Gravitational lensing

Gravitational lensing is the bending and magnification of light from a distant source by gravity along its path. It can produce distorted images, multiple images, or bright rings.

Gravitational deflection of light: The change in a light ray’s direction caused by gravity, described in general relativity. This deflection is the basic effect that makes a foreground mass act as a lens.
Dark matter: Matter inferred from gravitational effects that emits little or no detectable light. Lensing maps mass whether or not that mass shines.
General relativity: Einstein’s theory describing gravity as the curvature of spacetime caused by energy and matter. Its curved-spacetime description predicts the bending of light near mass.
Albert Einstein: The physicist who developed special and general relativity and reshaped twentieth-century physics. His theory predicted that gravity would deflect starlight.
Refraction: The change in a wave’s direction when it passes between media with different propagation speeds. Unlike refraction, gravitational lensing does not require light to enter a different material.
Einstein ring: A ring-shaped image formed when a source, lens, and observer are nearly perfectly aligned. Exact alignment turns the lensed source into a ring rather than separate images.
Galaxy cluster: A gravitationally bound system containing hundreds to thousands of galaxies, hot gas, and dark matter. Clusters are massive lenses that can magnify faint background galaxies.
Spacetime: The four-dimensional structure combining space and time in physical theories. Lensing follows paths through spacetime curved by intervening matter.
Arthur Eddington: The British astronomer who led the 1919 eclipse expedition testing the deflection of starlight. His expedition made the predicted bending a prominent observational claim.
Optical aberration: A defect in an optical system that distorts an image because rays do not focus ideally. Instrumental aberrations can mimic distortions, but arise in the telescope rather than along cosmic paths.

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