Astrophile is Joshua Sokol 's monthly column on curious cosmic objects, from the solar system to the far reaches of the multiverse

Lord of the rings ESA/Hubble & NASA

Picture a glimmering arc of light, the artefact of a lens made of warped space-time instead of glass. Einstein thought there was “no hope” of observing these bizarre cosmic illusions, predicted by his theory of general relativity – but we’ve been collecting so-called “Einstein rings” since 1987.

An Einstein ring is not a place to visit but a trick of perspective. Two distant galaxies have to line up just so, or we won’t see it. At its most perfect, it’s a big, unbroken circle: beautiful and scientifically valuable in equal measure.


A complete Einstein ring is one of the universe’s most accurate bathroom scales – the circle it draws around a galaxy lets us add up the mass of everything inside. This means they can help solve mysteries as diverse as dark matter and the ancient universe.

We know of only a few dozen arcs that approach a full circle, though. So it seems fair to ask: of the Einstein rings we’ve found hidden in the sky so far, is there one that rules them all?

A ring is born

Here’s the ideal scenario. We need a light source: a distant galaxy that is bright all over, not just at its core or in a knot of newly formed stars. We need a lens: another galaxy, a heavy one, which can be far away from the source but must sit directly in front of it along our line of sight.

As general relativity predicts, space sags around the mass of the lens galaxy, making light from the source bend on its way to us. As a result, the lens appears surrounded by multiple images of the background light source.

It’s common to see four distinct images of the source, as if at compass points around the lens, which is called an Einstein cross. In 2015, researchers using the Hubble Space Telescope even saw the same supernova four separate times through this configuration. Partial arcs of light around galaxy clusters are also somewhat common.

But for a true ring, we need the images to blend into each other, completely encircling the lens galaxy.

The more complete the ring, the more accurately astronomers can weigh all the mass enclosed inside the circle. That helps constrain theories of galaxy formation and dark matter. And by definition, an Einstein ring magnifies a faraway background galaxy, so it also helps us study the ancient universe.

“It satisfies that amateur astronomer drive to discover cool-looking things,” says Adam Bolton at the National Optical Astronomy Observatory in Tucson, Arizona. “But they’re cool-looking things that also have this unique and powerful quantitative utility.”

The contenders

Many of the most picturesque contenders have been found by mining the Sloan Digital Sky Survey, which was Bolton’s strategy to identify eight new ones – some verging on complete – in 2005.

There have also been lucky finds this year. In June, Margherita Bettinelli at the Instituto de Astrofísica de Canarias in Spain spotted a ring spanning 300 degrees during an unrelated project.

But in my opinion, the fairest of them all is the Cosmic Horseshoe found in the Sloan survey in 2007: light from an ancient blue galaxy draped 300 degrees around a red galaxy 10 times heavier than the Milky Way.

Other rings are more complete, but the Cosmic Horseshoe benefits from its large size – plus the fact that it has been imaged by the Hubble Space Telescope’s most powerful camera.

That will do until future telescopes reveal even more beautiful rings further out in deep space, where the shadows lie.