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Gravity helps astronomers see the unseen

Dense galaxy-size 'lenses' and the fact that light is bent by dark matter let researchers peer deeper into the universe.

By Robert C. Cowen Columnist / March 6, 2008



Einstein predicted that the gravity of a massive object such as a galaxy will bend light like a lens. In some cases, the lensing can image distant objects that lie behind the galaxy. Astronomers have studied such gravitational lenses for decades. Now they are ready to turn them into a powerful tool to test the latest theories of the structure and evolution of the universe.

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Far from being a cosmic oddity, gravitational lensing appears to be ubiquitous. New research suggests that there may be half a million strongly lensing galaxies scattered across the sky. Other research has traced strands of unseen dark matter by the way their gravity distorts light. These strands wrap the universe in an invisible web.

Some of this research is from the COSMOS Project that aims to survey thoroughly a small patch of sky about nine times the area of the full moon. Jean-Paul Kneib with the Lab­­or­­atoire d'Astronomique de Marseille, France, and Cécile Faure at the University of Heidelberg, Germany, led a team that sifted through 2 million galaxy images taken by the Hubble Space Telescope. They found 67 strong gravitational lenses in that small space of sky.

Such lensing usually occurs when light from a distant galaxy is magnified and distorted by a cluster of galaxies that lie between the distant galaxy and us. But, in the project's announcement last month, Dr. Kneib says that, "what we are observing here is a similar effect but on a much smaller scale – happening around a single but very massive galaxy."

Now the project plans to automate the search and have robots scan the entire Hubble image archive for similar lenses. If what they have found so far is typical, there could be half a million of these small strong gravity lenses out there. The images these produce of more distant galaxies could be used to create a census of galaxy masses throughout the universe. That could, in turn, be used to test predictions of cosmological theories.

Meanwhile, another international team is using the Canada-France-Hawaii Telescope to trace the cosmic distribution of dark matter. This is an unknown substance whose presence is revealed only by its gravitational effects. In this case, it is revealed by how it deflects light from distant galaxies. This is a very weak effect, which has been observed for the first time. Liping Fu, with the Institut d'Astrophysique de Paris, which leads this research, says, "Our observations extend the knowledge about the cosmic web beyond what was known before." This is a web of dark-matter structures that extend to more than 2,000 times the size of our Milky Way galaxy, according to the research.

Dr. Fu says that what they have observed so far "confirms" that the current model of dark matter structure is "correct even on those very large scales." His institute colleague Yannick Mellier adds, "These results show that weak gravitational lensing is a reliable and accurate technique for cosmology [research]."

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