New Light on Dark Energy (Forwarded)

ESO Education and Public Relations Dept.

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Science Contacts:

Luigi Guzzo
INAF-Osservatorio Astronomico di Brera, Merate, Italy
Phone: +39 328 8051158, +39 039 9991121

Olivier Le Fevre
Laboratoire d'Astrophysique de Marseille, France
Phone: +33 4 91 05 59 85

Enzo Branchini
Universita Roma III, Italy
Phone:+39 06 57337099

Under embargo till 30 January 2008, 19:00 CET

ESO Science Release 04/08

New Light on Dark Energy

Probing the cosmic Web of the Universe

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 Fevre, 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
Fevre 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," declares Le Fevre. "We have now collected more
than 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," explains
Guzzo.

"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 scientists to proceed with even more ambitious surveys of the
distant Universe," concludes Le Fevre.

More Information

"A test of the nature of cosmic acceleration using galaxy redshift
distortions", by L. Guzzo et al., Nature, 31 January 2008.

Notes

The VLT VIsible Multi-Object Spectrograph (VIMOS) can observe spectra of
about 1,000 galaxies in one single exposure. This cosmology science machine
is installed at the 8.2-m MELIPAL telescope, the third unit telescope of the
Very Large Telescope (VLT) at the ESO Paranal Observatory.

The VIMOS VLT Deep Survey (VVDS) is a breakthrough spectroscopic survey
aiming at providing a complete picture of galaxy and structure formation
over a large fraction of the Universe's history, covering sixteen square
degrees of the sky in four separate fields.

The team is composed of L. Guzzo, A. Iovino, and O. Cucciati
(INAF-Osservatorio Astronomico di Brera, Merate, Italy), M. Pierleoni, J.
Blaizot, G.De Lucia, and K. Dolag (Max Planck Institut f Astrophysik,
Germany), B. Meneux, B. Garilli, D. Bottini, D. Maccagni, M. Scodeggio, P.
Franzetti, P. Memeo, and D. Vergani (INAF-IASF, Milano, Italy), E. Branchini
(Universita Roma III, Italy), O. Le Fevre, V. LeBrun, L. Tresse, C. Adami,
S. Arnouts, A. Mazure, and S. de la Torre (Laboratoire d'Astrophysique de
Marseille, OAMP-CNRS - Universite de Provence, France), A. Pollo
(Laboratoire d'Astrophysique de Marseille, OAMP-CNRS - Universite de
Provence, France and Andrzej Soltan (Institute for Nuclear Research, Warsaw,
Poland), C. Marinoni (Centre de Physique Theorique, CNRS-Universite de
Provence, Marseille, France), S. Charlot (Institut d'Astrophysique de Paris,
CNRS-Universite de Paris 6, France), H. J. McCracken (Institut
d'Astrophysique de Paris, CNRS-Universite de Paris 6, and Laboratoire
d'etude du rayonnement et de la matiere en astrophysique, CNRS -
Observatoire de Paris, France), J. P. Picat, T. Contini, R. Pello, and E.
Perez-Montero (Laboratoire d'Astrophysique de Toulouse et Tarbes,
OMP-CNRS-Universite de Toulouse 3, France), G. Vettolani and A. Zanichelli
(INAF-IRA, Bologna, Italy), R. Scaramella (INAF- Osservatorio Astronomico di
Roma, Italy), S. Bardelli, M. Bolzonella, A. Cappi, P. Ciliegi, F.
Lamareille, R. Merighi, G. Zamorani, E. Zucca, and L. Pozzetti
(INAF-Osservatorio Astronomico di Bologna, Italy), A. Bongiorno and B.
Marano (Universita di Bologna, Italy), L. Moscardini (Universita di Bologna
and INFN-Sezione di Bologna, Italy), S. Foucaud (University of Nottingham,
UK), I. Gavignaud (Astrophysikalisches Institut Potsdam, Germany), O. Ilbert
(University of Hawaii, USA), S. Paltani (Geneva Observatory and Integral
Science Data Centre, Versoix, Switzerland), and M. Radovich
(INAF-Osservatorio Astronomico di Capodimonte, Napoli, Italy). L. Guzzo is
also associated with the MPE, MPA and ESO.

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