Background image: The galaxy cluster Abell 370, showing examples of gravitational lensing. Note how the images of galaxies appear stretched out in a roughly circular pattern around the centre of the image. Especially, note the very elongated image of a galaxy in the upper right-hand quadrant of the image.
Credit: NASA, ESA, and the Hubble SM4 ERO Team & ST-ECF
Large masses can curve spacetime, as described by Einstein’s General Relativity. As a result, light rays passing close to a large mass will have their path bent by the curved spacetime. This causes both magnification and distortion of the image of a background object (e.g. a galaxy). In this way, the large mass functions as a lens. Distortion features that can be observed are multiple images of one object, arcs/shape distortions, and so-called ‘Einstein rings’. Gravitational lensing has been observed for lensing objects such as the Sun, up to large clusters of galaxies with a mass of a million billion times the mass of the Sun.
Weak gravitational lensing of background galaxies and the cosmic microwave background by the foreground matter distribution are two tools to study cosmology that are gradually coming to the fore. Strong gravitational lensing also has the potential to provide important information about cosmology.