Credit: X-ray: NASA/CXC/M.Markevitch et al.
Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.
Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.
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.
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.
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.