fiery aftermath of the Big Bang to enormous scales. After inflation ended, gravity caused these seed fluctuations to grow into the galaxies and the galaxy clustering patterns observed in the SDSS.

Images of these seed fluctuations were released from the Wilkinson Microwave Anisotropy Probe (WMAP) in February, which measured the fluctuations in the relic radiation from the early Universe.

"We have made the best three-dimensional map of the Universe to date, mapping over 200,000 galaxies up to two billion light years away over six percent of the sky", said another lead author of the study, Michael Blanton from New York University. The gravitational clustering patterns in this map reveal the makeup of the Universe from its gravitational effects and, by combining their measurements with that from WMAP, the SDSS team measured the cosmic matter to consist of 70 percent dark energy, 25 percent dark matter and five percent ordinary matter.

They found that neutrinos couldn't be a major constituent of the dark matter, putting the strongest constraints to date on their mass. Finally, the SDSS research found that the data are consistent with the detailed predictions of the inflation model.


These numbers provide a powerful confirmation of those reported by the WMAP team. The inclusion of the new SDSS findings helps to improve measurement accuracy, more than halving the uncertainties from WMAP on the cosmic matter density and on the Hubble parameter (the cosmic expansion rate). Moreover, the new measurements agree well with the previous state-of-the-art results that combined WMAP with the Anglo-Australian 2dF galaxy redshift survey.

"Different galaxies, different instruments, different scientists and different analyses - but the results agree beautifully", says Max Tegmark from the University of Pennsylvania, first author on the two papers. "Carl Sagan was fond of saying that extraordinary claims require extraordinary evidence", Tegmark says, "but we now have extraordinary evidence for dark matter and dark energy and have to take them seriously no matter how disturbing they seem."

"The real challenge is now to figure what these mysterious substances actually are", said another author, David Weinberg from Ohio State University.


The SDSS is the most ambitious astronomical survey ever undertaken, with more than 200 astronomers at 13 institutions around the world.

"The SDSS is really two surveys in one", explained Project Scientist James Gunn of Princeton University. On the most pristine nights, the SDSS uses a wide-field CCD camera (built by Gunn and his team at Princeton University and Maki Sekiguchi of the Japan Participation Group) to take pictures of the night sky in five broad wavebands with the goal of determining the position and absolute brightness of more than 100 million celestial objects in one-quarter of the entire sky. When completed, the camera was the largest ever built for astronomical purposes, gathering data at the rate of 37 gigabytes per hour.

On nights with moonshine or mild cloud cover, the imaging camera is replaced with a pair of spectrographs (built by Alan Uomoto and his team at The Johns Hopkins University). They use optical fibers to obtain spectra (and thus redhsifts) of 608 objects at a time. Unlike traditional telescopes in which nights are parceled out among many astronomers carrying out a range of scientific programs, the special-purpose 2.5m SDSS telescope at Apache Point Observatory in New Mexico is devoted solely to this survey, to operate every clear night for five years.

The first public data release from the SDSS, called DR1, contained about 15 million galaxies, with redshift distance measurements for more than 100,000 of them. All measurements used in the findings reported here would be part of the second data release, DR2, which will be made available to the astronomical community in early 2004.

Strauss said the SDSS is approaching the halfway point in its goal of measuring one million galaxy and quasar redshifts.

"The real excitement here is that disparate lines of evidence from the cosmic microwave background (CMB), large-scale structure and other cosmological observations are all giving us a consistent picture of a Universe dominated by dark energy and dark matter", said Kevork Abazajian of the Fermi National Accelerator Laboratory and the Los Alamos National Laboratory.

(A complete list of authors and institutions can be found at www.sdss.org)

ILLUSTRATIONS: http://www.hep.upenn.edu/~max/sdss/release.html


The Sloan Digital Sky Survey (www.sdss.org) is a joint project of The University of Chicago, Fermilab, the Institute for Advanced Study, the Japan Participation Group, The Johns Hopkins University, the Los Alamos National Laboratory, the Max-Planck-Institute for Astronomy (MPIA), the Max-Planck- Institute for Astrophysics (MPA), New Mexico State University, University of Pittsburgh, Princeton University, the United States Naval Observatory, and the University of Washington.

Funding for the project has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Aeronautics and Space Administration, the National Science Foundation, the U.S. Department of Energy, the Japanese Monbukagakusho, and the Max Planck Society.


