Put a Ring on It: How Gravity Gives Astronomers a Powerful Lens on the Universe

In 1919, astronomers Arthur Eddington and Andrew Crumlin took photographs of a total solar eclipse. The Sun was in the constellation Taurus at the time, and a handful of its stars can be seen in the images. But the stars weren’t quite where they expected. The massive gravity of the Sun deflected the light of these stars, causing them to appear slightly out of place. This was the first evidence that gravity can alter the path of light, just as Albert Einstein had predicted in 1915.

The bending of light by the mass of a star or galaxy is one of the central predictions of general relativity. Although Einstein first predicted the diffraction of light from a single star, others such as Oliver Lodge argued that a large mass could act as a gravitational lens, distorting the path of light similar to the way a glass lens focuses light. By 1935, Einstein demonstrated how light from a distant galaxy can be distorted by a galaxy in front of it to form a ring of light. This Einstein ring, as it became known, would make a distant galaxy appear as a ring or arc of light around the nearest galaxy. But Einstein thought this effect would never be noticed. These arcs of light would be too faint for optical telescopes to pick up. Einstein was right until 1998 when the Hubble Space Telescope captured a ring around the galaxy B1938+666. This was the first optical ring observed, but it wasn’t Einstein’s first ring. The first episode was seen on radio, and was captured by the Very Large Array (VLA).

First picture of Einstein’s ring. It was taken over by the VLA in 1987. Credit: NRAO/AUI/NSF

In 1987, a team of students at the MIT Research Laboratory in Electronics under Professor Bernard Burke, led by doctoral student Jackie Hewitt, used the VLA to make detailed images of known objects emitting radio. One of them, known as MG1131 + 0456, showed a characteristic oval shape with two bright lobes. Hewitt and her team looked at several models to explain the unusual shape, but only Einstein’s ring matched the data. Einstein’s prediction of the galaxy was finally noted.

Radio astronomy is especially good at capturing lensed galaxies. They have become a powerful tool for radio astronomers. Just as a glass lens focuses light to make an object appear brighter and larger, so does a gravitational lens. By observing lenticular galaxies, radio astronomers can study galaxies that would be too far and too faint to be seen on their own. Einstein’s rings can be used to measure the mass of the nearest galaxy or galaxy cluster because the amount of gravitational lensing depends on the mass of the foreground galaxy.

One of the most interesting aspects of the gravitational lens is that it can be used to measure the expansion rate of the universe. Light from a distant galaxy can take many different paths as it passes through the foreground galaxy. Each of these paths can have different distances, which means that light can reach us at different times. We may see a burst of light from the galaxy multiple times, each one from a different path. Astronomers can use this to calculate the distance of the galaxy, and thus the scale of the universe.

SDP.81
ALMA’s highest resolution image reveals glowing dust within the distant galaxy SDP.81. The ring structure was created by a gravitational lens that distorted the view of the distant galaxy into a ring-like structure. Credit: ALMA (NRAO/ESO/NAOJ)

Since the first detection of Einstein’s ring by the VLA, radio astronomers have found more of it, and captured it in more detail. In 2015, for example, the Atacama Large Millimeter/submillimeter Array (ALMA) group made a detailed image of lens arcs from a distant galaxy called SDP.81. The image was sharp enough that astronomers were able to trace the arcs back to their source to study how stars formed within the galaxy.

Einstein’s rings are now commonly seen in astronomical images, particularly in deep field images, such as those of the James Webb Space Telescope and others. As radio astronomy has shown, they are more than just beauty. They give us a new lens on the universe.