Gravitational Lensing

Light Deflection

The General Theory of Relativity explains gravity in terms of assemblies of mass and energy curving space-time. This implies that masses deflect light in a way similar to a convex lens: they curve light rays towards themselves (right-hand image). Krümmung The deflection of light rays from their otherwise straight paths can give rise to multiple images because light rays can now pass masses in multiple ways. In addition, the deflection is differential, i.e. light bundles can be deformed and focused as they pass masses. Magnification, de-magnification and distortion of the observable images are among the possible consequences.


This so-called gravitational lensing effect can be observed with objects on all mass scales. Stars can produce multiple images whose angular separations are of order micro-arc seconds. Such image splittings are unobservable, but the magnification is which accompanies them. In that way, objects can be discovered which are of stellar mass but shine only weakly or not at all. For instance, this so-called microlensing effect was employed for demonstrating that there are dark, compact objects surrounding our Milky Way galaxy.


QSO 0957+561 Galaxies can produce multiple images with angular separations on the order of arc seconds. The very first gravitational lens to be detected in 1979 is an example: the quasar QSO 0957+561 (right-hand image) is split by a galaxy into two images separated by 6 arc seconds. Meanwhile, dozens of such multiply-imaged quasars have become known. In some of them, quadruple images occur in complicated configurations (image below). These images can be used for exactly measuring the masses and mass distributions of galaxies.

MG 0414 The ways which light has to travel from the source typically differ in length for the individual images of a multiply-imaged quasar. If the source is variable, as many quasars are, the variations occur at different times in the different images. If a model can be constructed for the mass distribution in the gravitational lens, the Hubble constant can be computed from the difference in light-travel time. This can be used for calibrating cosmological distances.

Galaxy Clusters

MS 2137 Even galaxy clusters can act as gravitational lenses. They contain some 100 up to of order 1000 galaxies, but consist mainly of dark matter. Some galaxy clusters can produce highly distorted multiple images of distant galaxies which then appear as arc-like features (arcs; the right-hand image shows the galaxy cluster MS 2137). This effect allows the mass distribution of the galaxy clusters to be probed, but also the cosmological model to be tested. In addition, occasionally the properties of very distant sources can only be measured because of the magnification by galaxy clusters.

Veritable maps of the dark-matter distribution in galaxy clusters can be constructed from the weaker distortion of very many galaxies in their background. This enables mass determinations, but also analyses of the structure of galaxy clusters.

The Universe as a Lens

cosmic Lens The entire Universe is traversed by a filamentary network of cosmic structures which reach lengths of some 10 million light years. Galaxy clusters are located at the intersections of the filaments. Those largest cosmic structures also exert a weak gravitational lensing effect on the light of very distant sources (the left-hand image shows an exaggerated simulation). Only in the past few years has it become possible to measure this lensing effect; in part due to magnification, but largely due to distortion effects. This so-called cosmic shear allows the distribution and amount of the dark matter in the Universe to be determined and the geometry of the Universe to be constrained.

Many more applications of the gravitational lensing effect have meanwhile been developed. As it is solely determined by the mass distribution of cosmic objects and neither depends on the type of matter nor on its physical state, it has developed into one of the most versatile tools in extragalactic astronomy and cosmology.

Verantwortlich: Matthias Bartelmann