Gravitational lensing occurs when a massive object—such as a galaxy, galaxy cluster, or black hole—bends and magnifies the light from a more distant background source. This bending happens because massive objects warp the fabric of spacetime, altering the path that light takes as it travels.
There are three main types of gravitational lensing. Strong lensing creates highly visible effects, such as arcs, multiple images, or complete Einstein rings. Weak lensing causes subtle distortions in the shapes of many background galaxies and is a valuable tool for mapping the distribution of dark matter. Microlensing results in brief changes in a star’s brightness when a smaller object, like a planet or brown dwarf, passes in front of it.
Strong gravitational lenses act as natural telescopes, allowing astronomers to observe extremely distant and faint galaxies by magnifying them. Some of the farthest known galaxies have been discovered this way.
Weak lensing surveys, such as those from the Dark Energy Survey and ESA’s Euclid mission, help trace large-scale structures and reveal how dark matter is distributed across the universe. This helps cosmologists understand the evolution of cosmic structures and the influence of dark energy.
Microlensing has been used to detect exoplanets and free-floating planets, especially in regions like the Galactic Bulge. These observations don’t require the planet to emit light—only that it briefly magnify a background star.
Gravitational lensing is not only a tool for discovery but also a powerful method to test general relativity on cosmic scales. It provides direct evidence of dark matter and helps refine measurements of the universe’s expansion rate.