Euclid space telescope reveals 900m galaxies in massive survey

The European Space Agency launched the Euclid space telescope on July 1, 2023, with the goal of constructing the largest and most comprehensive map of the universe ever attempted. The mission has now unveiled its first major data set, revealing approximately 900 million galaxies and 60 million quasars across a sky area nearly 10,000 times the size of the full moon.

“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. The instrument produces about 20 new images daily and aims to map one-third of the entire sky over its six-year mission. The initial release spans 2.5 trillion pixels and consumes 1.7 million gigabytes of digital data.

These observations allow researchers to look far back into cosmic history. Certain galaxies captured in the survey existed when the universe was only 700 million years old, representing just 5% of its current age. While scientists understand cosmic history well, roughly 95% of the universe remains unexplained by current physics models.

Unraveling Dark Matter and Dark Energy

“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. Visible matter, including stars and planets, accounts for only about 5% of total contents. The remaining composition consists of approximately 27% dark matter, which helps form galaxies, and roughly 68% dark energy, which drives the accelerated expansion of the universe.

Euclid assists cosmologists by mapping galaxy positions to show structural evolution and by measuring tiny distortions in galaxy shapes caused by gravitational lensing. These measurements help expose invisible matter. Researchers are also using the data to test whether our current understanding of gravity remains complete on cosmic scales, with the first results of these physics tests anticipated in June 2027.

“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,” Silvestri notes. Astronomers have already published around 20 scientific papers detailing the data, marking the beginning of a broader scientific effort.