Scientists from the Sloan Digital Sky Survey
announced the discovery of independent physical evidence for the existence of
dark energy.

The researchers found an imprint of dark energy by correlating millions of
galaxies in the Sloan Digital Sky Survey (SDSS) and cosmic microwave background
temperature maps from NASA’s Wilkinson Microwave Anisotropy Probe (WMAP). The
researchers found dark energy’s “shadow” on the ancient cosmic radiation, a
relic of cooled radiation from the Big Bang.

With the combination of results from these two large sky surveys, this
discovery provides physical evidence for the existence of dark energy; a
result that complements earlier work on the acceleration of the universe
as measured from distant supernovae. Observations from the Balloon
Observations of Millimetric Extragalactic Radiation and Geophysics
(BOOMERANG) of Cosmic Microwave Background (CMB) were also part of the
earlier findings.

Dark energy, a major component of the universe and one of the greatest
conundrums in science, is gravitationally repulsive rather than attractive.
This causes the universe’s expansion to accelerate, in contrast to the
attraction of ordinary (and dark) matter, which would make it decelerate.

“In a flat universe the effect we’re observing only occurs if you have a
universe with dark energy,” explained lead researcher Dr. Ryan Scranton of the
University of Pittsburgh’s Physics and Astronomy department. “If the
universe was just composed of matter and still flat, this effect wouldn’t
exist.”

As photons from the cosmic microwave background (CMB) travel to us from
380,000 years after the Big Bang, they can experience a number of physical
processes, including the Integrated Sachs-Wolfe effect. This effect is an
imprint or shadow of dark energy on microwaves. The effect also measures the
changes in temperature of cosmic microwave background due to the effects of
gravity on the energy of photons, added Scranton.

The discovery is “a physical detection of dark energy, and highly
complementary to other detections of dark energy” added Dr. Bob Nichol, an SDSS
collaborator and associate professor of physics at Carnegie Mellon University
in Pittsburgh. Nichol likened the Integrated Sachs-Wolfe effect to looking at
a person standing in front of a sunny window: “You just see their outline and
can recognize them from just this information. Likewise the signal we see has
the right outline (or shadow) that we’d expect for dark energy,” said Nichol.

“In particular the color of the signal is the same as the color of the cosmic
microwave background, proving it is cosmological in origin and not some
annoying contamination,” added Nichol.

“This work provides physical confirmation that one needs dark energy to
simultaneously explain both the CMB and SDSS data, independent of the
supernovae work. Such cross-checks are vital in science,” added Jim Gunn,
Project Scientist of the SDSS and Professor of Astronomy at Princeton
University.

Dr. Andrew Connolly of the University of Pittsburgh explained that photons
streaming from the cosmic microwave background pass through many concentrations
of galaxies and dark matter. As they fall into a gravitational well they gain
energy (just like a ball rolling down a hill). As they come out they lose
energy (again like a ball rolling up a hill). Photographic images of the
microwaves become more blue (i.e. more energetic) as they fall in toward
these supercluster concentrations and then become more red (i.e. less
energetic) as they climb away from them.

“In a universe consisting mostly of normal matter one would expect that the
net effect of the red and blue shifts would cancel. However in recent years we
are finding that most of the stuff in our universe is abnormal in that it is
gravitationally repulsive rather than gravitationally attractive,” explained
Albert Stebbins, a scientist at the NASA/Fermilab Astrophysics Center Fermi
National Accelerator Laboratory, an SDSS collaborating institution. “This
abnormal stuff we call dark energy.”

SDSS collaborator Connolly said if the depth of the gravitational well
decreases while the photon travels through it then the photon would exit with
slightly more energy. “If this were true then we would expect to see that the
cosmic microwave background temperature is slightly hotter in regions with more
galaxies. This is exactly what we found.”

Stebbins added that the net energy change expected from a single
concentration of mass is less than one part in a million and researchers had
to look at a large number of galaxies before they could expect to see the
effect. He said that the results confirm that dark energy exists in
relatively small mass concentrations: only 100 million light years across
where the previously observed effects dark energy were on a scale of 10
billion light years across.

A unique aspect of the SDSS data is its ability to accurately measure the
distances to all galaxies from photographic analysis of their photometric
redshifts. “Therefore, we can watch the imprint of this effect on the CMB
grow as a function of the age of the universe,” Connolly said. “Eventually
we might be able to determine the nature of the dark energy from
measurements like these, though that is a bit in the future.”

“To make the conclusion that dark energy exists we only have to assume that
the universe is not curved. After the Wilkinson Microwave Anisotropy Probe
results came in (in February, 2003), that’s a well-accepted assumption,”
Scranton explained. “This is extremely exciting. We didn’t know if we could
get a signal so we spent a lot of time testing the data against contamination
from our galaxy or other sources. Having the results come out as strongly as
they did was extremely satisfying.”

The discoveries were made in 3,400 square degrees of the sky surveyed by the
SDSS.

“This combination of space-based microwave and ground-based optical data gave
us this new window into the properties of dark energy,” said David Spergel, a
Princeton University cosmologist and a member of the WMAP science team. “By
combining WMAP and SDSS data, Scranton and his collaborators have shown that
dark energy, whatever it is, is something that is not attracted by gravity even
on the large scales probed by the Sloan Digital Sky Survey.

“This is an important hint for physicists trying to understand the mysterious
dark energy,” Spergel added.

In addition to principal investigators Scranton, Connolly, Nichol and
Stebbins, Istavan Szapudi of the University of Hawaii contributed to the
research. Others involved in the analysis include Niayesh Afshordi of
Princeton University, Max Tegmark of the University of Pennsylvania and
Daniel Eisenstein of the University of Arizona.

ABOUT THE SLOAN DIGITAL SKY SURVEY (SDSS)

The Sloan Digital Sky Survey (sdss.org) 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 telescopes.

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 Society.

The WILKINSON MICROWAVE ANISOTROPY PROBE (WMAP) is a NASA mission built in
partnership with Princeton University and the Goddard Space Flight Center to
measure the temperature of the cosmic background radiation, the remnant heat
from the Big Bang. The WMAP mission reveals conditions as they existed in the
early universe by measuring the properties of the cosmic microwave background
radiation over the full sky. (http://map.gsfc.nasa.gov)