An Introduction to Gravitational Lensing

smiley face in the stars
The light from distant objects passed through the gravitational field of closer galaxies to create a gravitational lens that looks like a "smiley face" to the Hubble Space Telescope. NASA/STScl

Most people are familiar with the tools of astronomy: telescopes, specialized instruments, and databases. Astronomers use those, plus some special techniques to observe distant objects. One of those techniques is called "gravitational lensing."

This method relies simply on the peculiar behavior of light as it passes near massive objects. The gravity of those regions, usually containing giant galaxies or galaxy clusters, magnifies light from very distant stars, galaxies, and quasars. Observations using gravitational lensing help astronomers explore objects that existed in the very earliest epochs of the universe. They also reveal the existence of planets around distant stars. In an uncanny way, they also unveil the distribution of dark matter that permeates the universe.

graphical view of gravitational lensing.
Gravitational lensing and how it works. Light from a distant object passes by a closer object with a strong gravitational pull. The light is bent and distorted and that creates "images" of the more distant object.  NASA

The Mechanics of a Gravitational Lens

The concept behind gravitational lensing is simple: everything in the universe has mass and that mass has a gravitational pull. If an object is massive enough, its strong gravitational pull will bend light as it passes by. A gravitational field of a very massive object, such as a planet, star, or galaxy, or galaxy cluster, or even a black hole, pulls more strongly at objects in nearby space. For example, when light rays from a more distant object pass by, they are caught up in the gravitational field, bent, and refocused. The refocused "image" is usually a distorted view of the more distant objects. In some extreme cases, entire background galaxies (for example) may end up distorted into long, skinny, banana-like shapes via the action of the gravitational lens.

The Prediction of Lensing

The idea of gravitational lensing was first suggested in Einstein's Theory of General Relativity. Around 1912, Einstein himself derived the math for how light is deflected as it passes through the Sun's gravitational field. His idea was subsequently tested during a total eclipse of the Sun in May 1919 by astronomers Arthur Eddington, Frank Dyson, and a team of observers stationed in cities across South America and Brazil. Their observations proved that gravitational lensing existed. While gravitational lensing has existed throughout history, it's fairly safe to say that it was first discovered in the early 1900s. Today, it is used to study many phenomena and objects in the distant universe. Stars and planets can cause gravitational lensing effects, although those are hard to detect. The gravitational fields of galaxies and galaxy clusters can produce more noticeable lensing effects. And, it now turns out that dark matter (which has a gravitational effect) also causes lensing.

Types of Gravitational Lensing

graphical view of gravitational lensing.
Gravitational lensing and how it works. Light from a distant object passes by a closer object with a strong gravitational pull. The light is bent and distorted and that creates "images" of the more distant object. NASA

Now that astronomers can observe lensing across the universe, they've divided such phenomena into two types: strong lensing and weak lensing. Strong lensing is fairly easy to understand — if it can be seen with the human eye in an image (say, from Hubble Space Telescope), then it's strong. Weak lensing, on the other hand, is not detectable with the naked eye. Astronomers have to use special techniques to observe and analyze the process.

Due to the existence of dark matter, all distant galaxies are a tiny bit weak-lensed. Weak lensing is used to detect the amount of dark matter in a given direction in space. It's an incredibly useful tool for astronomers, helping them understand the distribution of dark matter in the cosmos. Strong lensing also allows them to see distant galaxies as they were in the distant past, which gives them a good idea of what conditions were like billions of years ago. It also magnifies the light from very distant objects, such as the earliest galaxies, and often gives astronomers an idea of the galaxies' activity back in their youth.

Another type of lensing called "microlensing" is usually caused by a star passing in front of another one, or against a more distant object. The shape of the object may not be distorted, as it is with stronger lensing, but the intensity of the light wavers. That tells astronomers that microlensing was likely involved. Interestingly, planets can also be involved in microlensing as they pass between us and their stars.

Gravitational lensing occurs to all wavelengths of light, from radio and infrared to visible and ultraviolet, which makes sense, since they're all part of the spectrum of electromagnetic radiation that bathes the universe.

