Using observations of 3,000 quasars discovered by the
Sloan Digital Sky Survey (SDSS), scientists have made the most precise
measurement to date of the cosmic clustering of diffuse hydrogen gas. These
quasars–100 times more than have been used in such analyses in the past–
are at distances of eight to ten billion light years, making them among
the most distant objects known.

Filaments of gas between the quasars and the Earth absorb light in the
quasar’s spectra, allowing researchers to map the gas distribution and to
measure how clumpy the gas is on scales of one million light years. The degree of clumping of this gas, in turn, can answer fundamental questions such as whether neutrinos have mass and what the nature of dark energy is, hypothesized to be driving the accelerated expansion of the universe.

“Scientists have long studied the clustering of galaxies to learn about
cosmology,” explained Uros Seljak of Princeton University, one of the SDSS
researchers. “However, the physics of galaxy formation and clustering is very
complicated. In particular, because most of the mass of the universe is
made up of
dark matter, an uncertainty arises from our lack of understanding of the
relation between the distribution of galaxies (which we see) and the dark
matter
(which we can’t see but the cosmological models predict).” The gas
filaments seen
in the quasar spectra are thought to be distributed very much like the dark
matter, removing this source of uncertainty.

“We have known for several years that quasar spectra are a unique tool for
studying the distribution of dark matter in the early universe, but the
quantity
and quality of the SDSS data have made that vision a reality,” said David
Weinberg of Ohio State University, a member of the SDSS team. “It’s amazing
that we can learn so much about the structure of the universe 10 billion years
ago.”

Seljak and his collaborators on the SDSS combined the analysis of the quasar
spectra with measurements of galaxy clustering, gravitational lensing, and
ripples in the Cosmic Microwave Background observed by NASA’s Wilkinson
Microwave
Anisotropy Probe (WMAP). This gives the best determination to date of the
clustering of matter in the universe from scales of one million light years to
many billions of light years. This comprensive view allows detailed
comparison with theoretical models for the history and constituents of the
universe.

“This is the most rigorous test to date of the predictions of the
cosmological model of inflation; inflation passes with flying colors,”
added Seljak.

Inflationary theory states that right after the Big Bang the universe
underwent a period of extremely rapid acceleration, during which tiny
fluctuations
were transformed into astronomical-sized wrinkles in space-time, ultimately
observable in the clumping of astronomical objects. The theory of inflation
predicts a very specific dependence of the degree of clustering with scale,
which
the current analysis strongly supports. Other scenarios, such as the cyclic
universe theory, make very similar predictions and are also in agreement
with the latest results.

Early analyses by the WMAP team and others had hinted at deviations in cosmic
clustering from the prediction of inflation. If correct, this would have
required a major revision of the current paradigm for origin of structure
in the universe.

“The new data and the corresponding analysis substantially improves the
observational precision of this test,” said Patrick McDonald of Princeton
University and one of the finding’s authors. “The new results are in
nearly perfect agreement with inflation.”

“The clustering of matter is a precise and powerful test of cosmological
models, and the present analysis is consistent with, and extends our previous
studies,” agreed Adrian Pope of The Johns Hopkins University, who led an
earlier analysis of the clustering of SDSS galaxies.

The new analysis also provides the best information on the mass of the
neutrino. Terrestial experiments–resulting in the 2002 Nobel Prize in
Physics–have definitively shown that neutrinos have mass, but these
experiments could
only measure the difference in mass between the three different types of
neutrinos known. The presence of neutrinos would affect the cosmic
clustering on million-light-year scales, exactly the scales probed
with the quasar spectra.

The new analysis suggests that the lightest neutrino mass has to be less than
two times the previously measured mass difference. The new measurements also
eliminate the possibility of an additional massive neutrino family suggested
by some terrestrial experiments.

“Cosmology, the science of the very large, is able to tell us about
properties of fundamental particles, such as neutrinos,” said Lam Hui of
The U.S.
Department of Energy’s Fermi National Accelerator Laboratory, who has been
carrying out an independent analysis of these data, together with Scott
Burles of MIT and others.

The new analysis also provides further support for the existence of dark
energy, and suggests that dark energy is unchanging in time. This analysis
provides the best limits on its time evolution to date.

“No evidence of dark energy changing in time has emerged so far, and the
possibility that the universe will be torn apart by a big rip in the future is
substantially reduced by these new results,” said Alexey Makarov of Princeton
University, who also took part in this research.

ILLUSTRATION

This figure shows the SDSS spectrum of a quasar at a distance of 12 billion
light years. The middle panel shows the complete spectrum. The upper panel is
an expanded view of the region of the spectrum affected by the filaments of
gas whose clumping is the focus of the present study. Each of the hundreds of
dips in the spectrum corresponds to a different parcel of gas along the line
of sight between the quasar and the Earth. This is schematically shown in
the lower panel, which indicates a line of sight through a simulation 30
million
light years across of the distribution of gas in the universe. The clumpiness
of the gas is determined by, among other things, the constituents of the
universe, including dark matter, dark energy, and massive neutrinos.
Renyue Cen of Princeton University carried out the simulation.

The authors of the paper describing these results
(http://xxx.lanl.gov/list/astro-ph/new) are listed in full at www.sdss.org

ABOUT THE SLOAN DIGITAL SKY SURVEY

The Sloan Digital Sky Survey (http://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.