- Archis Anupam Mohapatra
- Apr 3
- 6 min read
Imagine a mystery novel where a brilliant detective is hunting an elusive criminal—one who leaves behind no fingerprints, no footprints, and no tangible evidence of their presence. Yet, the detective knows the culprit is there, influencing the world in unseen ways. In the vast expanse of the universe, cosmologists are those detectives, and their greatest unsolved mystery is dark matter—a mysterious substance that makes up a significant portion of the cosmos but remains invisible to all our instruments.
To crack this cosmic case, scientists launched a revolutionary space telescope: Euclid.
The Euclid Mission: A New Era in Cosmology
Euclid was launched in July 2023 aboard a SpaceX Falcon 9 rocket. It now orbits at the Lagrange Point L2, the same location as the James Webb Space Telescope (JWST). From this vantage point, Euclid has begun its ambitious mission: a six-year-long survey to map one-third of the sky.[1]
On 19th March , Euclid gave the first glimpse of 63 square degrees of the sky, the equivalent area of more than 300 times the full Moon, already gives an impressive preview of the scale of its grand cosmic atlas when the mission is complete. This atlas will cover one-third of the entire sky – 14 000 square degrees – in this high-quality detail.[2]
“Euclid is very quickly covering larger and larger areas of the sky thanks to its unprecedented surveying capabilities,” says Pierre Ferruit, ESA’s Euclid mission manager.
But Euclid’s true purpose extends beyond stunning imagery—it aims to unravel some of the deepest mysteries in physics.Euclid measures the huge variety of shapes and the distribution of billions of galaxies very precisely with its high-resolution imaging visible instrument (VIS), while its near-infrared instrument (NISP) is essential for unravelling galaxy distances and masses. The new images already showcase this capability for hundreds of thousands of galaxies, and start to hint at the large-scale organisation of these galaxies in the cosmic web.[3]
The Dark Matter Puzzle: Vera Rubin’s Discovery
To understand Euclid’s mission, we need to rewind to the 1980s when Vera Rubin made one of the most astonishing discoveries in astrophysics.
Rubin was observing galaxies, studying how stars orbit around their centers. According to Newtonian physics, stars farther from the galactic center should move more slowly than those closer in—similar to how planets in our solar system orbit the Sun. However, Rubin found something shocking: the outer stars were moving just as fast as the inner ones.
This defied all expectations. If only visible matter was influencing these stars, they should have flown off into space. The only explanation? An unseen force—an invisible "halo" of mass—was holding them in place.
This mysterious mass became known as dark matter, first theorized by Fritz Zwicky in the 1930s but confirmed by Rubin’s observations. Dark matter does not interact with light, making it completely invisible to telescopes. It does not absorb, reflect, or emit any part of the electromagnetic spectrum—no radio waves, no infrared, no X-rays—nothing.
So how do we study something that we cannot see? Enter Euclid.
Einstein Rings: Nature’s Cosmic Lens
One of Euclid’s most powerful tools for studying dark matter is gravitational lensing—a phenomenon predicted by Albert Einstein’s General Theory of Relativity.[4]
Just as a glass lens bends light to focus an image, gravity can bend light when it passes near a massive object like a galaxy or galaxy cluster. If a large mass lies directly between us and a distant light source, it can create a phenomenon called an Einstein Ring—a perfect halo of light formed by the bending of space itself.
Until now, space telescopes like Hubble and JWST had detected some Einstein Rings. In just two weeks, Euclid has pictured 497—double the amount of Einstein Rings captured by space based telescopes! However of the 497 captured only handful are new, still this marks a giant leap in technology. Scientists now estimate that Euclid will reveal 100,000 or more Einstein Rings over its lifetime.
By analyzing these rings, scientists can map the distribution of dark matter in unprecedented detail. This is because dark matter, despite being invisible, has mass—and mass bends light. By studying how much the light is distorted, we can determine the amount and distribution of matter and dark matter in galaxies and clusters.
