Space telescope Euclid sets the new standard for cosmology

On July 1, 2023, the European Space Agency (ESA) launched the Euclid space telescope into space. Its mission: to create the largest and most detailed map of the universe ever made.

“Euclid is a wide-field telescope. It sees more than 100 times as much of the sky in one image as the Hubble or James Webb Space Telescope,” explains Leiden astronomer Koen Kuijken. “It produces around 20 new images every day. Over its six-year mission, it will map a third of the entire sky.”

900 million galaxies and counting

Its first major data set, released now, already contains an astonishing amount of information: images covering an area of sky almost 10,000 times the size of the full moon, around 900 million galaxies and 60 million quasars. In total, the data set contains approximately 2.5 trillion pixels and takes up 1.7 million gigabytes of data.

These observations allow astronomers like Kuijken to look far back in time. Some of the galaxies in the data set existed when the universe was only 700 million years old—just 5% of its current age.

The universe’s biggest missing pieces

“We can describe the history of the universe remarkably well, but on the other hand we still do not understand what 95% of it is made of,” says Leiden professor Alessandra Silvestri. “These are some of the greatest mysteries in modern physics. And Euclid—alias our dark universe detective—is designed especially with this goal in mind, to help us unravel them.”

Everything we can see—from stars and planets to galaxies—accounts for only about 5% of its total contents. The rest consists of two mysterious ingredients: dark matter and dark energy. Dark matter makes up roughly 27% of the universe. We cannot see it directly, but its gravity helps galaxies form and holds cosmic structures together. Dark energy accounts for about 68% and appears to drive the accelerated expansion of the universe.

“Euclid is revolutionizing the way we study our universe, providing unprecedented insights and setting a new standard for cosmology,” says Silvestri.

But this is not all. According to Silvestri, there’s another big fundamental question to be answered: “Is our current understanding of gravity complete, or not?” By analyzing the data collected by Euclid, Silvestri wants to test if the laws of physics work as expected on the largest scales: the cosmic scale. The first results of these tests will be published in June 2027.

Billions of galaxies hold the clues

Euclid helps cosmologists investigate the universe in two ways. It maps the positions of billions of galaxies to reveal how cosmic structures have evolved. It also measures tiny distortions in their shapes caused by gravity bending light, an effect known as gravitational lensing. This reveals invisible matter, including dark matter.

“A single galaxy cannot tell us much about dark energy or the laws of gravity. But billions of galaxies can,” says Silvestri. By studying patterns in their positions and shapes, the researchers can reconstruct how the universe has evolved and test the physics behind it. “Statistics is the key. That is why our colleague Guadalupe Cañas Herrera, a postdoctoral researcher at Leiden Observatory, has already started developing the tools needed to tackle this statistical challenge.”

The real scientific adventure starts now

This first data release marks an important milestone, but it is only the beginning. Astronomers have already published around 20 scientific papers describing the data and their first analyses, with dozens more in the pipeline. The next major step will be to combine the observations into detailed cosmological analyses, expected in June 2027.

“Euclid’s images are breathtaking. Producing images with such extraordinary detail across such an enormous area of the sky has required a truly remarkable effort,” says Silvestri. “But we are only at the beginning of our journey. The real scientific adventure starts now.”

Who’s behind this story?

Lisa Lock

BA art history, MA material culture. Former museum editor, paramedic, and transplant coordinator. Editing for Science X since 2021.

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Robert Egan

Bachelor’s in mathematical biology, Master’s in creative writing. Well-traveled with unique perspectives on science and language.

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