The European Space Agency (ESA) launched the Euclid telescope on July 1, 2023. Its task is to build the largest and most detailed map of the Universe in the history of observations. The first major dataset has now been published, and it is already impressive.
Euclid is a wide-field telescope. In a single image it covers an area of sky more than 100 times larger than the Hubble or James Webb telescopes, explains Leiden University astronomer Koen Kuijken. According to him, the telescope takes about 20 new images per day, and over six years of operation it will map a third of the entire sky.
The first release includes images of a vast area of sky showing approximately 900 million galaxies and 60 million quasars - very bright nuclei of distant active galaxies. The total data volume is about 2.5 trillion pixels, or 1.7 million gigabytes.
These observations allow a look far into the past. Some galaxies in the dataset existed when the Universe was only 700 million years old - about 5% of its current age.
"We describe the history of the Universe quite well, but we still do not understand what 95% of its contents consists of," says Leiden University professor Alessandra Silvestri. "These are some of the greatest mysteries of modern physics. And Euclid - our dark Universe detective - was created precisely to help unravel them."
Everything we see - stars, planets, galaxies - makes up only about 5% of the contents of the Universe. The rest comes from two mysterious components. Dark matter accounts for about 27%. It cannot be seen directly, but its gravity helps galaxies form and holds cosmic structures together. Dark energy accounts for about 68%; observations suggest it is accelerating the expansion of the Universe.
Euclid studies the Universe in two ways. It maps the positions of billions of galaxies to show how cosmic structures evolved. It also measures tiny distortions in their shapes caused by gravity bending light. This phenomenon is called gravitational lensing; it makes it possible to detect invisible matter, including dark matter.
"One galaxy will tell us little about dark energy or the laws of gravity. But billions of galaxies can," notes Silvestri. By studying patterns in their positions and shapes, researchers can reconstruct how the Universe evolved and test the underlying physics. "Statistics are the key. That is why our colleague Guadalupe Canas Herrera, a postdoctoral researcher at Leiden Observatory, has already begun developing tools to solve this statistical problem."
Silvestri also poses a fundamental question: "Is our current understanding of gravity complete?" According to her, by analyzing Euclid data she wants to test whether the laws of physics work on the largest, cosmic scales as expected. The first results of these tests are planned for publication in June 2027.
The first data release is an important milestone, but only the beginning. Astronomers have already published about 20 scientific papers describing the data and initial analyses, and dozens more are in preparation. The next major step is to combine the observations into detailed cosmological analyses; these are expected in June 2027.
"Euclid's images are breathtaking. Obtaining images with such incredible detail over such a vast area of sky required truly outstanding efforts," says Silvestri. "But we are only at the beginning of the journey. The real scientific adventure is starting now."