Gravitational Lensing

If mass curves spacetime, then even light — following the straightest path — must bend as it passes a massive object. The first dramatic confirmation came in 1919, when Arthur Eddington measured starlight deflected by the Sun during a total eclipse, shifting the stars' apparent positions by exactly the amount Einstein predicted.^[1] Today this gravitational lensing is a routine tool: clusters of galaxies magnify and distort the light of far more distant ones behind them, and the amount of bending reveals how much unseen mass is there — one of our best ways to map dark matter.^[2]

What lensing looks like

The shape you see depends on how precisely the distant source, the lensing mass, and Earth line up. A near-perfect alignment wraps the source's light into a ring — an Einstein ring; a slight offset instead produces multiple separated images, or bright curved arcs.^[3]

Four bright points of light arranged in a cross around a central galaxy — the Einstein Cross. — The Einstein Cross: one distant quasar, its light split into four images by a galaxy almost directly in front of it. NASA / ESA / STScI · Hubble

A blue near-complete ring of light encircling a bright orange galaxy — the Cosmic Horseshoe. — The Cosmic Horseshoe: a background galaxy smeared into an almost complete Einstein ring by a massive foreground galaxy nearly on the same line of sight. ESA/Hubble & NASA

When the lens is a whole galaxy cluster, its enormous mass magnifies and stretches dozens of background galaxies at once into thin arcs. Because ordinary matter alone can't bend light this strongly, the arc pattern betrays the cluster's hidden dark matter — lensing lets us weigh and map mass we cannot see.

A dense field of galaxies threaded with many faint, thin curved arcs of light. — Galaxy cluster Abell 1689. The faint arcs threading the field are magnified, distorted images of galaxies far behind it; their arrangement traces the cluster's total mass, most of it dark matter. NASA / ESA · Hubble

Pushed to its extreme, the same light-bending is what gives a black hole its silhouette: rays skimming the photon sphere are lensed around the hole into the bright ring the Event Horizon Telescope imaged.