Press Release

Sloan Digital Sky Survey Study Confirms Dark Matter

By SpaceRef Editor
May 21, 2003
Filed under ,

Dr. Francisco Prada, Instituto de Astrofisica de Canarias in Spain

Dr. Anatoly Klypin, New Mexico State University at Las Cruces

(505) 646-1400,

Gary S. Ruderman, Public Information Officer, Sloan Digital Sky Survey


A new study using the Sloan Digital Sky Survey provides the most direct
evidence yet that galaxies reside at the center of giant, dark matter
concentrations that may be 50 times larger than the visible galaxy

The study very directly supports the generally accepted astronomical
theories on dark matter and contradicts an alternative theory of gravity
known as Modified Newtonian Dynamics (MOND).

Lead investigator, Francisco Prada, of the Max Planck Institute for
Astronomy in Germany and the Instituto de Astrofisica de Canarias in
Spain, and a team of colleagues from New Mexico State University at Las
Cruces and at other Sloan Digital Sky Survey (SDSS) institutions, will
present the results of the team’s research at a joint conference of the
Isaac Newton Group of Telescopes and the Instituto de Astrofisica de
Canarias, May 26-30 in La Palma, Canary Islands, Spain.

Prada and his colleagues studied the motion of about 3,000 satellites
orbiting isolated bright galaxies and found strong evidence of dark
matter gravitational effects.

The study “is important because it is a direct measurement of some of
the properties predicted for dark matter,” said Anatoly Klypin of New
Mexico State University.

Although it cannot be observed directly, dark matter is believed to
account for about 27 percent of the total mass of the universe,
compared with only about 3 percent for normal, observable matter. The
rest, according to standard models of the structure and evolution of
the universe, consists of dark energy and radiation.

Prada and his colleagues observed 250,000 galaxies from the Sloan
Digital Sky Survey (SDSS) telescope to find good candidates for a
study of the gravitational effects of dark matter. From the SDDS data,
they identified about 3,000 satellites — generally small galaxies
orbiting large ones — for which they could measure velocities.

The velocity of a satellite declines predictably as the satellite moves
Away from the object it is orbiting, due to the effects of gravity. In
the case of planets in our solar system, where there is too little dark
matter to have a gravitational effect, the decline is rapid because
there is no mass between the planets and the sun, he said. But in the
outer reaches of galaxies, where dark matter is believed to be
clustered, the decline in velocity would be much more gradual if
standard cosmological models are correct.

“Our results imply the presence of dark matter,” Prada said. The
findings provide strong evidence against the MOND alternative theory of
gravity, he said. MOND, controversial since it was proposed in 1983,
eliminates the need for dark matter in explaining the nature of the
universe, by changing the law of gravity in areas such as the outskirts
of galaxies.

Researchers said the SDSS study probes dark matter in a way that cannot
be done by any other current experiment. This type of research could
not have been done without the unique capabilities of the SDSS.

The researchers determined the velocity of each satellite relative to
the galaxy it orbits by measuring the redshift in the light spectra from
the objects. Redshift, a change in light waves as an object moves away
from an observer, is a standard means of measuring the speed of a distant

The next stage of the research will be to extend the sample of galaxies
and satellites studied to improve the accuracy of the results.

Participating institutions in this discovery are: The Max Planck
Institute for Astronomy, Germany; Centro Astronomico Hispano-Aleman; New
Mexico State University; Princeton University Observatory; Apache Point
Observatory; University of Michigan; Eotvos University, Budapest; and
Johns Hopkins University.

(A complete list of people contributing to the discovery is posted on
the NEWS Section of the Sloan Digital Sky Survey Web site at


The Sloan Digital Sky Survey ( will map in detail one-quarter
of the entire sky, determining the positions and absolute brightness of
100 million celestial objects. It will also measure the distances to more
than a million galaxies and quasars. The Astrophysical Research
Consortium (ARC) operates Apache Point Observatory, site of the SDSS

SDSS 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

Additional information about the conference is at:

ILLUSTRATION available from Ruderman (above) at SDSS

SpaceRef staff editor.