Light does not always travel in straight lines. As Einstein predicted in his Theory of General Relativity, massive objects will deform the fabric of space itself. When light passes one of these massive objects, such as a cluster of galaxies, its path is changed slightly.
This effect, called gravitational lensing, is only visible in rare cases and only the best telescopes can observe the related phenomena.
Hubble’s sensitivity and high resolution allow it to see faint and distant gravitational lenses that cannot be detected with ground-based telescopes whose images are blurred by the Earth’s atmosphere. The gravitational lensing results in multiple images of the original galaxy each with a characteristically distorted banana-like shape or even into rings.
Hubble was the first telescope to resolve details within these multiple banana-shaped arcs. Thanks to its sharp vision, it can reveal the shape and internal structure of the lensed background galaxies directly. In this way one can easily match the different arcs coming from the same background object — be it a galaxy or even a supernova — by eye.
Gravitational lensing can be used to ‘weigh’ clusters because the amount of lensing depends on the total mass of the cluster. This has substantially improved our understanding of the distribution of the ‘hidden’ dark matter in galaxy clusters and hence in the Universe as a whole. The effect of gravitational lensing also allowed a first step towards revealing the mystery of the dark energy.
As gravitational lenses function as magnification glasses it is possible to use them to study distant galaxies from the early Universe, which otherwise would be impossibly faint to see due to their great distance from Earth.
“When we first observed the galaxy cluster Abell 2218 with Hubble in 1995 we mainly aimed at studying the cluster and its galaxies. But we got a surprise. The images showed dozens and dozens of gravitationally lensed arcs. When we showed these ultrasharp images to our colleagues they could immediately see the importance of using gravitational lensing as a cosmological tool.”
— Richard Ellis, Astronomer, University of Cambridge and California Institute of Technology