Every day you play with the light of the universe.
Subtle visitor, you arrive in the flower and the water…
These lines from one of Pablo Neruda’s poems captures the essence of light bending—gravitational lensing—that is ubiquitous in the cosmos. Re-conceptualizing gravity in his theory of general relativity, Einstein postulated the existence of space-time, a four-dimensional sheet to describe the universe. This is a beautiful marriage between geometry and physics, wherein all matter, both ordinary and exotic, in the universe would cause potholes in the fabric of space-time. So that matter would dictate how space-time curved, and space-time in-turn would determine how matter moves.
One important consequence of this formulation is the impact that the rumpled fabric of space-time would have on the propagation of light in the universe. Light emitted by distant galaxies will get deflected by the potholes generated by all the mass en-route that it encounters. This phenomenon of the bending of light is referred to as gravitational lensing.
The consequence of lensing is that we would see the shapes of distant galaxies to be systematically distorted, and they would appear to be more elongated than their actual, true shapes. The strength of the lensing distortion is directly proportional to the number and depth of potholes encountered, namely, the detailed distribution of matter along the line of sight as well as cosmic distances. Also, the alignment matters, how the distant galaxy, the intervening matter that is causing the deflection and we are located. The strength of the light bending therefore also depends on the geometrical properties of space-time that are encapsulated in a set of cosmological parameters that characterize our universe.
Gravitational lensing by matter is somewhat similar to the optical focusing and defocusing produced convex and concave glass lenses that we are all familiar with from high school science experiments. Unlike the light bending produced by glass lenses though, in the case of gravitational lensing by matter in the universe, occasionally when the alignment in perfect, a single light beam might be cleaved into two—causing the appearance of a pair of images when in reality there is a single distant object that is the source of light. Production of multiple images of the same object is referred to as strong gravitational lensing. This happens only occasionally though. Most of the time, what is observed are weak distortions in the shapes of distant galaxies when viewed through a cosmic lens.
Lensing was one of the key predictions of the theory of general relativity proposed in 1915. It was proved in 1919, when light bending during a solar eclipse was detected. During a solar eclipse the pothole generated by the sun and the earth in space-time line up cause a measureable deflection in light from stars in the field. In this instance, there are no multiple images produced or distortions since stars are point sources. But stars in the field have a displaced apparent position due to the curvature of space-time during the line-up. And once the eclipse is over, the stars appear where they really are.
General relativity accurately predicted the displacement between the real and apparent positions. It was the verification of this prediction by the British Astronomer Arthur Eddington that made Einstein a celebrity and a household name. The more dramatic predictions of gravitational lensing—the strong distortions in the shapes of lensed distant galaxies, the production of multiple image of the same distant object have all been verified with data from ground based telescopes and the Hubble Space Telescope. In fact, the most convincing evidence for the existence of copious amounts of dark matter in the universe comes from the lensing effects produced by the unseen matter on the shapes of distant galaxies.