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.
Clicking on each image below will bring up MPEG movies (800 kB) showing the evolution of the distortion as the clusters move against the background over half billion years. A full description of the simulation process is available. Courtesy J. A. Tyson, UC Davis.
Two simulations of strong lensing by a massive cluster of galaxies. In the upper image, all the dark matter is clumped around individual cluster galaxies (orange), causing a particular distortion of the background galaxies (white and blue). In the lower image, the same amount of mass is more smoothly distributed over the cluster, causing a very different distortion pattern.