Science › Space, time & gravity
Gravitational lensing
1979· Discovery / experiment
ProvenTested many times; part of working physics.
Massive objects bend light from things behind them, creating magnified, distorted or multiple images, like a lens made of gravity.
See it on the Science map →How it works
A galaxy or galaxy cluster curves spacetime around it. Light from a more distant galaxy passing nearby bends, so we may see it as arcs, a ring, or several copies. Strong lensing makes obvious arcs; weak lensing slightly stretches the shapes of millions of galaxies; microlensing briefly brightens a star when another passes in front. The amount of bending reveals the total mass of the lens, visible or not.
Why it matters
It weighs things we cannot see, maps dark matter, magnifies the most distant galaxies, and has found planets around other stars.
Think of it like
Like the wavy glass at the base of a wine glass distorting what is behind it; the shape of the distortion tells you the shape of the glass.
How we know
The first lensed quasar, a "twin quasar," was found in 1979. Einstein rings, Hubble and Webb arc images, and microlensing planet discoveries have followed by the hundreds.
Still a mystery
Some lensing measurements of how clumpy matter is disagree slightly with other cosmology data, a tension not yet resolved.
Connections (4)
▸General relativity: gravity is curved spacetime predicts Gravitational lensingProven
The relationship
Any mass curves the spacetime around it, and light follows that curve. A galaxy or cluster sitting between us and a more distant source acts like a giant, imperfect lens, bending the background light into arcs, rings or several copies, and often magnifying it. How strongly the image is bent tells us how much mass the lens holds.
How the link was found
Einstein worked out lensing by single stars in 1936 but thought it unobservable. Fritz Zwicky argued in 1937 that whole galaxies would make visible lenses.
Why it's proven
Walsh, Carswell and Weymann found the first lensed quasar in 1979. Thousands of lensed systems are now known, and the bending matches general relativity.
Open questions and mysteries
Lensing surveys of how clumpy matter is come out slightly lower than predictions from the early universe; whether that is a measurement issue or new physics is open.
▸Gravitational lensing maps the hidden mass of Dark matterProven
The relationship
Lensing measures all the mass in a galaxy cluster, visible or not, by how much it bends light from galaxies behind it. Comparing that with the visible stars and gas shows roughly five times more mass than we can see. Weak-lensing surveys turn the subtle distortions of millions of galaxies into maps of where the hidden mass sits.
How the link was found
Cluster-lensing studies grew through the 1990s. Douglas Clowe's team mapped the Bullet Cluster in 2006, showing mass separated from the visible gas.
Why it's proven
In the Bullet Cluster, lensing mass sits where the galaxies are, not where most normal matter (hot gas) is, which strongly favors unseen dark matter.
Open questions and mysteries
Lensing shows where dark matter is but not what it is. No dark-matter particle has been detected in any laboratory.
▸Gravitational lensing mapped the hidden mass in The Bullet Cluster: dark matter caught in the actProven
The relationship
Light from thousands of faint galaxies behind the Bullet Cluster is slightly stretched by its gravity, so their shapes line up in arcs around each concentration of mass. Averaging those shapes produced a map of where the mass is, regardless of whether it shines. A few strongly bent arcs pinned down the cores. Laying that map over the X-ray picture of the gas showed the mismatch.
How the link was found
Douglas Clowe, Anthony Gonzalez and Maxim Markevitch (2004), refined in 2006 with Hubble, Magellan and European Southern Observatory images; Maruša Bradač's team (2006) added the strongly lensed arcs.
Why it's proven
Lensing follows directly from general relativity and is routinely checked against clusters' X-ray and galaxy-motion masses. The independent weak and strong lensing maps agree on where the Bullet Cluster's mass sits.
Open questions and mysteries
Lensing adds up all mass along the line of sight, so unrelated foreground or background matter can blur the map slightly. How the dark matter is distributed in fine detail inside the cluster is still unclear.
▸Fritz Zwicky predicted galaxies would produce observable Gravitational lensingProven
The relationship
In 1936 Einstein calculated that one star could lens another but judged it essentially unobservable. In 1937 Zwicky argued that a whole galaxy, billions of times heavier, would bend light enough to be seen. He listed three payoffs: testing relativity, magnifying galaxies too distant to see, and weighing galaxies to check his dark-matter estimates. All three are now routine.
How the link was found
Fritz Zwicky, in two short 1937 papers in Physical Review, written months after Einstein's 1936 note in Science, which an amateur, Rudi Mandl, had urged Einstein to publish.
Why it's proven
Dennis Walsh, Robert Carswell and Ray Weymann found the first galaxy-lensed quasar in 1979. Giant arcs in galaxy clusters followed in 1986–87, and lensing now weighs clusters routinely.
Open questions and mysteries
Lensing now hunts dark-matter clumps too small to hold any stars. How small such clumps go would reveal what dark matter is made of.