What is Gravitational Lensing?
Image Credit: ESA/Hubble & NASA
From time to time you may see photos like the one above, which shows an object (or group of objects) appearing to have a kind of "halo" surrounding it.
Also sometimes referred to as an "Einstein Ring", gravitational lensing occurs when light from a distant source is warped by a large mass, such as a galaxy or black hole. The light gets bent by the mass (such as a lens would), and forms into a ring shape.
This helpful video shows how this phenomenon works using a wine glass:
Why is gravitational lensing important?
According to One Minute Astronomer:
Astronomers can do more than take pretty pictures of gravitational lenses. Using the ideas developed by Einstein and others, if the distance to the foreground and background objects can be determined using Hubble’s Law, and if the degree of deflection by the “lens” is measured, then astronomers can calculate the mass of the foreground object. This is, of course, simply amazing. That we can determine the mass of, for example, a cluster of hundreds of galaxies billions of light years away by measuring some light with a telescope and applying some inspired mathematics. And yet science enables such things.
After measuring gravitational lensing by dozens of galaxy clusters, astronomers made an unexpected discovery. The masses of galaxy clusters are much larger than can be explained by the visible light in the clusters. There are simply not enough stars in these galaxies to account for the amount of gravitational lensing observed. The conclusion? Most of the mass of these clusters consists of matter which emits no light, a type of “dark matter”.
The universe is full of light-bending mysteries like the gravitational lens - but each discovery adds another piece to the puzzle.