The press release above refers to two papers appearing simultaneously on astro-ph in the mailing of Tuesday October 28, 2003:
  1. The 3D power spectrum of galaxies from the SDSS, Max Tegmark, Michael R. Blanton, Michael A. Strauss, Fiona S. Hoyle, David Schlegel, Roman Scoccimarro, Michael S. Vogeley, David H. Weinberg, Idit Zehavi Andreas Berlind, Tamas Budavari, Andrew Connolly, Scott Dodelson, Daniel J. Eisenstein, Joshua A. Frieman, James E. Gunn, Lam Hui, Bhuvnesh Jain, David Johnston, Stephen Kent, Huan Lin, Reiko Nakajima, Robert C. Nichol, Adrian Pope, Ryan Scranton, Uros Seljak, Ravi K. Sheth, Albert Stebbins, Alexander S. Szalay, Istvan Szapudi, Yongzhong Xu, James Annis, Neta A. Bahcall, J. Brinkmann, Istvan Csabai, Jon Loveday, Mamoru Doi, Masataka Fukugita, Richard Gott III, Greg Hennessy, David Hogg, Zeljko Ivezic, Gillian R. Knapp, Don Q. Lamb, Brian C. Lee, Robert H. Lupton. Timothy A. McKay. Peter Kunszt. Jeffrey A. Munn. Liam O'Connell. Jeremiah P. Ostriker, John Peoples, Jeffrey R. Pier, Michael Richmond, Constance Rockosi, Christopher Stoughton, Douglas L. Tucker, Brian Yanny and Donald G. York, for the SDSS Collaboration, submitted to the Astrophysical Journal (40 ApJ pages, 40 figures, 3 tables)
    Download: pdf, ps
  2. Cosmological parameters from SDSS and WMAP, Max Tegmark, Michael Strauss, Michael R. Blanton, Kev Abazajian, Scott Dodelson, Havard Sandvik, Xiaomin Wang, David H. Weinberg, Idit Zehavi, Neta A. Bahcall, Fiona Hoyle, David Schlegel, Roman Scoccimarro, Michael S. Vogeley, Andreas Berlind, Tamas Budavari, Andrew Connolly, Daniel J. Eisenstein, Douglas Finkbeiner, Joshua A. Frieman, James E. Gunn, Lam Hui, Bhuvnesh Jain, David Johnston, Stephen Kent, Huan Lin, Reiko Nakajima, Robert C. Nichol, Adrian Pope, Ryan Scranton, Uros Seljak, Ravi K. Sheth, Albert Stebbins, Alexander S. Szalay, Istvan Szapudi, Yongzhong Xu, James Annis, J. Brinkmann, Scott Burles, Francisco J. Castander, Istvan Csabai, Jon Loveday, Mamoru Doi, Masataka Fukugita, Greg Hennessy, David W. Hogg, Zeljko Ivezic, Gillian R. Knapp, Don Q. Lamb, Brian C. Lee, Robert H. Lupton, Timothy A. McKay, Peter Kunszt, Jeffrey A. Munn, Liam O'Connell, Jeremiah P. Ostriker, John Peoples, Jeffrey R. Pier, Michael Richmond, Constance Rockosi, Donald P. Schneider, Christopher Stoughton, Douglas L. Tucker, Daniel E. Vanden Berk, Brian Yanny and Donald G. York, submitted to Physical Review D (26 PRD pages, 18 figures, 8 tables)
    Download: pdf, ps
Here are the abstracts of these two papers:

The 3D Power Spectrum of Galaxies from the SDSS

We measure the large-scale real-space power spectrum P(k) using a sample of 205,443 galaxies from the Sloan Digital Sky Survey, covering 2417 effective square degrees with mean redshift z~0.1. We employ a matrix-based method using pseudo-Karhunen-Loeve eigenmodes, producing uncorrelated minimum-variance measurements in 22 k-bands of both the clustering power and its anisotropy due to redshift-space distortions, with narrow and well-behaved window functions in the range 0.02h/Mpc < k < 0.3h/Mpc. We pay particular attention to modeling, quantifying and correcting for potential systematic errors, nonlinear redshift distortions and the artificial red-tilt caused by luminosity-dependent bias. Our results are robust to omitting angular and radial density fluctuations and are consistent between different parts of the sky. Our final result is a measurement of the real-space matter power spectrum P(k) up to an unknown overall multiplicative bias factor. Our calculations suggest that this bias factor is independent of scale to better than a few percent for k < 0.1h/Mpc, thereby making our results useful for precision measurements of cosmological parameters in conjunction with data from other experiments such as the WMAP satellite. The power spectrum is not well-characterized by a single power law, but unambiguously shows curvature. As a simple characterization of the data, our measurements are well fit by a flat scale-invariant adiabatic cosmological model with h Omega_m = 0.213 +/- 0.023 and sigma_8 = 0.89 +/- 0.02 for L* galaxies, when fixing the baryon fraction Omega_b/Omega_m=0.17 and the Hubble parameter h=0.72.

Cosmological parameters from SDSS and WMAP

We measure cosmological parameters using the three-dimensional power spectrum P(k) from over 200,000 galaxies in the Sloan Digital Sky Survey (SDSS) in combination with WMAP and other data. Our results are consistent with a ``vanilla'' flat adiabatic Lambda-CDM model without tilt (n=1), running tilt, tensor modes or massive neutrinos. Adding SDSS information more than halves the WMAP-only error bars on some parameters, tightening 1 sigma constraints on the Hubble parameter from h~0.74+0.18-0.07 to h~0.70+0.04-0.03, on the matter density from Omega_m~0.25+/-0.10 to Omega_m~0.30+/-0.04 (1 sigma) and on neutrino masses from <11 eV to <0.6 eV (95%). SDSS helps even more when dropping prior assumptions about curvature, neutrinos, tensor modes and the equation of state. Our results are in substantial agreement with the joint analysis of WMAP and the 2dF Galaxy Redshift Survey, which is an impressive consistency check with independent redshift survey data and analysis techniques. In this paper, we place particular emphasis on clarifying the physical origin of the constraints, i.e., what we do and do not know when using different data sets and prior assumptions. For instance, dropping the assumption that space is perfectly flat, the WMAP-only constraint on the measured age of the Universe tightens from t0~16.3+2.3-1.8 Gyr to t0~14.1+1.0-0.9 Gyr by adding SDSS and SN Ia data. Including tensors, running tilt, neutrino mass and equation of state in the list of free parameters, many constraints are still quite weak, but future cosmological measurements from SDSS and other sources should allow these to be substantially tightened.
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