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Sloan Digital Sky Survey astronomers measure role of dark matter,dark energy and gravity in the distribution of galaxies (Forwarded)



 
 
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Old January 25th 05, 02:48 AM
Andrew Yee
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Default Sloan Digital Sky Survey astronomers measure role of dark matter,dark energy and gravity in the distribution of galaxies (Forwarded)

Sloan Digital Sky Survey

CONTACTS:

Daniel Eisenstein
AT AAS MEETING
(703) 966-4237

At the University of Arizona
(520) 621-5904,

Bob Nichol
Institute of Cosmology & Gravitation (ICG)
University of Portsmouth (UK)
(44) 023 9284 3117,


Michael Strauss
Scientific Spokesperson, Sloan Digital Sky Survey
(609) 258-3808,


Gary S. Ruderman
Public Information Officer, Sloan Digital Sky Survey
(312) 320-4794,


January 11, 2005

THE COSMIC YARDSTICK -- Sloan Digital Sky Survey astronomers measure role of
dark matter, dark energy and gravity in the distribution of galaxies

SAN DIEGO -- In the largest galaxy survey ever, the Sloan Digital Sky Survey
(SDSS) confirmed the role of gravity in growing structures in the universe,
using the result to precisely measure the geometry of the universe.

The SDSS researchers from the University of Arizona, New York University, the
University of Portsmouth (UK), the University of Pittsburgh and the
Massachusetts Institute of Technology, detected ripples in the galaxy
distribution made by sound waves generated soon after the Big Bang.

"These sound waves left their imprint in the Cosmic Microwave Background,
remnant radiation from the Big Bang seen when the universe was 400,000 years
old. We are now seeing the corresponding cosmic ripples in the SDSS galaxy maps.
Seeing the same ripples in the early universe and the relatively nearby galaxies
is smoking-gun evidence that the distribution of galaxies today grew via
gravity," explains lead investigator Daniel Eisenstein of the University of Arizona.

Eisenstein made the announcement today during a press conference at the winter
meeting of the American Astronomical Society in San Diego. The paper, "Detection
of the Baryon Acoustic Peak in the Large-Scale Correlation Function of SDSS
Luminous Red Galaxies" was submitted for publication to the Astrophysical
Journal on December 31, 2004.

RIPPLES AS YARDSTICKS

The early Universe was smooth and homogeneous, quite a contrast from the clumpy
array of galaxies and clusters of galaxies observed today. One of the major
goals of cosmology is to understand how these structures grew out of the
initially smooth universe.

The galaxies we see today consist of ordinary matter, made up of the atoms of
our familiar world. However, astronomers have long known that there is roughly
five times more 'dark matter' than ordinary or 'baryonic' matter. Understanding
how gravity causes the clumps that will become galaxies and clusters of galaxies
to grow as the universe expands requires studying the interaction between
ordinary and dark matter.

"In the early Universe, the interaction between gravity and pressure caused a
region of space with more ordinary matter than average to oscillate, sending out
waves very much like the ripples in a pond when you throw in a pebble," explains
SDSS scientist and co-author Bob Nichol, an astrophysicist at the Institute of
Cosmology & Gravitation at the University of Portsmouth (UK), the most recent
institution to join the SDSS collaboration. "These ripples in the matter grew
for a million years until the Universe cooled enough to freeze them in place.
What we now see in the SDSS galaxy data is the imprint of these ripples billions
of years later."

Or gravity's signature could be likened to a ringing bell's resonance in time
and space, adds Idit Zehavi of the University of Arizona. "This last ring gets
forever quieter and deeper in tone as the Universe expands. It is now so faint
as to be detectable only by the most sensitive surveys," Zehavi explains. "The
SDSS has measured the tone of this last ring very accurately.

"Comparing the measured value with that predicted gives a yardstick that enables
us to determine the rate at which the universe expands, which in turn depends on
the amount of both dark matter and dark energy," Zehavi explains. Dark energy is
the still mysterious substance driving the acceleration of the Universe today.

WAVES SEPARATED BY 500 MILLION LIGHT YEARS

The sound waves propagated for the first million years of the Universe's
history. Their existence was first predicted in 1970 and they were first seen in
1999 in fluctuations in the remnant light from the hot glow of the Big Bang
known as the Cosmic Microwave Background. It had long been suggested that these
sound waves should also be present in the distribution of galaxies, but the
signal was predicted to be subtle and difficult to discern.

To find the signal, the SDSS team mapped more than 46,000 very luminous red
galaxies over a volume of space roughly five billion light years in diameter.
They found a slight excess of galaxies with separations of 500 million light
years, exactly the predicted signature of the sound waves.

"This is just the scale predicted for these ripples", explained David Hogg of
New York University, a member of the team. "The precise determination of the
distance between ripples allows us to set the scale of the expansion of the
universe, which in turn allows us to constrain the properties of both dark
matter and dark energy."

SDSS team member Kazuhiro Yahata of the University of Tokyo led a complementary
analysis of quasar clustering and credits the huge volume of SDSS data that
allows such findings. While Yahata's analysis did not directly detect the 500
million light year yardstick in the quasar distribution, its results are fully
consistent with the presence of sound waves.

A similar analysis on a different dataset by the Two Degree Field Galaxy
Redshift Survey has also detected the sound waves. "It is impressive
verification of the standard cosmological model that two groups with independent
data have both made significant detections of the baryon induced features in
large-scale galaxy clustering," said Shaun Cole of the University of Durham
(UK), lead author of the Two Degree Field study.

"The amazing thing about all these results is that they are in perfect accord
with the predictions of our standard cosmological model, including both dark
matter and dark energy," says Eisenstein. "So while it all fits together, it
still leaves us 'in the dark' about the nature of these two mysterious
components which dominate the energy of the universe."

AUTHORS

Authors of the paper "Detection of the Baryon Acoustic Peak in the Large-Scale
Correlation Function of SDSS Luminous Red Galaxies" a

Daniel J. Eisenstein
Steward Observatory, University of Arizona
933 N. Cherry Ave., Tucson, AZ 85121

David W. Hogg
Department of Physics, New York University
4 Washington Pl., NY, NY 10003

Roman Scoccimarro
New York University

Idit Zehavi
University of Arizona

Michael R. Blanton
New York University

Bob Nichol
Institute of Cosmology and Gravitation (ICG)
University of Portsmouth, Portsmouth, PO1 2EG, UK

Ryan Scranton
Department of Physics and Astronomy
University of Pittsburgh, Pittsburgh, PA 15260

Hee-Jong Seo
University of Arizona

Max Tegmark
University of Pennsylvania, Philadelphia, PA 15260;
Massachusetts Institute of Technology, Cambridge, MA 02139

Zheng Zheng
School of Natural Sciences
Institute for Advanced Studies, Princeton, NJ 08540

Figure Caption:
[
http://www.sdss.org/news/releases/Acoustic_Figure3.png (294KB)]
A map of the galaxies in a portion of the Sloan Digital Sky Survey (SDSS). The
position of the earth is at the bottom, represented by a picture of the SDSS
telescope at Apache Point Observatory in New Mexico. Each dot marks the position
of a galaxy, such as the example displayed on the left.

In the first million years after the Big Bang, sound waves are driven into the
cosmic gas (bottom right). SDSS researchers have used to map of galaxies to
detect the remnants of these waves. The bullseye shows the present-day scale of
the sound waves; however, the imprint is too subtle to see by eye. (Credit:
Eisenstein, Sloan Digital Sky Survey)

 




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