Euclid Space Warps: help spot galaxies bending spacetime

Strong gravitational lenses captured by Euclid

A collage of fourteen by eight squares containing examples of gravitational lenses. Each example typically comprises a bright centre with smears of stacredirs in an arc or multiple arcs around it as a result of light travelling towards Euclid from distant galaxies being bent and distorted by normal and dark matter in the foreground. In some rare cases the smearing is in a complete ring, crea,brting a so-called Einstein Ring. Credit: ESA/Euclid/Euclid Consortium/NASA, image processing by M. Walmsley, M. Huertas-Company, J.-C. Cuillandre.Hi-res JPG

Licence: CC BY-SA 3.0 IGO or ESA Standard Licence (content can be used under either licence)

Infographic explaining how an Einstein Ring is formed

Against a dark blue background, this infographic contains a paragraph of text in the top left corner, the logo of ESA in the top right corner and a succession of graphics in the bottom half of the image. The text paragraph explains the principle behind Einstein rings, and it can be read in the image caption. The graphics below it illustrate this astrophysical phenomenon, and by looking at them from left to right we can understand the process of how Einstein rings are formed.

The left-most element in the bottom half of the image is a graphic representation of a galaxy, labelled ‘distant galaxy’. To the right of it, another galaxy is shown, labelled ‘Foreground galaxy acting as a magnifying lens’. The third illustration, to the right of the previous one, shows ESA’s Euclid space telescope and is labelled ‘Telescope’. The ‘distant galaxy’ and the ‘Telescope’ are connected by two lines that form an elongated diamond-shape around the ‘Foreground galaxy’. This line is labelled ‘Gravity bends the light rays of the distant galaxy’. The fourth and last illustration in the line shows a ring of light around a central disk and is labelled ‘What the telescope sees’. Credit: ESA. Hi-res JPG

Licence: CC BY-SA 3.0 IGO or ESA Standard Licence (content can be used under either licence)

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In brief

With the launch of Space Warps, a new citizen science project on the Zooniverse platform, you can now join in the search to find rare and elusive strong gravitational lenses in never-before-seen images captured by the European Space Agency’s Euclid space telescope. The project aims at shining a light on dark matter in galaxies and providing clues about mysterious dark energy.

In-depth

Warps in spacetime do not only show up in science fiction movies like Interstellar. In real life, we can see the warping effect that gravity has on spacetime in the form of gravitational lensing.

The enormous gravity of a massive object – such as a galaxy or cluster of galaxies – distorts the shape of spacetime and can bend the light rays coming from a distant galaxy behind. By warping spacetime, the foreground galaxy acts like a magnifying glass.

Light from the background object that would be obscured doesn’t travel in a straight line anymore. Instead, it curves around the intervening mass, often producing multiple images, stretched arcs, or even a complete ring known as ‘Einstein ring’, like the one recently discovered by Euclid.

Strong gravitational lenses offer a striking demonstration of Einstein’s theory of general relativity, showing that matter in the Universe can act as a natural telescope, bringing distant objects into sight.

ESA’s Euclid telescope is revolutionising the studies of strong gravitational lensing by providing very sensitive imaging over large swaths of the sky in unprecedented detail. This is exactly what is needed to identify rare gravitational lenses.

In March 2025, 500 galaxy-galaxy strong lenses were found nestled in just the first 0.04% of Euclid data, most of them previously unknown. This pioneering catalogue was created thanks to the combined effort from citizen scientists, artificial intelligence (AI) and researchers.

Early glimpse of new Euclid images

As Euclid continues its survey, sending around 100 GB of data back to Earth every day, ESA and the Euclid Consortium once again need help from citizen scientists to identify strong gravitational lenses in a large data set.

For this, the Space Warps team has launched a citizen science project based on new Euclid images, which will be part of the future Euclid Data Release 1. While this data is not public yet, by participating in this new citizen science project you can get an early glimpse of these new images of galaxies captured by the telescope.

For this project, you will be inspecting new high quality imaging data from Euclid in which many previously unknown strong lenses are hiding. About 300 000 images pre-selected by AI algorithms will be shown, which are fine-tuned with the results from the initial citizen-science Euclid strong lens search. These are the highest ranked candidates from a whopping 72 million galaxies from DR1 that were classified by the AI algorithms. Scientists expect that this exquisite high-quality data will reveal more than 10 000 new lenses.

What can we learn from strong lenses.

