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An international team of scientists from the Sloan
Digital Sky Survey (SDSS) announced that they have completed a map of the locations
of 200,000 galaxies—the most precise map to date of the location
of visible matter in the universe. The distribution of the galaxies
in space reveals clues as to the composition and evolution of the
universe, and has provided independent confirmation that the bulk
of the material in the universe consists of a quantity which as
been dubbed “Dark Energy.” Donald
Schneider, professor
of astronomy at Penn State, the Chair of the Sloan Digital Sky
Survey’s Quasar Science Group, and the project’s Scientific
Publications Coordinator, was a coauthor of this investigation.
“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 both the
Astrophysical Journal and to Physical
Review D.
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 2003, 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.
In the three-dimensional map each galaxy
is shown as a single point, the color representing the luminosity—this
shows only those 66,976 out of 205,443 galaxies in the map that
lie near the plane of Earth’s equator. They found that neutrinos
couldn’t
be a major constituent of the dark matter, putting among 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 people, and different analysis—but the results
agree,” says Max
Tegmark from the University of Pennsylvania, first author on the
two papers. “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 redshifts, 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.
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.
Barbara K. Kennedy and Sloan Digital Sky Survey
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