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New Light on Dark Energy...

Von: Ken Kubos (kubos@execpc.com) [Profil]
Datum: 31.01.2008 01:21
Message-ID: <13q2548kp614110@corp.supernews.com>
Newsgroup: uk.rec.ufoalt.ufo.reports

New Light on Dark Energy

Astronomers have used ESO's Very Large Telescope to measure the distribution
and motions of thousands of galaxies in the distant Universe. This opens
fascinating perspectives to better understand what drives the acceleration
of the cosmic expansion and sheds new light on the mysterious dark energy
that is thought to permeate the Universe.

"Explaining why the expansion of the Universe is currently accelerating is
certainly the most fascinating question in modern cosmology," says Luigi
Guzzo, lead author of a paper in this week's issue of Nature, in which the
new results are presented. "We have been able to show that large surveys
that measure the positions and velocities of distant galaxies provide us
with a new powerful way to solve this mystery."

Ten years ago, astronomers made the stunning discovery that the Universe is
expanding at a faster pace today than it did in the past.

"This implies that one of two very different possibilities must hold true,"
explains Enzo Branchini, member of the team. "Either the Universe is filled
with a mysterious dark energy which produces a repulsive force that fights
the gravitational brake from all the matter present in the Universe, or, our
current theory of gravitation is not correct and needs to be modified, for
example by adding extra dimensions to space."

Current observations of the expansion rate of the Universe cannot
distinguish between these two options, but the international team of 51
scientists from 24 institutions found a way that could help in tackling this
problem. The technique is based on a well-known phenomenon, namely the fact
that the apparent motion of distant galaxies results from two effects: the
global expansion of the Universe that pushes the galaxies away from each
other and the gravitational attraction of matter present in the galaxies'
neighbourhood that pulls them together, creating the cosmic web of
large-scale structures.

"By measuring the apparent velocities of large samples of galaxies over the
last thirty years, astronomers have been able to reconstruct a
three-dimensional map of the distribution of galaxies over large volumes of
the Universe. This map revealed large-scale structures such as clusters of
galaxies and filamentary superclusters ", says Olivier Le Fèvre, member of
the team. "But the measured velocities also contain information about the
local motions of galaxies; these introduce small but significant distortions
in the reconstructed maps of the Universe. We have shown that measuring this
distortion at different epochs of the Universe's history is a way to test
the nature of dark energy."

Guzzo and his collaborators have been able to measure this effect by using
the VIMOS spectrograph on Melipal, one of the four 8.2-m telescopes that is
part of ESO's VLT. As part of the VIMOS-VLT Deep Survey (VVDS), of which Le
Fèvre is the Principal Investigator, spectra of several thousands of
galaxies in a 4-square-degree field (or 20 times the size of the full Moon)
at epochs corresponding to about half the current age of the Universe (about
7 billion years ago) were obtained and analysed.

"This is the largest field ever covered homogeneously by means of
spectroscopy to this depth," says Le Fèvre. "We have now collected more
13,000 spectra in this field and the total volume sampled by the survey is
more than 25 million cubic light-years."

The astronomers compared their result with that of the 2dFGRS survey that
probed the local Universe, i.e. measures the distortion at the present time.

Within current uncertainties, the measurement of this effect provides an
independent indication of the need for an unknown extra energy ingredient in
the 'cosmic soup', supporting the simplest form of dark energy, the
so-called cosmological constant, introduced originally by Albert Einstein.
The large uncertainties do not yet exclude the other scenarios, though.

"We have also shown that by extending our measurements over volumes about
ten times larger than the VVDS, this technique should be able to tell us
whether cosmic acceleration originates from a dark energy component of
exotic origin or requires a modification of the laws of gravity," said

"VIMOS on the VLT would certainly be a wonderful tool to perform this future
survey and help us answer this fundamental question. This strongly
encourages our team to proceed with even more ambitious surveys of the
distant Universe," says Le Fèvre.

Source: ESO



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