The First Gravitational Lens

gravitational lensing
The pair of bright objects in the center of this image were once thought to be twin quasars. They are actually two images of a very distant quasar being gravitationally lensed. NASA/STScI

The first gravitational lens (other than the 1919 eclipse lensing experiment) was discovered in 1979 when astronomers looked at something dubbed the "Twin QSO".QSO is shorthand for "quasi-stellar object" or quasar. Originally, these astronomers thought this object might be a pair of quasar twins. After careful observations using the Kitt Peak National Observatory in Arizona, astronomers were able to figure out that there weren't two identical quasars (distant very active galaxies) near each other in space. Instead, they were actually two images of a more distant quasar that were produced as the quasar's light passed near a very massive gravity along the light's path of travel. That observation was made in optical light (visible light) and was later confirmed with radio observations using the Very Large Array in New Mexico.

Einstein Rings

gravitational lensing
A partial Einstein Ring known as the Horseshoe. It shows the light from a distant galaxy being warped by the gravitational pull of a closer galaxy. NASA/STScI

Since that time, many gravitationally lensed objects have been discovered. The most famous are Einstein rings, which are lensed objects whose light makes a "ring" around the lensing object. On the chance occasion when the distant source, the lensing object, and telescopes on Earth all line up, astronomers are able to see a ring of light. These are called "Einstein rings," named, of course, for the scientist whose work predicted the phenomenon of gravitational lensing.

Einstein's Famous Cross

gravitational lensing
The Einstein Cross is actually four images of a single quasar (the image in the center is not visible to the unaided eye). This image was taken with the Hubble Space Telescope's Faint Object Camera. The object doing the lensing is called "Huchra's Lens" after the late astronomer John Huchra. NASA/STScI

Another famous lensed object is a quasar called Q2237+030, or the Einstein Cross. When the light of a quasar some 8 billion light-years from Earth passed through an oblong-shaped galaxy, it created this odd shape. Four images of the quasar appeared (a fifth image in the center is not visible to the unaided eye), creating a diamond or cross-like shape. The lensing galaxy is much closer to Earth than the quasar, at a distance of about 400 million light-years. This object has been observed several times by the Hubble Space Telescope.

Strong Lensing of Distant Objects in the Cosmos

gravitational lensing
This is Abell 370, and shows a collection of more distant objects being lensed by the combined gravitational pull of a foreground cluster of galaxies. The distant lensed galaxies are seen distorted, while the cluster galaxies appear fairly normal. NASA/STScI

On a cosmic distance scale, Hubble Space Telescope regularly captures other images of gravitational lensing. In many of its views, distant galaxies are smeared into arcs. Astronomers use those shapes to determine the distribution of mass in the galaxy clusters doing the lensing or to figure out their distribution of dark matter. While those galaxies are generally too faint to be easily seen, gravitational lensing makes them visible, transmitting information across billions of light-years for astronomers to study.

Astronomers continue to study the effects of lensing, particularly when black holes are involved. Their intense gravity also lenses light, as shown in this simulation using an HST image of the sky to demonstrate.

computer simulation of a supermassive black hole
This computer-simulated image shows a supermassive black hole at the core of a galaxy. The black region in the center represents the black hole's event horizon, where no light can escape the massive object's gravitational grip. The black hole's powerful gravity distorts space around it like a funhouse mirror, in a process known as gravitational lensing. Light from background stars is stretched and smeared as the stars skim by the black hole. NASA, ESA, and D. Coe, J. Anderson, and R. van der Marel (Space Telescope Science Institute), Science Credit: NASA, ESA, C.-P. Ma (University of California, Berkeley), and J. Thomas (Max Planck Institute for Extraterrestrial Physics, Garching, Germany).
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Petersen, Carolyn Collins. "An Introduction to Gravitational Lensing." ThoughtCo, Aug. 1, 2021, Petersen, Carolyn Collins. (2021, August 1). An Introduction to Gravitational Lensing. Retrieved from Petersen, Carolyn Collins. "An Introduction to Gravitational Lensing." ThoughtCo. (accessed June 7, 2023).