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 Fèvre
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 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," declares Le Fèvre. "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 Fèvre.

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 für 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 Fèvre, V. LeBrun, L. Tresse,
C. Adami, S. Arnouts, A. Mazure, and S. de la Torre (Laboratoire
d'Astrophysique de Marseille, OAMP-CNRS - Université de Provence, France),
A. Pollo (Laboratoire d'Astrophysique de Marseille, OAMP-CNRS - Université
de Provence, France and Andrzej Soltan (Institute for Nuclear Research,
Warsaw, Poland), C. Marinoni (Centre de Physique Théorique,
CNRS-Université de Provence, Marseille, France), S. Charlot (Institut
d'Astrophysique de Paris, CNRS-Université de Paris 6, France), H. J.
McCracken (Institut d'Astrophysique de Paris, CNRS-Université de Paris 6,
and Laboratoire d'étude du rayonnement et de la matière en astrophysique,
CNRS - Observatoire de Paris, France), J. P. Picat, T. Contini, R. Pellò,
and E. Perez-Montero (Laboratoire d'Astrophysique de Toulouse et Tarbes,
OMP-CNRS-Université 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 (Universitá di Bologna, Italy), L. Moscardini (Universitá 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.

National contacts for the media:

Belgium: Dr. Rodrigo Alvarez, +32-2-474 70 50
Czech Republic: Pavel Suchan, +420 267 103 040
Finland: Ms. Tiina Raivo, +358 9 7748 8369
Denmark: Dr. Michael Linden-Vørnle, +45-33-18 19 97
France: Dr. Daniel Kunth, +33-1-44 32 80 85
Germany: Dr. Jakob Staude, +49-6221-528229
Italy: Dr. Leopoldo Benacchio, +39-347-230 26 51
The Netherlands: Ms. Marieke Baan, +31-20-525 74 80
Portugal: Prof. Teresa Lago, +351-22-089 833
Spain: Dr. Miguel Mas-Hesse, +34918131196
Sweden: Dr. Jesper Sollerman, +46-8-55 37 85 54
Switzerland: Dr. Martin Steinacher, +41-31-324 23 82
United Kingdom: Mr. Peter Barratt, +44-1793-44 20 25
USA: Dr. Paola Rebusco, +1-617-308-2397

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