From: LARRY KLAES (ljk4_at_msn.com)
Date: Fri Oct 12 2007 - 06:25:40 PDT
>From: "AAS Press Officer Dr. Steve Maran" <Steve.Maran_at_aas.org>
>To: "AAS Press Officer Dr. Steve Maran" <steve.maran_at_aas.org>
>Subject: CfA: TESTING EINSTEIN: IS DARK ENERGY CONSTANT?
>Date: Thu, 11 Oct 2007 11:42:25 -0400
>
THE FOLLOWING RELEASE WAS RECEIVED FROM THE HARVARD-SMITHSONIAN CENTER FOR
ASTROPHYSICS, IN CAMBRIDGE, MASSACHUSETTS, AND IS FORWARDED FOR YOUR
INFORMATION. (FORWARDING DOES NOT IMPLY ENDORSEMENT BY THE AMERICAN
ASTRONOMICAL SOCIETY.) Steve Maran, American Astronomical Society
steve.maran_at_aas.org 1-202-328-2010 x116
David Aguilar
1-617-495-7462
daguilar_at_cfa.harvard.edu
Christine Pulliam
1-617-495-7463
cpulliam_at_cfa.harvard.edu
CfA Release No: 2007-25
For Release: Thursday, October 11, 2007
TESTING EINSTEIN: IS DARK ENERGY CONSTANT?
Nearly a decade ago, astronomers discovered the surprising existence of dark
energy-a mysterious force that pushes galaxies apart and accelerates the
expansion of the universe. Also known as the energy density of the vacuum,
dark energy is a property of space itself. Scientists have many questions
about the nature of dark energy. One question that soon may be answered: Is
the energy density of the vacuum constant over cosmic time?
Theorists Stuart B. Wyithe (University of Melbourne) and Avi Loeb
(Harvard-Smithsonian Center for Astrophysics) suggest answering that
question by studying the distribution of distant hydrogen clumps, which will
yield clues to the history of dark energy.
"The simplest expectation is that the energy density of the vacuum was
steady over time-a 'cosmological constant'-but we need to check it out. We
might be surprised by the answer," said Loeb.
Dark energy made its first appearance in Einstein's General Theory of
Relativity. Einstein believed that the universe was static so he inserted a
constant, repulsive force into his equations to counteract the inexorable
pull of gravity on all the galaxies. When Edwin Hubble found that the
universe is expanding, Einstein threw out the cosmological constant and is
rumored to have called it his "biggest blunder."
In 1998, two teams of astronomers discovered that the universe is speeding
up, not slowing down under the pull of gravity. They resurrected Einstein's
cosmological constant in the form of dark energy. While dark energy clearly
exists and its effects are visible to astronomers, no one knows what causes
it or whether it is truly constant over time.
"The origin of dark energy is the biggest unsolved problem in astrophysics,"
said Wyithe.
-- Investigating Dark Energy --
To study the past behavior of dark energy, astronomers must look into the
distant universe, to a region whose light took billions of years to travel
to Earth. At those great distances, individual galaxies and supernovae - the
signposts used to study dark energy in our neighborhood - fade to
near-invisibility. A new signpost is needed.
Wyithe and Loeb propose studying the radio emission from neutral hydrogen,
whose wavelength is stretched from its starting value of 21 centimeters by
the expansion of the universe (a process called redshifting).
After the universe was re-ionized by the first galaxies (sometime in the
first billion years), a small fraction of hydrogen remained neutral,
surviving in dense pockets. Astronomers had not realized before this work
that 21-cm signals from the leftover hydrogen might be detectable.
Wyithe and Loeb showed that, in fact, upcoming observatories will be capable
of detecting 21-cm signals from the distant, young universe, even after it
gets mostly ionized. Moreover, while the signal strength decreases after
re-ionization, the noise also decreases. In principle, the 21-cm signal from
neutral hydrogen can be measured from the present epoch all the way up to a
redshift of z=15, when the universe was only 200 million years old.
"There is no other viable technique to study dark energy at high redshifts,"
stated Loeb.
-- Universal Sound Waves --
In the universe's earliest moments, small fluctuations in energy density and
pressure caused oscillations, sending out sound waves that expanded across
space like ripples in a pond. The size of the outermost "ripple" is about
500 million light-years across today. These universal sound waves influenced
the large-scale structure in the distribution of galaxies, and indeed their
signature was detected recently in galaxy surveys at low redshifts.
Neutral hydrogen gas should show the same distribution patterns as galaxies
due to primordial acoustic oscillations. By studying the large-scale
distribution of hydrogen in the early universe, astronomers can learn how
dark energy influenced the growth of structure in the crucial first few
billion years.
Theoretically, instruments now under construction such as the Murchison
(formerly Mileura) Wide-field Array (MWA) and its future extensions could
detect 21-cm signals from hydrogen in the first 1 to 4 billion years of the
universe's history, corresponding to redshift factors of 1.5 to 6.
"The broad range of redshifts we can reach is important because we can pick
up the signal regardless of when the universe was re-ionized," explained
Wyithe.
Two journal papers describing Wyithe and Loeb's research are available
online at http://arxiv.org/abs/0708.3392 and http://arxiv.org/abs/0709.2955
(the latter with co-author Paul Geil of the University of Melbourne).
Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for
Astrophysics (CfA) is a joint collaboration between the Smithsonian
Astrophysical Observatory and the Harvard College Observatory. CfA
scientists, organized into six research divisions, study the origin,
evolution and ultimate fate of the universe.
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