The Euclid mission explores how the Universe has expanded and how its structure has changed through cosmic history using mainly two methods: weak lensing and baryonic acoustic oscillations. From this, scientists can learn more about the role of gravity and the nature of dark matter and dark energy.

Strong gravitational lenses can also provide insights into these central questions. For example, strong lensing features can ‘weigh’ individual galaxies and clusters of galaxies. This reveals the total matter (whether dark or light) and traces the distribution of dark matter. By studying strong lenses across cosmic time, scientists can trace the expansion of the Universe and its apparent acceleration. This will provide additional insight into the role of dark energy..

“We’ve already seen the success of combining AI with visual inspection by citizen volunteers and scientists on Space Warps, efficiently finding hundreds of high‑probability lens candidates in an initial small Euclid search in 2024”, explains Aprajita Verma, Space Warps’ co-founder and project lead at the University of Oxford, UK..

“In this brand new DR1 data, 30 times larger than the initial search and together with our improved AI algorithms, we are expecting to find more than 10 000 high quality lens candidates. This is more than four times the number of lenses than we have been able to find since the first gravitational lens was discovered nearly 50 years ago.”.

This step-change is possible thanks to Euclid. The mission can map large areas of the sky with unique sharpness, an ideal combination for finding rare objects like strong gravitational lenses..

“We can’t wait to see what we will find within this unprecedented dataset. Join us on Space Warps to take part in this exciting search!” concludes Aprajita.

Euclid: ESA’s mission into the unknown
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Source: ESA/Science Exploration

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About Euclid

Euclid was launched in July 2023 and started its routine science observations on 14 February 2024. The goal of the mission is to reveal the hidden influence of dark matter and dark energy on the visible Universe. Over a period of six years, Euclid will observe the shapes, distances and motions of billions of galaxies out to 10 billion light-years. Euclid is a European mission, built and operated by ESA, with contributions from NASA. The Euclid Consortium – consisting of more than 2000 scientist from 300 institutes in 15 European countries, the USA, Canada, and Japan – is responsible for providing the scientific instruments and scientific data analysis. ESA selected Thales Alenia Space as prime contractor for the construction of the satellite and its service module, with Airbus Defence and Space chosen to develop the payload module, including the telescope. NASA provided the detectors of the Near-Infrared Spectrometer and Photometer, NISP. Euclid is a medium-class mission in ESA’s Cosmic Vision Programme.

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Spiral in Serpens

PGC 54493

Credit: ESA/Hubble & NASA
Acknowledgement: Judy Schmidt (geckzilla.com)

This new NASA/ESA Hubble Space Telescope image shows a beautiful spiral galaxy known as PGC 54493, located in the constellation of Serpens (The Serpent). This galaxy is part of a galaxy cluster that has been studied by astronomers exploring an intriguing phenomenon known as weak gravitational lensing.

This effect, caused by the uneven distribution of matter (including dark matter) throughout the Universe, has been explored via surveys such as the Hubble Medium Deep Survey. Dark matter is one of the great mysteries in cosmology. It behaves very differently from ordinary matter as it does not emit or absorb light or other forms of electromagnetic energy — hence the term "dark".

Even though we cannot observe dark matter directly, we know it exists. One prominent piece of evidence for the existence of this mysterious matter is known as the "galaxy rotation problem". Galaxies rotate at such speeds and in such a way that ordinary matter alone — the stuff we see — would not be able to hold them together. The amount of mass that is "missing" visibly is dark matter, which is thought to make up some 27% of the total contents of the Universe, with dark energy and normal matter making up the rest. PGC 55493 has been studied in connection with an effect known as cosmic shearing. This is a weak gravitational lensing effect that creates tiny distortions in images of distant galaxies.

A version of this image was entered into the Hubble's Hidden Treasures image processing competition by contestant Judy Schmidt.

Links

• Hubble Advanced Camera for Surveys (ACS)

 

Source:  ESA/Hubble - Space Telescope

Weak Lensing Gains Strength

Visible-light images from the Hubble Space Telescope populate this tiny section of the full two-square-degree Cosmic Evolution Survey (COSMOS), which combines data in many wavelengths from space and ground-based telescopes around the world. COSMOS was the basis of a new extension of the mass-luminosity relation for weak lensing studies.

Berkeley, CA — Weak gravitational lensing is a uniquely promising way to learn how much dark matter there is in the Universe and how its distribution has evolved since the distant past. New work by a team led by a cosmologist from the U.S. Department of Energy’s Lawrence Berkeley National Laboratory has made major progress in extending the use of gravitational lensing to the study of much older and smaller structures than was previously possible.

