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What is gravitational lensing?

What is gravitational lensing? Two center lights inside a ring of light with four dots on ring.
NASA released the Hubble Space Telescope image above on August 27, 2021. The 2 bright lights inside the ring are galaxies. The gravity of the 2 galaxies acts as a gravitational lens in space. That is, their combined extreme mass has caused space to curve. The ring and 4 points aren’t physically connected to the 2 galaxies. They’re light originating from a yet-more-distant quasar. The quasar’s light has been bent while traveling on the curved space – the gravitational lens – around the galaxy pair. Image via ESA/ Hubble/ NASA.

What is gravitational lensing?

In November 1915, Albert Einstein published his theory of general relativity. A key point in his theory is that massive objects distort the fabric of space-time, the way a bowling ball on a trampoline would stretch and warp the fabric around it. In order to prove Einstein’s theory right, scientists traveled the globe to be under the solar eclipse of 1919. There they witnessed the sun bending the light of background stars by the amount Einstein predicted. Nowadays, scientists use the same concept – gravitational lensing – to learn more about galaxies and quasars in the early universe.

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Gravitational lensing occurs when massive foreground objects, such as the two galaxies in the above image, bend and warp the fabric of space itself. The more distant light of the quasar – a young active galaxy – traveling toward us reaches this warped space, which then acts as a lens, bending and magnifying the light. This is why we see a ring with the four different points of light from one bright, very distant light source.

Diagram with paths of light from point on left around galaxy in middle to Earth on right.
This diagram shows how light from a distant quasar bends around the warped fabric of space in the vicinity of a massive galaxy. This results in multiple images of the quasar from Earth’s point of view. Image via R. Hurt (IPAC/ Caltech)/ The GraL Collaboration/ ESA.

Gravitational lenses and dark matter

The concept of gravitational lensing lets astronomers learn more about the amount of mass and dark matter that is present in the foreground galaxies. Dark matter is, as the name alludes to, dark. The only way for astronomers to study it is to see how its gravity affects visible objects. Astronomers don’t know what dark matter is yet. It is quite a conundrum, because they estimate that dark matter makes up as much as 85% of the total mass in the universe. So gravitational lensing provides more information about foreground objects and gives astronomers a tool to see where the dark matter must lie, based on its effects on the background galaxies.

Left side black and gold galaxies, right side blue overlain in smoky rings.
View larger. | On the left is galaxy cluster Cl 0024+17 in visible light. In the artist’s concept on the right, blue shading indicates the location of dark matter that scientists mathematically determined must exist to account for the nature and placement of the gravitationally lensed galaxies. Image via NASA/ ESA/ M.J. Jee and H. Ford (Johns Hopkins University).

Scientists searching for more gravitational lenses to study have enlisted the help of artificial intelligence. In 2017, astronomers used an algorithm and artificial intelligence to detect 56 gravitational lens candidates. As of February 2021, AI has now found more than 1,200 gravitational lens candidates.

Gravitational lenses can create phenomena in a wide variety of shapes and sizes, but two of the most well-known are the Einstein cross and Einstein ring.

Einstein crosses

While there are many Einstein crosses, there is only one with the proper name of the Einstein Cross, discovered in 1985. It provides an excellent example of a quasar that we see through a gravitational lens. The quasar sits directly behind a galaxy and we see it as four points of light surrounding the nearer galaxy.

Center point of light with four lights to the top, bottom, right and left.
The Einstein Cross is a gravitationally lensed object called G2237+0305, a quasar located 8 billion light-years away. Image via HubbleSite.

As of April 2021, the Gaia survey has now found a dozen Einstein crosses.

Twelve images: each with central bright dot with dots around it on dark background.
The Gaia Survey discovered these 12 Einstein crosses. Image via The GraL Collaboration/ ESA.

Einstein rings

When a galaxy or a quasar is off center from the foreground object, the lensing creates a ring or arcs instead of points. Einstein rings appear around many distant galaxies.

A collection of light rings seen around small, bright galaxies against dark backgrounds.
These giant elliptical galaxies all have rings around them. The rings are more distant galaxies lensed by the foreground galaxy. Image via NASA/ ESA/ and the SLACS Survey team.

More beautiful examples of gravitational lensing

Each gravitationally lensed object has a beauty and mystique all its own.

Blue ring around bright, reddish central object.
This luminous red galaxy has a strong gravitational field that has warped light from a much more distant blue galaxy into a ring shape. Scientists discovered this object, LRG 3-757, in data observed in 2007, from the Sloan Digital Sky Survey. In 2011, the Hubble Space Telescope’s Wide Field Camera 3 took the follow-up observation we see here. Image via ESA/ Hubble & NASA/ APOD.
Purple glow resembling two bright eyes, small nose, curved mouth and side of head.
In 2015, astronomers combined optical and X-ray data for this image of the Cheshire Cat group of galaxies. The two bright eyes are large galaxies racing toward each other at more than 300,000 miles (483,000 km) per hour. The curved arcs around the galaxy that make up the cat’s face and grin are 4 distant background galaxies that we see through gravitational lensing. Image via NASA/ CXC/ UA/ STScI.
Black background dotted with spots of light, stars and galaxies, with large arcs of light.
View larger. | This image contains 12 images of a background galaxy. The Sunburst Arc galaxy that appears in these arcs is 11 billion light-years away. A gravitational lens made from the foreground cluster of galaxies 4.6 billion light-years away enables us to see the more distant galaxy. Image via ESA/ NASA/ E. Rivera-Thorsen et al.
Black background with very many small golden galaxies and numerous arcs.
Scientists named this region crowded with galaxies Abell 370. This galaxy cluster was one of the very first astronomers used to observe gravitational lensing. The many arcs and streaks in the picture are stretched images of background galaxies. Image via NASA/ ESA/ Hubble.

Bottom line: Gravitational lensing occurs when a massive foreground object warps the space around it, distorting the image of a background object into rings, arcs or multiple points of light. Gravitational lenses help reveal dark matter in galaxies.

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