In the middle of this image, the fuzzy-looking bulb of light in a warm shade of yellow extends around a small bright spot, nestled within a thin light circle that appears to be drawn closely around it. The circle is an Einstein ring.
| Photo Credit: ESA/Euclid/Euclid Consortium/NASA
More than a century ago, Albert Einstein predicted that massive objects like large galaxies and clusters of galaxies act like giant lenses in space by bending light from distant objects.
As seen from an observer on the earth, a rare alignment of a background object with such a lens in the foreground can lead to a visual spectacle. Because of the lensing, the observer sees arc-like structures skirt the foreground lens. Sometimes these arcs are arranged in a circular pattern, which is called an Einstein ring.
Altieri’s ring
Recently, the Euclid space mission of the European Space Agency (ESA) spotted an Einstein ring in the galaxy NGC 6505, just 590 million lightyears from the earth. This may sound like a long distance, but on the astronomical scale the galaxy is veritably in our cosmic backyard.
An astronomer named Bruno Altieri first noticed this Einstein ring in September 2023 in a blurry image captured by Euclid, which ESA had launched only two months earlier.
The image was unfocused by design because in the initial days of the mission, scientists were taking data to test if all of Euclid’s systems were functioning properly. Subsequent images of the galaxy yielded focused images, using which scientists confirmed the presence of the ring. It has since been nicknamed Altieri’s ring in honour of the scientist who stumbled upon it.
A quirk of the light
Einstein predicted that light will not travel on a straight path when moving in the vicinity of massive objects. He argued that a large object distorts spacetime — the fabric of space and time around it — just like the curvature of a hammock is determined by the mass of the person sitting in it.
This idea forms the basis of Einstein’s famous general theory of relativity, which the American physicist John Wheeler summed up perfectly in the following words: “Matter tells spacetime how to curve, and curved spacetime tells matter how to move”.
The massive object in the foreground, called a gravitational lens, distorts and amplifies the light coming from background sources in the same way a magnifying glass distorts the path of light scattered by a background object, like small lettering on a piece of paper.
This said, a gravitational lens is not as perfectly shaped as a magnifying glass and may produce multiple images of the background object. The number of images depends on the relative distance between the lens and the observer, between the lens and the background object, and the latter’s alignment with the lens.
This quirky cosmic phenomenon is called strong gravitational lensing. The multiple images can appear in a variety of configurations around the lens, and can assume slightly different shapes and sizes depending on the distribution of matter in it.
An Einstein ring is a special case of strong gravitational lensing. Astronomers discovered the first Einstein ring in 1998, more than 80 years after Einstein predicted their existence. An Einstein ring is created when a gravitational lens distorts light coming from a distant background object, like a star or a galaxy, in such a way that the multiple images created in the foreground form a circular pattern around the lens. This requires a near-perfect alignment between the distant object, the lens, and the observer.
A new set of eyes
All Einstein rings have great scientific value but Altieri’s ring is extra-special because scientists have observed it in a well studied nearby galaxy, NGC 6505. Scientists have found only five other gravitational lenses at similar distances so far. Altieri’s ring is composed of the distorted images of another galaxy 4.5 billion lightyears away.
Since NGC 6505 has been known to astronomers since the 19th century, the ring’s discovery shows how turning new telescopes to old targets can still yield valuable new knowledge.
The study of Einstein rings can also provide new insights into the universe’s expansion, and provide opportunities to test the theory of general relativity and investigate distant objects that are otherwise obscured.
They can also help astronomers understand the nature of dark matter, a mysterious form of matter that comprises around 30% of the total mass-energy budget of the universe yet remains undetected because it doesn’t interact with the normal matter of which you and I are made. The presence of dark matter can only be inferred from the gravitational effect it has on matter surrounding it — or by bending light around itself.
One of a kind, probably
Following the discovery of Altieri’s ring in September 2023, Euclid scientists further investigated this system for more insights using other telescopes. This way, for example, data from the Keck Cosmic Web Imager (KCWI) obtained in March 2024 confirmed the lensed nature of the images.
Together with data from the archives of the Canada-France-Hawaii Telescope and the Dark Energy Spectroscopic Instrument, scientists also confirmed the total mass of stars and the distance to NGC 6505 and the lensed galaxy. They found that the latter is an old galaxy no longer forming stars.
While the discovery of Altieri’s ring so early in Euclid’s life is exciting for the mission, its scientists sounded caution in a paper published in Astronomy & Astrophysics on February 10: “… the exceptional nature of Altieri’s lens means it is unlikely that Euclid will find another lens” closer than around 680 million light-years “with a ring as bright as that observed here”.
Euclid began to scan the sky formally on February 14, 2024, and is expected to discover 100,000 new gravitational lenses in the universe. Its chances of discovering lenses so close to the earth, however, remain slim because of the smaller volume of universe available to look in.
This said, the discovery of Altieri’s ring highlights Euclid’s potential and the role it can play in advancing our understanding of dark matter.
Smriti Mahajan is an astronomer and science communicator promoting STEM education through astronomy.
Published – February 18, 2025 04:10 pm IST
