Gravitational Lensing (Astronomy)
Gravitational lensing is the deflection of light by the curvature of spacetime near a mass, as predicted by general relativity; for a point mass M and an impact parameter b, the deflection angle is α = 4GM/(c²b), twice the Newtonian value. Astronomers use strong, weak and micro lensing to weigh galaxies and clusters, magnify distant sources, and detect dark or isolated objects.
Curious
Einstein's theory of general relativity says that mass curves space, and light follows the curved paths (see Curved Spacetime). Light grazing the edge of the Sun should therefore be bent by about 1.75 arcseconds. In 1919, expeditions led by Arthur Eddington and others measured star positions during a total solar eclipse and reported deflections consistent with Einstein's prediction rather than the Newtonian half-value, which made Einstein world famous.
When the alignment of source, lens and observer is close, the bending can create multiple images, arcs, or a full ring of light, called an Einstein ring. Lensing by galaxy clusters gives astronomers a natural telescope, letting the James Webb Space Telescope see galaxies that would otherwise be too faint.
Exploring
| Regime | What is seen | Typical use |
|---|---|---|
| Strong lensing | Multiple images, arcs, rings | Mass maps of cluster cores; magnified distant galaxies |
| Weak lensing | Small coherent distortions of many galaxies | Mapping dark matter; cosmic shear and cosmology |
| Microlensing | Temporary brightening, no resolved images | Detecting planets and dark compact objects |
Deep Dive
Theory. In the thin-lens approximation, the lens equation relates the true source angle β and image angle θ by β = θ − θE²/θ. For a point lens, a perfectly aligned source gives a ring of radius θE; otherwise there are two images. Magnification equals the ratio of image area to source area and can be large near the critical curves. Lensing conserves surface brightness, so it enlarges rather than brightens per unit area. Time delays between the images of a variable source depend on the lens model and on the Hubble constant, which allows an independent measurement of H0 (see Hubble Constant).
History. The 1919 result has been debated by historians: the data were noisy, and some critics argued the team selected plates favourably. Later eclipse and radio-interferometric measurements, with far greater accuracy, confirm the general-relativistic value to a fraction of a percent, so the physical conclusion is secure even if the 1919 precision was overstated. Fritz Zwicky proposed in 1937 that galaxies might act as lenses, and the first lensed quasar, Q0957+561, was identified in 1979.
Applications and limits. Weak lensing surveys map dark matter without assuming it is luminous, and the Bullet Cluster analysis relies on them. Microlensing surveys such as OGLE and KMTNet detect planets that other methods miss (see Exoplanets). Lensing mass estimates still depend on model assumptions, such as the mass-sheet degeneracy, which can bias H0 measurements, a live issue in the Hubble tension discussion.
Concept Relationship Map
Requires: curved spacetime, galaxy clusters
Enables: dark-matter mapping, microlensing planet searches, H0 measurements
Commonly confused with: gravitational waves
Sources
- Dyson, F.W., Eddington, A.S., Davidson, C. (1920). Philosophical Transactions of the Royal Society A, 220, 291–333.
- Zwicky, F. (1937). “Nebulae as Gravitational Lenses.” Physical Review, 51, 290.
- Schneider, P., Ehlers, J., Falco, E.E. (1992). Gravitational Lenses. Springer.