You may be familiar with how lenses bend light to create images, such as the biconvex lens shown below. The lens is made of a material such as glass or plastic that is denser than air, so light will bend at the interface between the air and the lens.
But did you know that any massive object can be used as a lens? As Einstein’s general relativity tells us, gravity can be seen as massive objects bending spacetime. If you imagine the Earth as a bowling ball on a trampoline, the weight of the bowling ball pulls down the fabric of the trampoline, just as the mass of the Earth bends the spacetime around it.
In a phenomenon called gravitational lensing, large massive objects such as galaxy clusters bend light from distant sources, creating distorted images that we can see here on Earth. The diagram below from the European Southern Observatory (home of the creatively-named European Extremely Large Telescope) shows how this works. The foreground galaxy is massive enough that it bends the light from the background galaxy around it, much like an ordinary lens does. The telescopes on Earth see a distorted image of the background galaxy that contains multiple images, often in an arc around the foreground galaxy. An image below from the Hubble Telescope has several examples of gravitational lensing. There are three distorted images of a lensed galaxy and five of a lensed quasar.
Besides being really neat, this technique is especially useful for detecting dark matter. Since dark matter doesn’t interact with light, it can’t be seen directly. However, since dark matter is very massive, it can be detected indirectly by the distorted images it creates of normal matter through gravitational lensing. Experiments like the Large Synoptic Survey Telescope aim to take advantage of gravitational lensing to map the dark matter in the universe and provide clues to its nature. For a bit of added intrigue, it turns out that mass-energy equivalence (what E=mc2 is all about) means that energy can bend light just as mass can—so in addition to providing information about dark matter, gravitational lensing can also be used to study dark energy! The effect is much smaller, so it’s called weak gravitational lensing, but experiments such as the Dark Energy Survey are using gravitational lensing to study dark energy in much the same way as it can be used to study dark matter.