Until recently, weak lensing had been limited to calculating the total mass of relatively nearby groups and clusters of galaxies. Their total mass includes both ordinary, visible matter like stars and dust – what astronomers call “baryonic” matter – plus the much more massive invisible concentrations of dark matter that form groups and clusters by pulling galaxies together.

Astronomers were able to establish an important scaling relationship for nearby clusters between their total masses, determined by gravitational lensing, and the brightness of their x-ray emissions, an indication of the mass of the ordinary matter alone. A new study in the Astrophysical Journal (ApJ) now continues this important relationship to distant objects.

“We’ve been able to extend measurements of mass to much smaller structures, which existed much earlier in the history of the Universe,” says Alexie Leauthaud, a Chamberlain Fellow in Berkeley Lab’s Physics Division and first author of the ApJ study. “This helps us gain a better understanding of the relationship between the normal matter in dense structures, which are seen through the x-ray luminosity, and the total dark-matter mass of these structures, as measured by the weak lensing.” Leauthaud is a member of the Berkeley Center for Cosmological Physics (BCCP) at UC Berkeley and Berkeley Lab.

Mass as a lens

Gravitational lensing occurs because mass curves the space around it, bending the paths along which rays of light travel: the more mass (and the closer to the center of mass), the more space bends, and the more the image of a distant object is displaced and distorted. Thus measuring distortion, or “shear,” is key to measuring the mass of the lensing object.

At least this is so for “strong” lensing. A very massive object or collection of objects, like a nearby galaxy cluster and the invisible dark matter that encloses it, distorts the apparent shape and position of bright objects beyond it so much that the distant images are bent and may even be smeared into rings around the foreground cluster. The visible distortion is a direct measure of the mass of the lens and points to its center.

A spectacular example of strong gravitational lensing is the nearby galaxy cluster Abell 2218 (top), in which the visible distortion of individual background galaxies can be used to measure the mass of the lensing structure. The weak lensing of fainter and more distant structures must be detected by statistical averaging (bottom). (Abell 2218 image by NASA, weak lensing simulation by Bhuvnesh Jain, Uroš Seljak, and Simon White)

Weak lensing works the same way, except that the shear is too subtle to be seen directly. Most of the apparent shear isn’t distortion at all – a galaxy has its own distinct shape, and we often see it from an angle that makes it look elongated. Apparent shear may also be due to the telescope, the detector, or the atmosphere.

Nevertheless, faint additional distortions in a collection of distant galaxies can be calculated statistically, and the average shear due to the lensing of some massive object in front of them can be computed. Yet to calculate the lens’s mass from average shear, one needs to know its center.

“The problem with low-mass, high-redshift clusters is that it is difficult to determine which exact galaxy lies at the center of the cluster,” says Leauthaud. “That’s where x-rays help. The x-ray luminosity from a galaxy cluster can be used to find its center very accurately.”

The hot intracluster medium of gas or plasma that fills almost all galaxy clusters emits x-rays, making x-ray emission a convenient way to find distant galaxy structures in the night sky. But how does this emission help find the center of mass in a galaxy cluster? For the same reason that dark matter is dark.

Why dark matter is dark

Except through gravitation, dark matter does not interact (or interacts only very weakly) with itself or with ordinary matter. Indeed, that’s why it’s dark: to emit light it would have to interact via the electromagnetic force.

With ordinary matter, electromagnetism affects everything from chemistry to luminosity to electric and magnetic fields and even the pressure of stellar winds; thus electromagnetism plays an important role in determining the arrangement of ordinary matter, which is often irregular.

Because electromagnetism plays no role in the distribution of dark matter, however, dark matter forms large, smooth, spherical clumps, usually filled by ordinary galaxies plus hot gas or plasma, which it has trapped and retained solely through gravitation.

“Gas density follows the dark matter density, and because x-ray emission scales as the square of the gas density, the x-ray light shines very strongly in the core of the structure,” Leauthaud explains. “So x-rays are an excellent way to determine the center of even a distant, fuzzy galaxy cluster.”

“Basically the more mass, the more heat,” says Jean-Paul Kneib, a lead author of the ApJ paper from the Laboratory of Astrophysics of Marseilles (LAM) and France’s National Center for Scientific Research (CNRS). “But the plasma is baryonic matter, which is only a small part of the total mass of the cluster. While the x-radiation tells you something about the total mass, you need to get the scaling just right.”

