The largest lenses in the world are not built by humans. The most powerful telescopes are provided to us for free by the universe when we manage to observe a nearby galaxy aligned with a more distant one, causing one of nature's most spectacular phenomena: a gravitational lens.
On July 1, 2023, the European Euclid space telescope took off from Earth to begin its six-year mission to explore the dark Universe. Before the spacecraft could start its journey, a team of scientists and engineers verified that everything was working correctly. During this testing phase in September 2023, Euclid deliberately sent some blurry images to Earth, and in one of those blurry images, scientist Bruno Altieri saw a hint of a strange phenomenon that he wanted to observe more closely. "From that first observation, I could see it, it was a perfect Einstein ring, and for me, who has always been interested in gravitational lenses, it was amazing," says Altieri.
The ring was hidden in plain sight in a not-so-distant galaxy called NGC 6505, about 590 million light-years from Earth, which is a cosmic stone's throw away. It was the first time that the light ring surrounding its center had been detected thanks to Euclid's high-resolution instruments.
The ring surrounding the foreground galaxy is formed by the light from a brighter galaxy that is even further away and still unnamed, as it had not been observed before. This background galaxy is 4.420 billion light-years away and its light appears distorted due to gravity on its way to us.
"The Einstein ring is an example of strong gravitational lensing," explains Conor O'Riordan from the Max Planck Institute for Astrophysics (Germany) and lead author of the first scientific article analyzing the ring. "All strong lenses are special because they are very rare and incredibly useful from a scientific point of view. This one is particularly special because it is very close to Earth, and its alignment makes it very beautiful."
Albert Einstein's general theory of relativity predicts that light will bend around objects in space, so they will focus it like a giant lens. This gravitational lensing effect is greater with more massive objects, such as galaxies and galaxy clusters, allowing us to see the light from distant galaxies that would otherwise be hidden.
If the alignment is correct, the light from the distant galaxy curves to form a spectacular ring around the foreground object. These Einstein rings are a rich laboratory for scientists, as their gravitational effects can help us learn more about the expansion of the Universe.
It is a phenomenon similar to when we observe light through the bottom of a glass of liquid and see the distorted image of the light source. The nearest galaxy acts as a colossal lens that redirects the rays of light emitted from behind to create a distorted image.
"Astronomers have known the foreground galaxy for a long time, as it was discovered in 1884, and yet, this ring had never been observed before. This demonstrates how powerful Euclid is, as it discovers new things in places we thought we knew well, showcasing its fantastic capabilities," says Valeria Pettorino, a scientist from the ESA's Euclid project.
The phenomenon was predicted by Albert Einstein almost a century ago, which is why the circular shapes that form are called Einstein rings. Einstein himself, explains Rafael Bachiller, director of the National Astronomical Observatory, considered it a very subtle phenomenon and doubted that a day would come when it could be observed, so he considered it more of a curiosity than a working tool.
Experts believe that by exploring how the Universe has expanded and formed throughout its cosmic history, Euclid will reveal more about the role of gravity and the nature of dark energy and dark matter. The space telescope will map more than a third of the sky, observing billions of galaxies up to 10,000 million light-years away. It is expected to find around 100,000 powerful gravitational lenses, but they hardly believe it will find one as spectacular and as close to home. So far, less than 1,000 lenses were known, and high-resolution images of even fewer had been obtained.
Although this Einstein ring is impressive, Euclid's main task is to search for the most subtle effects of weak gravitational lensing, where background galaxies appear only slightly stretched or displaced. To detect this effect, scientists will need to analyze billions of galaxies. Euclid began its detailed study of the sky on February 14, 2024, and is gradually creating the most extensive 3D map of the Universe to date. Such an astonishing and early finding in its mission means for researchers that Euclid is on track to discover many more hidden secrets.