Gravitational lenses: when galaxies bend the light of other galaxies

A lens without glass

In general relativity, light moves along paths determined by the geometry of spacetime. A large mass can therefore change the direction from which the light of a background object reaches us. With suitable alignment, we see multiple images of the same source, elongated arcs or approximately ring-shaped structures. The foreground mass then acts as a gravitational lens. Unlike an ordinary glass lens, it has no precisely ground surface. Its effect depends on the entire spatial distribution of mass and the distances between source, lens and observer. The effect does not mean that light is deflected by an ordinary electric charge. Rather, it is a consequence of gravity and affects visible light as well as other electromagnetic wavelengths, provided they propagate through the same large-scale geometry.

Amplification brings opportunities and distortions

A gravitational lens can amplify the observed light of a distant source and increase its apparent size. This makes objects accessible that would be harder to study without the effect. However, the result is not an unaltered portrait. Shapes are distorted, different regions are magnified to different degrees, and sometimes multiple images are produced. To reconstruct the original source, researchers need a model of the foreground mass. Uncertainties in this model affect conclusions about the size and brightness of the background galaxy. Multiple images also provide additional information: they show the same source along different light paths. For variable objects, this can make time delays measurable. The lens is therefore neither merely an optical trick nor a perfect cosmic telescope, but a physical system whose effect must itself be carefully measured.

Invisible mass is mapped statistically

Not every gravitational lens produces conspicuous arcs. Often, the changes in the shapes of individual background galaxies are so small that they can only be detected statistically across very large numbers of objects. This weak lensing helps to study the distribution of gravitating mass on large scales. It is also sensitive to dark matter, because its gravity acts regardless of whether it emits light. The challenge is to separate the cosmic effect being sought from intrinsic differences in galaxy shapes and instrumental distortions. Large sky surveys are particularly well suited to this. Lensing maps therefore do not directly show a new type of particle, but rather the spatial effect of mass under the model assumptions used. Combined with stellar motions, gas observations and cosmological data, this produces a much more robust picture of the distribution of matter than any single measurement method alone.

Keep exploring

• ALMA: How radio telescopes see the cold nurseries of stars
• Dark energy: what accelerating expansion really means
• Dark matter: why astronomers infer invisible mass