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An international team of astronomers from French and Canadian institutes has analysed Megacam images from the Canada-France-Hawaii Telescope Legacy Survey. They observed for the first time very weak gravitational lensing effects produced by extremely large cosmic structures of the Universe.  The distribution of dark matter inside filaments extending up to 270 million light-years in size provides unprecedented information on the cosmic history of structure formation, properties of the dark Universe and the cosmological parameters that characterize the Universe.

Astronomers have known for some time that the Universe is full of mysterious dark matter. This invisible matter forms giant structures of filaments, sheets and clusters. How exactly this dark "cosmic web" is distributed throughout the Universe has long eluded scientists.

An international team of astronomers from France and Canada has recently managed to detect for the first time dark matter structures up to 270 million light years in size. These structures extend to more than 2000 times the size of our Milky Way. The explored scales are three times larger than what any previous analysis showed.

To accomplish this feat, the scientists used a relatively new technique called weak gravitational lensing. The light from distant galaxies is deflected by dark matter while it travels through the Universe to us.  Just like the bone structure of a human body is visible on an X-ray radiograph, the dark matter leaves its imprint on the galaxy light pattern revealing itself through gravity. The careful measurement of this effect, predicted by Einstein, has been the major goal of a large project, the Canada-France-Hawaii Telescope Legacy Survey.

The team of 19 researchers from 11 institutes led by the Institut d'Astrophysique de Paris (IAP, CNRS, Université Pierre & Marie Curie), the University of British Columbia  (UBC) and the University of Vistoria (UVic) has spent several years developing tools to analyse images obtained by the largest digital camera in the world. The results are a major leap forward, as such large scales and small signals have never been probed before.

"Our observations extend the knowledge about the cosmic web far beyond what was known before," says Liping Fu. "We confirmed that our model about the Universe is correct even on those very large scales."  The measurements on very large scales, adds Fu, have the advantage to be easily comparable to theoretical predictions.  The measurement of the composition of the Universe is crucial for the understanding of the history and evolution of the Universe. It also allows to predict its fate in the far future.

"These results show that weak gravitational lensing is a reliable and accurate technique for cosmology," emphasizes Dr. Yannick Mellier from the IAP. Next-generation telescopes and cameras will measure weak gravitational lensing over the whole sky using billions of galaxies. Those surveys will help to unveil the nature of dark matter and to shed light on the even bigger mystery of dark energy.

These results will appear soon in Astronomy & Astrophysics Main Journal and is already available on-line at http://dx.doi.org/10.1051/0004-6361:20078522 .  The international team of astronomers is composed of  L. Fu (IAP, UPMC, SHNU Shanghai), E. Semboloni (IAP, UPMC, AIfA Bonn), H. Hoekstra (UVic), M. Kilbinger (IAP, UPMC, AIfA Bonn) L. van Waerbeke (UBC), I. Tereno (IAP, UPMC, AIfA Bonn), Y. Mellier (IAP, UPMC), C. Heymans (UBC, IAP,UPMC), J. Coupon (IAP, UPMC), K. Benabed (IAP, UPMC), J. Benjamin (UBC), E. Bertin (IAP, UPMC), O. Doré (CITA), M.J. Hudson (Univ. Waterloo), O. Ilbert (LAM Marseille, IfA Honolulu), R. Maoli (IAP, UPMC, Univ. La Sapienza Roma), C. Marmo (IAP, UPMC), H.J. McCracken (IAP, UPMC), B.Ménard (CITA).

Contact persons:
Liping Fu and Yannick Mellier (IAP and UPMC) (33) 1 44 32 8140
Ludovic Van Waerbeke (UBC) (1) 604-822-5515
Henk Hoekstra (UVic) (1) 250-721-7743

CFHT is a joint facility of National Research Council of Canada, Institut National des Science de l'Univers of Centre National de la Recherche Scientifique of France, and University of Hawaii.  TERAPIX is a French national data center funded by the CNRS Institut National des Sciences de l'Univers, the Programme National de Cosmologie and IAP.

Based on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council (NRC) of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique (INSU-CNRS) of France, and the University of Hawaii. This work is based in part on data products produced at Terapix and the Canadian Astronomy Data Centre as part of the Canada-France-Hawaii Telescope Legacy Survey, a collaborative project of NRC and CNRS.