gravitational-lensing Astronomy

Strong lensing produces multiple images, Einstein rings, and giant arcs, letting astronomers weigh galaxy clusters and magnify faint background galaxies; weak lensing shows up as tiny coherent shears in the shapes of background galaxies and underpins maps of the cosmic web as well as tests of cosmological parameters; microlensing detects unseen compact objects, from free-floating planets to stellar-mass black holes, through the brief brightening of background stars. All three turn gravity itself into a natural telescope.

When the gravitational field of a foreground galaxy or cluster curves spacetime, light from a more distant source reaches the observer along several geodesic paths at once, so one target can appear as two to four separate images; under favorable geometry they merge into an Einstein ring. The image separation is set by the Einstein radius, which depends on the lens mass and the distances among source, lens, and observer, so measuring that angle yields a direct estimate of the lens system's mass. Strong lensing also magnifies the brightness and apparent size of background objects, bringing otherwise invisible high-redshift galaxies into reach.

General relativity lets massive bodies bend light. When a foreground galaxy sits almost precisely between quasar and Earth, rays travel two symmetric paths around it and arrive together — two images of one source, spectra identical but delayed by days to hundreds of days. Near-perfect alignment stretches the image into a luminous ring around the lens: an Einstein ring. The 1979 double quasar Q0957+561 was the first confirmed case; lensing since became a weighing tool, because image separation and ring radii measure total lens mass including dark matter, and time delays even probe the universe's expansion rate.