Light does not always travel in a straight line. As Einstein predicted in his theory of general relativity, massive objects will warp the fabric of space itself. When light passes in front of one of these massive objects, such as a cluster of galaxies, its trajectory is slightly altered.
This effect, called gravitational lensis visible only in rare cases and only the best telescopes can observe the associated phenomena.
Hubble’s sensitivity and high resolution allow it to see weak and distant gravitational lenses which cannot be detected with ground-based telescopes whose images are blurred by the Earth’s atmosphere. Gravitational lensing yields multiple images of the original galaxy, each with a characteristic distorted banana shape or even in rings.
Hubble was the first telescope to resolve the details of these multiples banana shaped bows. With its sharp vision, it can directly reveal the shape and internal structure of lensed background galaxies. This way one can easily match the different arcs coming from the same background object – be it a galaxy or even a supernova – at the eye.
The gravitational lens can be used to ‘weigh’ the grapes because the quantity of lenses depends on the total mass of the cluster. This has greatly improved our understanding of the distribution of “hidden” dark matter in galaxy clusters and therefore in the Universe as a whole. The gravitational lensing effect has also allowed a first step towards reveal the mystery of dark energy.
As gravitational lenses work like magnifying glasses it is possible to use them for study distant galaxies of the early Universe, which would otherwise be incredibly faint to see due to their great distance from Earth.
“When we first observed the galaxy cluster Abell 2218 with Hubble in 1995, our main objective was to study the cluster and its galaxies. But we had a surprise. The images showed dozens and dozens of gravitationally lensed arcs. When we showed these ultra-sharp images to our colleagues, they immediately understood the importance of using gravitational lensing as a cosmological tool.
— Richard Ellis, Astronomer, University of Cambridge and California Institute of Technology