Weak gravitational lensing

Weak gravitational lensing is the small, coherent distortion of background galaxies’ apparent shapes by foreground mass. Statistical patterns in these distortions reveal the distribution of matter, including dark matter.

Gravitational lensing: The bending and distortion of light from a background source by gravity along its path. Weak lensing is the subtle, statistical regime of this broader gravitational effect.
Dark matter halo: A gravitationally bound distribution of dark matter surrounding a galaxy or galaxy cluster. Weak lensing measures halo mass without requiring the dark matter to emit light.
General relativity: Einstein’s theory of gravity, in which mass and energy curve spacetime and influence the motion of matter and light. The curvature of spacetime underlies the deflection measured in lensing.
Strong gravitational lensing: Gravitational lensing strong enough to produce conspicuous arcs, multiple images, or Einstein rings. Unlike weak lensing, it is identified through dramatic distortions around individual lenses.
Shear (astronomy): The anisotropic stretching of images caused by gravitational lensing. Weak lensing is measured chiefly through the coherent shear of galaxy images.
Mass mapping: The reconstruction of projected matter distributions from gravitational-lensing observations. Weak-lensing shear can be converted into maps of otherwise invisible foreground mass.
Gravitational potential: A scalar field describing the gravitational potential energy per unit mass at each point in space. Its spatial variations determine how foreground structures deflect light rays.
Microlensing: Gravitational lensing in which a compact foreground object magnifies a background source without usually producing resolved multiple images. It detects transient brightness changes rather than coherent galaxy-shape distortions.
Convergence (gravitational lensing): The isotropic magnification or demagnification of images produced by gravitational lensing. Together with shear, convergence describes the local distortion field.
Galaxy cluster: A gravitationally bound system containing hundreds to thousands of galaxies, hot gas, and dark matter. Weak lensing estimates cluster masses independently of their light and gas.

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