From: Eugen Leitl (eugen@leitl.org)
Date: Sun Apr 14 2002 - 02:52:20 MDT
On Sun, 14 Apr 2002, Colin Hales wrote:
> John K Clark
> > results that does not need to invoke the acceleration of the universe.
> > If true most of the predictions about the ultimate fate of the universe
> > would turn out to be wrong.
Refurbished tired light is neat, but there are other reports which seem to
confirm dark energy:
http://www.sciencemag.org/cgi/content/full/295/5564/2341b
Four years ago, cosmologists astonished their colleagues by announcing
that the universe appears to be expanding at ever-increasing speed--and
that a mysterious antigravity force must be doing the pushing. Since then,
other scientists have scanned space in vain for evidence that the
unexpected acceleration might be an illusion. Now an international
consortium of astronomers has confirmed the original finding by taking a
completely different approach. "A compelling case has been made that the
universe is accelerating," says Max Tegmark, a cosmologist at the
University of Pennsylvania in Philadelphia.
"This is an important piece of work," says Neil Turok, a cosmologist at
the University of Cambridge, U.K. In combination with earlier results,
Turok says, the new research adds to the mounting evidence that ordinary
matter alone cannot mold space into the geometry that cosmologists believe
it has. Instead, many now believe, "dark energy" must be added to the
mix--a repulsive force similar to one that Albert Einstein once considered
and then forcefully rejected.
The original finding and the new study reflect the two different
strategies that scientists use to map the structure and geometry of
far-flung corners of the universe. One is to take a "standard candle"--an
object of known brightness--and then calculate its velocity by measuring
how much the redshift "stretches" its light as it traverses the universe.
The 1998 announcements that the universe's expansion is speeding up
(Science, 30 January 1998, p. 651 ) relied on this technique, using
exploding stars dubbed type IA supernovae as standard candles. Distant
supernovae recede more slowly than expected, researchers found, suggesting
a cosmic acceleration.
The new work, by a 27-person team from 14 institutions around the globe,
takes the alternative route. "We have used a 'standard ruler' test," says
George Efstathiou of the Institute of Astronomy in Cambridge, who led the
study. By starting with something of known size, he explains, cosmologists
can calculate how much matter the universe must contain to make it appear
the way astronomers see it from Earth.
The researchers used an esoteric ruler: the size of the lumpiness of the
universe. They started relatively close to home, by calculating the
variations in clustering within a huge swarm of galaxies surveyed by the
Anglo-Australian Observatory in Siding Spring, Australia--the so-called
Two Degree Field Galaxy Redshift Survey (2dFGRS). This lumpiness, says
Efstathiou, can be traced all the way back to the ripples in the afterglow
of the big bang, the cosmic microwave radiation. In recent years,
balloon-borne and ground-based detectors have given scientists a good look
at those ripples in patches of the sky (Science, 28 April 2000, p. 595; 22
June 2001, p. 2236).
"The cosmic microwave background gives us a picture on the sky of the
fluctuations as they were when the universe was 400,000 years old," says
Efstathiou. "So the test is, we have a three-dimensional picture of the
present day, and we can compare it with the angular picture at 400,000
years." Working from that information, the team calculated how much matter
must be sprinkled through the cosmos to transform the primordial ripples
into multigalactic clumps as the universe aged.
The results, published in the Monthly Notices of the Royal Astronomical
Society , confirm earlier findings that the universe is geometrically
flat, or subject to the rules of Euclid on a large scale. More important,
there is not enough matter to create that flatness. Taken together, the
ordinary matter that astronomers can observe and the so-called dark matter
that they infer from the pull of its gravity provide just a third of the
energy required for a flat universe. It's the Enron problem, says Tegmark:
"Most of the budget is missing." The budget deficit, the remaining
two-thirds, is what cosmologists call dark energy.
"What we are measuring is the energy associated with empty space," says
Efstathiou. And some features of that energy, Tegmark says, are "really
weird." For example, whereas ordinary matter pulls on other matter and
attempts to reign in the expansion of the universe, "dark energy has this
strange property that it's essentially repulsive, so that it pushes
everything away and makes the universe accelerate faster and faster."
The new study gives a valuable boost to the earlier results based on
supernovae, says Paul Steinhardt, a cosmologist at Princeton University in
New Jersey. "We are uncertain how reliable type IA supernovae are as
standard candles when you talk about supernovae that exploded a long, long
time ago," he says. The strategy used by Efstathiou and his colleagues "is
much more secure since we have many cross-checks, [although] it is more
indirect and less intuitive," says Steinhardt. Crucially, Tegmark says,
the new study "finds the same amount of dark energy as the supernova
analysis did, but in a completely independent way." In an online paper
(xxx.lanl.gov/abs/astro-ph/0105091) scheduled to appear in Physical Review
D, Tegmark and two colleagues provide further backing with a similar study
based on a different, smaller galaxy survey.
Even so, Tegmark says, the case for acceleration fueled by dark energy
"certainly hasn't been established beyond any reasonable doubt." Turok
agrees. Data from the forthcoming MAP and Planck satellites and the
million-galaxy Sloan Digital Sky Survey should help firm up the case and
enable cosmologists to check out some of the assumptions underlying these
tests.
Andrew Watson is a writer in Norwich, U.K.
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