Gravitational lens

A massive object whose gravity bends light from a more distant source, producing distorted, magnified, or multiple images.

Gravitational lensing: The deflection and distortion of light by gravity, as described by general relativity. This broader phenomenon explains how the lens changes a background source’s apparent image.
Dark matter: Matter inferred from gravitational effects but not detected through emitted or absorbed light. Lens distortions map mass that emits too little light to see directly.
Einstein Cross: A quasar appearing as four images around a foreground galaxy due to gravitational lensing. Its four bright images make a compact galaxy-scale lens especially recognizable.
Optical lens: A transparent device that refracts light to form or alter images. Both can magnify distant sources, but an optical lens bends light through material rather than gravity.
Einstein radius: The characteristic angular scale of a gravitational lens, set by the lens and source geometry. It marks the scale at which a lens can produce a ring or widely separated images.
Galaxy cluster: A gravitationally bound system containing many galaxies, hot gas, and dark matter. Clusters often provide enough mass to act as powerful lenses.
Twin Quasar: A quasar seen in two images because a foreground galaxy deflects its light. This early identified example helped establish that duplicate quasars can be lens images.
Gravitational redshift: A change in light’s frequency as it moves through a gravitational potential. Gravity can alter light’s energy as well as its path, producing a distinct observable effect.
Einstein ring: A ring-shaped image formed when a source, lens, and observer are nearly aligned. Near-perfect alignment reveals the lens’s symmetric image pattern.
Time-delay cosmography: A method for estimating cosmic distances and expansion using delays between variable lensed images. Different image paths can arrive at different times, adding a distance-measuring signal.

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