#strong lensing

Two Kinds Of Gravitational Lenses Both Reveal Dark Matter

“There needs to be more mass than normal matter alone to explain what we see. Some novel, exotic, invisible “dark matter” is the leading theoretical idea. It’s a radical proposition to presume dark matter’s existence, but gravitational lenses can reveal it. In Einstein’s General Relativity, the presence of mass curves the fabric of spacetime. A large collection of foreground mass distorts the background light through the process of gravitational lensing.”

When we look out at the light-emitting objects in the Universe, they get affected by the entirety of the matter that they pass through along the line-of-sight, as that light travels from the source to our eyes. If there are large collections of mass along that line-of-sight, that mass will bend the fabric of spacetime, creating an effect known as gravitational lensing.

Move Over Hubble: Gravity Is The Best Cosmic Telescope Of All

"If you learn nothing else about General Relativity, learn this: its central idea is that space and time are not independent, but rather make up a single, continuous and inseparable fabric known as spacetime, that every particle travels through this spacetime, and that the presence of matter and energy warps the fabric of spacetime itself. It was realized by Fritz Zwicky in the 1930s that if you got a large enough amount of mass together in one location in space — something like an ultramassive galaxy or a cluster of galaxies — it could acts as a strange sort of magnifying glass on the objects behind it: as a gravitational lens."

Want a recipe for seeing as far into the distant Universe as you can? Because of how distance and brightness are inversely related, you need to make the most of every photon, build as large a telescope as possible and observe for long periods of time in order to collect the most light. But there's an extra bit of magnification that Einstein's General Relativity gives to us for free: gravitational lensing, where any large mass magnifies the light from the objects directly behind it. Thanks to this technique, we've discovered the most distant galaxies ever.