Visible matter follows an underlying dark matter scaffolding. At left, blue indicates the mass of stars in galaxies in a given area, yellow the number of galaxies, and red the sources of brightest x-ray emission. Contours at right are the distribution of dark matter, from gravitational lensing. (Richard Massey et al, Nature 2007.

To pin down the scaling relation between x-ray brightness and total mass, Leauthaud and her colleagues first used x-ray luminosity to identify the center of mass of 206 galaxy groups and clusters, including numerous faint, distant clusters listed in the Hubble Space Telescope’s Cosmic Evolution Survey (COSMOS), which is curated by Nick Scoville of the California Institute of Technology, an author of the ApJ paper.

X-ray imaging came from the European Space Agency’s XMM-Newton satellite and from NASA’s Chandra satellite, whose principal investigator is Martin Elvis of the Harvard-Smithsonian Center for Astrophysics, an author of the ApJ paper. Elvis says, “I never thought our Chandra data would enable such a great measurement. In fact I was astonished when Alexie first showed me the results. It’s quite a tour de force of analysis, and really convincing.”

The X-ray analysis itself was performed by Alexis Finoguenov of the Max Planck Institute for Extraterrestrial Physics and the University of Maryland, one of the paper’s lead authors. Knowing the centers of mass from analysis of x-ray emission, the researchers could now use weak lensing to estimate the total mass of the distant groups and clusters with greater accuracy than ever before.

Finally they calculated the mass-luminosity relation for the new collections of groups and clusters and found that it was consistent with previous relations established by surveys of much closer structures – including some studied with strong gravitational lensing. Within calculable uncertainty, the relation follows the same straight slope from nearby galaxy clusters to distant ones; a simple, consistent scaling factor relates a group or cluster’s total mass to its x-ray brightness, or “baryonic tracer.”

“By confirming the mass-luminosity relation and extending it to high redshifts,” Leauthaud says, “we have taken a small step in the right direction toward using weak lensing as a powerful tool to measure the evolution of structure.”

In the beginning

The origin of galaxies can be traced back to slight differences in the density of the hot, liquid-like early universe; traces of these differences can still be seen as minute temperature differences in the cosmic microwave background (CMB).

“The variations we observe in the ancient microwave sky represent the imprints that developed over time into the cosmic dark-matter scaffolding for the galaxies we see today,” says BCCP director and UC Berkeley physics professor George Smoot of Berkeley Lab’s Physics Division, who shared the 2006 Nobel Prize in Physics for measuring anisotropies in the CMB and is one of the authors of the ApJ paper. “It is very exciting that we can actually measure with gravitational lensing how the dark matter has collapsed and evolved since the beginning.”

Dark matter shapes visible matter in a way that reflects the nature of dark energy. How galaxies are distributed in a Universe with no dark energy (left) would differ measurably from one in which dark energy is significant (right).

One goal in studying the evolution of structure is to understand dark matter itself, and how it interacts with the ordinary matter we can see. Another goal is to learn more about dark energy, the mysterious something that is pushing matter apart and causing the Universe to expand at an accelerating rate. Is dark energy constant, or is it dynamic? Or is it unreal, merely an illusion caused by a limitation in Einstein’s General Theory of Relativity?

The tools provided by the extended mass-luminosity relationship will do much to answer these questions about the opposing roles of gravity and dark energy in the once and future shape of the Universe.

“A Weak Lensing Study of X-Ray Groups in the COSMOS Survey: Form and Evolution of the Mass-Luminosity Relation,” by Alexie Leauthaud, Alexis Finoguenov, Jean-Paul Kneib, James E. Taylor, Richard Massey, Jason Rhodes, Olivier Ilbert, Kevin Bundy, Jeremy Tinker, Matthew R. George, Peter Capak, Anton M. Koekemoer, David E. Johnston, Yu-Ying Zhang, Nico Cappelluti, Richard S. Ellis, Martin Elvis, Catherine Heymans, Oliver Le Fèvre, Simon Lilly, Henry J. McCracken, Yannick Mellier, Alexandre Réfrégier, Mara Salvato, Nick Scoville, George Smoot, Masayuki Tanaka, Ludovic Van Waerbeke, and Melody Wolk, appears in the Astrophysical Journal and is available online to subscribers.

Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research for DOE’s Office of Science and is managed by the University of California. Visit our website at http://www.lbl.gov.

Additional Information

More about weak gravitational lensing

More about the COSMOS survey

Alexie Leauthaud demonstrates how dark matter causes gravitational lensing

Scientific contact: Alexie Leauthaud