Gravitational lens

Gravitational lensing is a phenomenon in which light from a distant astronomical source is bent as it passes near a massive object on its way to the observer. The deflection arises from the curvature of spacetime, a central prediction of Albert Einstein’s general theory of relativity. Although Newtonian physics can also account for the bending of light if light is treated as corpuscular in nature, it predicts only half of the value obtained under general relativity. Gravitational lensing has become an essential tool for probing the distribution of matter in the Universe, including both luminous and dark matter components.

Description and Theoretical Foundations

Gravitational lensing occurs when mass causes a curvature in spacetime sufficient to alter the path of light. Unlike an optical lens, which has a finite focal length, a point-like gravitational lens produces its strongest deflection for light rays passing closest to the centre of mass and a weaker effect for those travelling further away. Consequently, the focusing effect forms not a single point but rather a focal line.
When a distant light source, a massive intervening lens and an observer lie in near-perfect alignment, the lensed source may appear as a ring of light surrounding the lens, termed an Einstein ring. This configuration was mentioned by Orest Khvolson in 1924 and later quantified by Einstein in 1936. Small deviations from perfect alignment transform the ring into an arc, while more complex mass distributions—such as galaxy clusters—produce multiple distorted images or arc-like features scattered around the lens.
The magnitude and geometry of gravitational lensing depend on the mass, shape and distribution of the lensing object, as well as on the relative distances between the observer, lens and source. Importantly, gravitational lenses act on all forms of electromagnetic radiation, including visible light, radio waves and X-rays, and can also influence gravitational waves.

Types of Gravitational Lensing

Gravitational lensing is categorised according to the degree of distortion it imparts on the background source.
Strong gravitational lensingStrong lensing occurs when the distortions are pronounced and readily visible. Characteristic signatures include Einstein rings, elongated arcs and multiple images of the same object. Even very massive galaxies generate image separations of just a few arcseconds, whereas galaxy clusters may produce separations of several arcminutes. These systems are typically located at cosmological distances, often many hundreds of megaparsecs from the Milky Way.
Weak gravitational lensingWeak lensing results in subtle distortions of background galaxies, often at the level of a few per cent. Individual distortions are too small to be measured reliably, but statistical analyses of many thousands or millions of galaxies reveal coherent patterns of shear. These shear fields can be used to infer the projected mass distribution of the lensing region and are instrumental in mapping the dark matter content of the Universe. Weak lensing surveys require careful treatment of systematic effects such as the intrinsic ellipticity of galaxies, atmospheric seeing and camera point-spread distortions. They play a central role in improving constraints on cosmological parameters, in particular those underpinning the ΛCDM model and theories of dark energy.
Gravitational microlensingMicrolensing refers to the temporary brightening of a background source caused by the passage of a compact foreground object, without a resolvable change in the source’s shape. Such lenses may be stars in the Milky Way, with the background source lying in the Magellanic Clouds or even in a distant galaxy. Extreme cases include one star in a galaxy lensing another far more distant star, as observed for MACS J1149 Lensed Star 1 (Icarus). Microlensing enables the detection of otherwise invisible objects, including exoplanets and compact stellar remnants.
A key aspect of multi-image strong lenses is the “time delay” between the arrival of light along different paths. The differences arise because the paths have different lengths and pass through varying gravitational potentials, meaning that the same event can be observed at slightly different times in each image.

Historical Development

The idea that gravity might deflect light dates back to Isaac Newton, who speculated on the matter in 1704. Henry Cavendish computed such an effect in an unpublished manuscript in 1784, and Johann Georg von Soldner published a Newtonian calculation in 1804. In 1911, Einstein independently derived an identical value based on the equivalence principle, but later recognised that this was only half the value predicted by the completed theory of general relativity in 1915.
The first observational confirmation came during the solar eclipse of 29 May 1919. Expeditions led by Sir Arthur Eddington, Frank Watson Dyson and their collaborators measured shifts in stellar positions as their light passed near the Sun. The observed bending matched Einstein’s prediction and propelled general relativity—and Einstein himself—into global prominence.
Einstein had speculated in 1912 that a mass could produce multiple images of a background source. However, he believed such alignments would be too improbable to observe, and early physicists shared this view. Orest Khvolson’s 1924 paper provided the first printed discussion of the “halo” effect when the alignment is nearly perfect, now known as the Einstein ring. In 1936, prompted by Rudi W. Mandl, Einstein published a short article discussing the “lens-like action” of a star. A year later, Fritz Zwicky extended the idea to galaxies and galaxy clusters, proposing that their large masses would render gravitational lensing observable.
In the 1960s, independent work by Yu. G. Klimov, S. Liebes and Sjur Refsdal established quasars as ideal background sources. The first confirmed gravitational lens—a double image of quasar SBS 0957+561, often referred to as the “Twin QSO”—was discovered in 1979, validating Zwicky’s prediction.

Modern Applications and Observations

Gravitational lensing is now a cornerstone of observational cosmology. Strong lenses provide high-resolution magnified views of distant galaxies and enable detailed studies of mass distributions in galaxies and clusters. Weak lensing surveys are fundamental for reconstructing the dark matter distribution across large areas of the sky and for constraining cosmological parameters. Microlensing contributes to the discovery of exoplanets, the study of stellar populations and the detection of compact masses such as black holes.
Lensing effects are analysed across the electromagnetic spectrum, including optical, radio, X-ray and microwave observations. Weak lensing signatures have been detected in the cosmic microwave background, providing an integrated measure of structure formation over cosmic time. Lensing of gravitational waves has also become a prospective frontier, offering new possibilities for multimessenger astronomy.