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.
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.
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.
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.
While there are many Einstein crosses, there is only one with the proper name of theEinstein 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.
As of April 2021, the Gaia survey has now found a dozen Einstein crosses.
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.
More beautiful examples of gravitational lensing
Each gravitationally lensed object has a beauty and mystique all its own.
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.
Kelly Kizer Whitt has been a science writer specializing in astronomy for more than two decades. She began her career at Astronomy Magazine, and she has made regular contributions to AstronomyToday and the Sierra Club, among other outlets. Her children’s picture book, Solar System Forecast, was published in 2012. She has also written a young adult dystopian novel titled A Different Sky. When she is not reading or writing about astronomy and staring up at the stars, she enjoys traveling to the national parks, creating crossword puzzles, running, tennis, and paddleboarding. Kelly lives with her family in Wisconsin.
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