Transcript: It was a surprise to astronomers twenty or so years ago when clusters of galaxies began to be detected in significant numbers in x-ray emission, a surprise because astronomers did not expect to find gas in clusters of galaxies. This is because the galaxies that are dominant in clusters, elliptical galaxies, tend to have very little gas, and what gas they would have will be swept out by the rapid motion of the galaxies through the cluster gravitational potential. But it turns out...

Transcript: Purely by chance nature has created several hundred little optics experiments with gravity. This occurs whenever a single galaxy or cluster of galaxies lies directly along the sight line to a more distant object like a quasar, an active galaxy, or another galaxy. Gravitational optics is directly analogous to optics with light. The radiation can be bent, focused, magnified, or demagnified. Gravitational lensing always creates an odd number of images, but the central image is...

Transcript: The most massive bound objects in the universe, clusters of galaxies, can also deflect light. Lensing by clusters produces the interesting phenomenon of multiple images of background galaxies along with distorted images of the galaxies where they form little arcs of light. Cluster lensing has now been observed in dozens of cases. It’s best seen with the Hubble Space Telescope. With its extraordinarily sharp images, the tiny little arcs are easily visible. Typically a...

Transcript: In Einstein’s theory of relativity mass bends light, and this leads to the phenomenon of gravitational lensing. A single massive galaxy can deflect light by a very small angle, about one arcsecond. However, this can be resolved from the ground and especially with the Hubble Space Telescope, and so many situations of gravitational lensing have been discovered. Elliptical galaxies are massive and concentrated enough in their centers to cause multiple image formation of background...