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ESA Previews Euclid Mission’s Deep View of ‘Dark Universe’

With contributions from NASA, the mission is looking back into the universe’s history to understand how the universe’s expansion has changed. The Euclid mission — led by ESA (European Space Agency) with contributions from NASA — aims to find out why our universe is expanding at an accelerating rate. Astronomers use the term “dark energy” […]

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Mar 19, 2025 - 22:00

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ESA Previews Euclid Mission’s Deep View of ‘Dark Universe’

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A sea of galaxies of many different shapes and sizes, oriented in all directions and many with spiral arms visible. A large, nearby face-on spiral galaxy draws attention at the bottom center of the image. Two bright stars in the center and to the left are seen with prominent diffraction spikes.

This image shows about 1.5% of Euclid’s Deep Field South, one of three regions of the sky that the telescope will observe for more than 40 weeks over the course of its prime mission, spotting faint and distant galaxies. One galaxy cluster near the center is located almost 6 billion light-years away from Earth.

ESA/Euclid/Euclid Consortium/NASA; image processing by J.-C. Cuillandre, E. Bertin, G. An-selmi

With contributions from NASA, the mission is looking back into the universe’s history to understand how the universe’s expansion has changed.

The Euclid mission — led by ESA (European Space Agency) with contributions from NASA — aims to find out why our universe is expanding at an accelerating rate. Astronomers use the term “dark energy” to refer to the unknown cause of this phenomenon, and Euclid will take images of billions of galaxies to learn more about it. A portion of the mission’s data was released to the public by ESA released on Wednesday, March 19.

This new data has been analyzed by mission scientists and provides a glimpse of Euclid’s progress. Deemed a “quick” data release, this batch focuses on select areas of the sky to demonstrate what can be expected in the larger data releases to come and to allow scientists to sharpen their data analysis tools in preparation.

The data release contains observations of Euclid’s three “deep fields,” or areas of the sky where the space telescope will eventually make its farthest observations of the universe. Featuring one week’s worth of viewing, the Euclid images contain 26 million galaxies, the most distant being over 10.5 billion light-years away. Launched in July 2023, the space telescope is expected to observe more than 1.5 billion galaxies during its six-year prime mission.

On a black background lies a rectangular shape with stepped edges all around, oriented with its long axis pointing from bottom left to top right.

The entirety of the Euclid mission’s Deep Field South region is shown here. It is about 28.1 square degrees on the sky. Euclid will observe this and two other deep field regions for a total of about 40 weeks during its 6-year primary mission.

ESA/Euclid/Euclid Consortium/NASA; image processing by J.-C. Cuillandre, E. Bertin, G. An-selmi

By the end of that prime mission, Euclid will have observed the deep fields for a total of about 40 weeks in order to gradually collect more light, revealing fainter and more distant galaxies. This approach is akin to keeping a camera shutter open to photograph a subject in low light.

The first deep field observations, taken by NASA’s Hubble Space Telescope in 1995, famously revealed the existence of many more galaxies in the universe than expected. Euclid’s ultimate goal is not to discover new galaxies but to use observations of them to investigate how dark energy’s influence has changed over the course of the universe’s history.

In particular, scientists want to know how much the rate of expansion has increased or slowed down over time. Whatever the answer, that information would provide new clues about the fundamental nature of this phenomenon. NASA’s Nancy Grace Roman Space Telescope, set to launch by 2027, will also observe large sections of the sky in order to study dark energy, complementing Euclid’s observations.

The location of the Euclid deep fields are shown marked in yellow on this all-sky view from ESA’s Gaia and Planck missions. The bright horizontal band is the plane of our Milky Way galaxy. Euclid’s Deep Field South is at bottom left.

ESA/Euclid/Euclid Consortium/NASA; ESA/Gaia/DPAC; ESA/Planck Collaboration

Looking Back in Time

To study dark energy’s effect throughout cosmic history, astronomers will use Euclid to create detailed, 3D maps of all the stuff in the universe. With those maps, they want to measure how quickly dark energy is causing galaxies and big clumps of matter to move away from one another. They also want to measure that rate of expansion at different points in the past. This is possible because light from distant objects takes time to travel across space. When astronomers look at distant galaxies, they see what those objects looked like in the past.

For example, an object 100 light-years away looks the way it did 100 years ago. It’s like receiving a letter that took 100 years to be delivered and thus contains information from when it was written. By creating a map of objects at a range of distances, scientists can see how the universe has changed over time, including how dark energy’s influence may have varied.

But stars, galaxies, and all the “normal” matter that emits and reflects light is only about one-fifth of all the matter in the universe. The rest is called “dark matter” — a material that neither emits nor reflects light. To measure dark energy’s influence on the universe, astronomers need to include dark matter in their maps.

Bending and Warping

Although dark matter is invisible, its influence can be measured through something called gravitational lensing. The mass of both normal and dark matter creates curves in space, and light traveling toward Earth bends or warps as it encounters those curves. In fact, the light from a distant galaxy can bend so much that it forms an arc, a full circle (called an Einstein ring), or even multiple images of the same galaxy, almost as though the light has passed through a glass lens.

In most cases, gravitational lensing warps the apparent shape of a galaxy so subtly that researchers need special tools and computer software to see it. Spotting those subtle changes across billions of galaxies enables scientists to do two things: create a detailed map of the presence of dark matter and observe how dark energy influenced it over cosmic history.

It is only with a very large sample of galaxies that researchers can be confident they are seeing the effects of dark matter. The newly released Euclid data covers 63 square degrees of the sky, an area equivalent to an array of 300 full Moons. To date, Euclid has observed about 2,000 square degrees, which is approximately 14% of its total survey area of 14,000 square degrees. By the end of its mission, Euclid will have observed a third of the entire sky.

The dataset released this month is described in several preprint papers available today. The mission’s first cosmology data will be released in October 2026. Data accumulated over additional, multiple passes of the deep field locations will also be included in the 2026 release.

More About Euclid

Euclid is a European mission, built and operated by ESA, with contributions from NASA. The Euclid Consortium — consisting of more than 2,000 scientists from 300 institutes in 15 European countries, the United States, 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. Euclid is a medium-class mission in ESA’s Cosmic Vision Programme.

Three NASA-supported science teams contribute to the Euclid mission. In addition to designing and fabricating the sensor-chip electronics for Euclid’s Near Infrared Spectrometer and Photometer (NISP) instrument, JPL led the procurement and delivery of the NISP detectors as well. Those detectors, along with the sensor chip electronics, were tested at NASA’s Detector Characterization Lab at Goddard Space Flight Center in Greenbelt, Maryland. The Euclid NASA Science Center at IPAC (ENSCI), at Caltech in Pasadena, California, supports U.S.-based science investigations, and science data is archived at the NASA / IPAC Infrared Science Archive (IRSA). JPL is a division of Caltech.

For more information about Euclid go to:

science.nasa.gov/mission/euclid/