In a moment of serendipity, after fixing an icing problem on the lens, scientists decided to check the lens by taking a picture of Galaxy NGC 6505 and witnessed the Einstein Ring at the center for the first time.
“I find it very intriguing that this ring was observed within a well-known galaxy, which was first discovered in 1884,” says Valeria Pettorino, ESA Euclid Project Scientist. “The galaxy has been known to astronomers for a very long time. And yet this ring was never observed before. This demonstrates how powerful Euclid is, finding new things even in places we thought we knew well. This discovery is very encouraging for the future of the Euclid mission and demonstrates its fantastic capabilities.”[5]
Scientists differentiate between strong and weak gravitational lensing. Strong lensing creates highly visible distortions in background galaxies or other light sources, forming arcs, multiple images, or Einstein rings. In contrast, weak lensing results in only slight stretching or displacement of background sources, making it detectable only through statistical analysis of large data sets.
The deeper we observe into the Universe, the more pronounced weak gravitational lensing becomes, as an increasing number of dark matter structures act as lenses, altering the light from distant sources.
Euclid will map the distorted shapes of billions of galaxies spanning over 10 billion years of cosmic history, creating a three-dimensional representation of dark matter distribution. This will offer new insights into the nature of this elusive component.
Additionally, by mapping the distribution of galaxies across cosmic time, Euclid will help scientists understand dark energy, which influences the Universe’s expansion rate. By charting the large-scale structure of the cosmos in unprecedented detail, Euclid will allow researchers to track how cosmic expansion has evolved over time.(Credit:ESA)
The Role of Machine Learning and Citizen Science
The amount of data Euclid is collecting is staggering. Every day, it beams back 100 gigabytes of data. Over its lifetime, Euclid will observe billions of galaxies. The sheer volume of data is beyond what any team of scientists could analyze manually.
To solve this problem, machine learning is being used to categorize and analyze the images. AI algorithms are trained to recognize galaxies, Einstein Rings, active galactic nuclei (AGNs), and other cosmic structures.
But how does machine train on the data? Enter citizen scientists. Through projects like Zooniverse, over 400,000 volunteers (over 20 years) have helped train AI models by classifying astronomical and other images. These efforts allow Euclid’s data to be processed much faster—turning what would have taken centuries into just a few years.
This data release showcases the remarkable potential of uniting Euclid’s capabilities with AI, citizen science, and expert analysis into a powerful discovery engine—one that will be crucial for handling the immense volume of data generated by Euclid.
The catalogue is created by the ‘Zoobot’ AI algorithm. During an intensive one-month campaign on Galaxy Zoo last year, 9976 human volunteers worked together to teach Zoobot to recognise colliding galaxy, einstein rings, Active Galactic Nuclei (AGN) and other features by classifying Euclid images.
"Every answer a volunteer in Galaxy zoo gives gets multiplied by 1000 folds" says Mike Walmsley, a Dunlap Fellow at the University of Toronto, while showing the power citizen Zooniverse on Cosmology Talks (a YouTube chanel).
There is a brilliant video on Youtube by a channel named Cosmology Talks, where PhD scholars and scientist working on Euclid data talk about this Q1 data . Link is available at the end of the article.
What’s Next for Euclid?
Euclid is gearing up for Data Release 1, which will cover much more area of the proposed total survey area. Will contain far more Einstein Rings, AGN, and what not and help push cosmology to the next generation.
Want to see Euclid’s stunning images and latest discoveries Click Here?
To read more about Vera Rubin and her discovery of Dark Matter Click here.
Want to become a Citizen scientist and help Euclid and other such projects Click Here!
Link to the Cosmology Talk video,Click Here.
I am an ordinary reader, not a scholar of physics or Astronomy. This article is very interesting for me because I am able to understand difficult things of cosmic science. The writer has explained difficult things in a simple and understandable manner. Great work. Keep writing.👏👏👏
A very nice compilation of all the related topics. Very beautifully covered.