Astronomers at the Atacama Large Millimeter Array (ALMA), located in Chile’s Atacama Desert, have reported a rare form of gravitational lensing, known as the Einstein Ring – one that Albert Einstein predicted over 100 years ago in his theory of relativity.
What is the Einstein Ring?
Popularized by past observations made from the Hubble Space Telescope, gravitational lensing is fairly common. It takes place when a massive galaxy—or a cluster of galaxies—bends the light emitted from a smaller, less visible galaxy behind it. When this happens, powerful telescopes are given a boost in their capabilities, thus giving astronomers a unique opportunity to gaze upon very distant universes.
But what makes this most recent gravitational lensing so special is that it is the finest example of an Einstein Ring—an unusual ring-like structure predicted by Einstein in his theory of relativity.
In his renowned theory, Einstein detailed how spacetime bends and curves around a cluster of galaxies and how the light from a background source can become magnified when it passes around the bowed and sagging spacetime. He also predicted that a ring of light would form when the light from two galaxies line up perfectly—unfortunately, it was nearly 24 years after Einstein’s death when gravitational lensing was first empirically observed.
The ancient galaxy seen in ALMA’s most recent observation is known as SDP .81, and it is 12 billion light years away from Earth. SDP .81 is an active star-forming galaxy and is viewed at a time when the universe was only 15 percent of its current age.
The image of the Einstein ring captured is of the highest quality of its kind and were taken as part of ALMA’s Long Baseline Campaign, a plan to test the upper limits of the telescope’s resolution, which requires ALMA’s antennas to be placed as far apart as they can go—up to 9 miles (15 kilometers) apart.
To achieve these observations, ALMA accomplished a mind-boggling maximum resolution of 23 milliarcseconds, which according to the National Radio Astronomy Observatory, is like being on the observing deck of the Empire State Building and looking at a basketball hoop atop the Eiffel Tower.