Einstein's general relativity predicts that mass curves spacetime itself. Light, following the shortest path through curved spacetime, bends around massive objects — an effect confirmed during the 1919 solar eclipse by Arthur Eddington, which made Einstein world-famous overnight.
When a massive foreground object sits between us and a distant light source, it acts as a cosmic lens. The background source can appear stretched into arcs, duplicated into multiple images, or — if the alignment is perfect — smeared into a complete ring of light called an Einstein ring. The radius of that ring depends on the mass of the lens and the distances involved.
Gravitational lensing has become one of astronomy's most powerful tools. Strong lensing, where arcs and rings are visible, lets astronomers directly measure the mass of galaxy clusters including the dark matter that makes up most of their mass. Weak lensing, a statistical distortion across millions of background galaxies, maps the large-scale structure of dark matter across the universe.
Microlensing — where a single star lenses a background star, briefly brightening it — is used to hunt for exoplanets and rogue planets drifting through the galaxy with no host star.
The deep field images you see here are real Hubble and JWST observations. Almost every smear, arc, and elongated blob in a deep field is a galaxy — and many of those distorted shapes are caused by real gravitational lensing from foreground clusters millions of light years away.
