"Lighthouses in the Sky" Yield Biggest-Ever 3-D Map of the Cosmos (Forwarded)

Lawrence Berkeley National Laboratory

Contact:
Paul Preuss, (510) 486-6249

May 15, 2006

A Ruler to Measure the Universe

"Lighthouses in the Sky" Yield Biggest-Ever 3-D Map of the Cosmos

BERKELEY, CA -- A team of astronomers led by Nikhil Padmanabhan and David
Schlegel has published the largest three-dimensional map of the universe
ever constructed, a wedge-shaped slice of the cosmos that spans a tenth of
the northern sky, encompasses 600,000 uniquely luminous red galaxies, and
extends 5.6 billion light-years deep into space, equivalent to 40 percent of
the way back in time to the Big Bang.

Schlegel is a Divisional Fellow in the Physics Division of Lawrence Berkeley
National Laboratory, and Padmanabhan will join the Lab's Physics Division as
a Chamberlain Fellow and Hubble Fellow in September; presently he is at
Princeton University. They and their coauthors are members of the Sloan
Digital Sky Survey (SDSS), and have previously produced smaller 3-D maps by
using the SDSS telescope in New Mexico to painstakingly collect the spectra
of individual galaxies and calculate their distances by measuring their
redshifts.

"What's new about this map is that it's the largest ever," says Padmanabhan,
"and it doesn't depend on individual spectra."

The principal motive for creating large-scale 3-D maps is to understand how
matter is distributed in the universe, says Padmanabhan. "The brightest
galaxies are like lighthouses -- where the light is, is where the matter
is."

Schlegel says that "because this map covers much larger distances than
previous maps, it allows us to measure structures as big as a billion
light-years across."

A natural ruler in space

The variations in galactic distribution that constitute visible large-scale
structures are directly descended from variations in the temperature of the
cosmic microwave background, reflecting oscillations in the dense early
universe that have been measured to great accuracy by balloon-borne
experiments and the WMAP satellite.

The result is a natural "ruler" formed by the regular variations (sometimes
called "baryon oscillations," with baryons as shorthand for ordinary
matter), which repeat at intervals of some 450 million light-years.

"Unfortunately it's an inconveniently sized ruler," says Schlegel. "We had
to sample a huge volume of the universe just to fit the ruler inside."

Says Padmanabhan, "Although the universe is 13.7 billion years old, that
really isn't a whole lot of time when you're measuring with a ruler that's
marked only every 450 million light-years."

The distribution of galaxies reveals many things, but one of the most
important is a measure of the mysterious dark energy that accounts for some
three-fourths of the universe's density. (Dark matter accounts for roughly
another 20 percent, while less than 5 percent is ordinary matter of the kind
that makes visible galaxies.)

"Dark energy is just the term we use for our observation that the expansion
of the universe is accelerating," Padmanabhan remarks. "By looking at where
density variations were at the time of the cosmic microwave background" --
only about 300,000 years after the Big Bang -- "and seeing how they evolve
into a map that covers the last 5.6 billion years, we can see if our
estimates of dark energy are correct."

The new map shows that the large-scale structures are indeed distributed the
way current ideas about the accelerating expansion of the universe would
suggest. The map's assumed distribution of dark matter, which although
invisible is affected by gravity just like ordinary matter, also conforms to
current understanding.

Dead, red galaxies

What made the big new 3-D map possible were the Sloan Digital Sky Survey's
wide-field telescope, which covers a three-degree field of view (the full
moon is about half a degree), plus the choice of a particular kind of
galactic "lighthouse," or distance marker: luminous red galaxies.

"These are dead, red galaxies, some of the oldest in the universe -- in
which all the fast-burning stars have long ago burned out and only old red
stars are left," says Schlegel. "Not only are these the reddest galaxies,
they're also the brightest, visible at great distances."

The Sloan Digital Sky Survey astronomers worked with colleagues on the
Australian Two-Degree Field team to average the color and redshift of a
sample of 10,000 red luminous galaxies, relating galaxy color to distance.
They then applied these measurements to 600,000 such galaxies to plot their
map.

Padmanabhan concedes that "there's statistical uncertainty in applying a
brightness-distance relation derived from 10,000 red luminous galaxies to
all 600,000 without measuring them individually. The game we play is, we
have so many that the averages still give us very useful information about
their distribution. And without having to measure their spectra, we can look
much deeper into space."

Schlegel agrees that the researchers are far from achieving the precision
they want. "But we have shown that such measurements are possible, and we
have established the starting point for a standard ruler of the evolving
universe."

He says "the next step is to design a precision experiment, perhaps based on
modifications to the SDSS telescope. We are working with engineers here at
Berkeley Lab to redesign the telescope to do what we want to do."

"The Clustering of Luminous Red Galaxies in the Sloan Digital Sky Survey
Imaging Data," by Nikhil Padmanabhan, David J. Schlegel, Uros Seljak, Alexey
Makarov, Neta A. Bahcall, Michael R. Blanton, Jonathan Brinkmann, Daniel J.
Eisenstein, Douglas P. Finkbeiner, James E. Gunn, David W. Hogg, Zeljko
Ivezic, Gillian R. Knapp, Jon Loveday, Robert H. Lupton, Robert C. Nichol,
Donald P. Schneider, Michael A. Strauss, Max Tegmark, and Donald G. York,
will appear in the Monthly Notices of the Royal Astronomical Society and is
now available online at
http://arxiv.org/abs/astro-ph/0605302

SDSS is managed by the Astrophysical Research Consortium for the
Participating Institutions, which are the American Museum of Natural
History, Astrophysical Institute Potsdam, University of Basel, Cambridge
University, Case Western Reserve University, University of Chicago, Drexel
University, Fermilab, the Institute for Advanced Study, the Japan
Participation Group, Johns Hopkins University, the Joint Institute for
Nuclear Astrophysics, the Kavli Institute for Particle Astrophysics and
Cosmology, the Korean Scientist Group, the Chinese Academy of Sciences
(LAMOST), Los Alamos National Laboratory, the Max-Planck-Institute for
Astronomy (MPIA), the Max-Planck-Institute for Astrophysics (MPA), New
Mexico State University, Ohio State University, University of Pittsburgh,
University of Portsmouth, Princeton University, the United States Naval
Observatory, and the University of Washington.

SDSS funding is provided by the Alfred P. Sloan Foundation, the
Participating Institutions, the National Science Foundation, the U.S.
Department of Energy, the National Aeronautics and Space Administration, the
Japanese Monbukagakusho, the Max Planck Society, and the Higher Education
Funding Council for England. Visit the SDSS web site at
http://www.sdss.org/

Berkeley Lab is a U.S. Department of Energy national laboratory located in
Berkeley, California. It conducts unclassified scientific research and is
managed by the University of California. Visit our website at
http://www.lbl.gov

IMAGE CAPTIONS:

[Image 1:
http://www.sdss.org/news/releases/20..._enlarge.html]
A schematic view of the new SDSS three-dimensional map, which includes
regular galaxies (black points) and luminous red galaxies (red points) and
extends 5.6 billion light-years, 40 percent of the distance to the edge of
the visible universe.

[Image 2:
http://www.lbl.gov/Science-Articles/..._telescope.jpg
(18KB)]
The SDSS 2.5 meter telescope at Apache Point, New Mexico was used to create
the new map of the universe.

--
Andrew Yee
ayee @ nova.astro.utoronto.ca