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Illustration of strong gravitaional lensing.

Strong gravitational lensing happens when there is so much mass contrast in the lens that the light rays from a distant source bend around both sides of the lens and cross near Earth. Then multiple images of the source may be seen. This was first seen in a quasar lensed by a galaxy in 1979. More commonly, the huge dark matter concentrations in clusters of galaxies create typical bending angles of 30 arcseconds, and multiple highly distorted images of a source galaxy.

About 90% of the Universe is dark—we can't see it except through its gravitational pull. Although this was suspected more than 60 years ago, we are just now in a position to explore the dark matter in large areas of the Universe through a technique called weak gravitational lensing.

As the light from a distant source passes by a mass concentration its ray path is bent, causing the distant source to appear at an altered place on the sky and resulting in a tell-tale distortion of its shape. This gravitational lensing effect provides the first, and currently only, way to directly "weigh" cosmic mass. Lensing in its strong form results in some striking images, but it is relatively rare. To learn about more typical parts of the Universe, we use weak lensing.

At the faint magnitudes reached by large telescopes, the sky is studded with tens of billions of faint distant blue galaxies. In recent years astronomers have become adept at mapping the dark matter associated with known galaxy clusters using these background galaxies as a cosmic wallpaper for weak gravitational lensing analyses.

^{This is an image of dark matter in a 2 degree by 2 degree field of the sky from the Deep Lens Survey. Many mass clusters may be seen in projection. With color redshift information on the background galaxies, three-dimensional maps can be constructed. The LSST is needed to make these tomographic mass maps over a cosmologically significant area.}

With multi-wavelength deep imaging of the faint blue galaxies, we can construct photometric redshifts for them and go beyond a simple foreground/background paradigm. Photometric redshifts enable tomographic analysis of slices of the projected sky in redshift bins. By obtaining weak lensing maps for sources at a variety of redshifts, we can obtain a three-dimensional mass map of the universe back to half its current age. Only the Large Synoptic Survey Telescope with its combination of huge field and large light grasp would enable such a survey in our lifetimes.

Structures as large as 500 million light-years are known to exist. The Large Synoptic Survey Telescope will for the first time map the evolution of these mass structures over cosmic time. This will directly test theories of the evolution of our universe and the nature of dark matter and dark energy.

^{32-Mpc simulation of large-scale structure in a standard cold-dark matter Universe. Courtesy R. Cen, Princeton University.}

To undertake tomographic gravitational lens reconstruction of dark matter images at high redshift and large look-back times requires superb imaging of distant background galaxies. At 29th magnitude per square arcsecond surface brightness, there is a distant blue galaxy every several arcseconds on the sky. One requires good angular resolution over a 10 square degree field of view, coupled with the light gathering power of an 8-meter class mirror.

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