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The consortium running the European Space Agency's (ESA) Euclid mission has published the most extensive simulation of the cosmos to date. The modeling was based on algorithms developed by UZH professor Joachim Stadel.

The Euclid Consortium has released the Flagship 2 galaxy mock—the largest synthetic simulation of the universe ever created. It contains 3.4 billion galaxies, each with 400 modeled properties such as brightness, position, velocity and shape. The simulation is designed to help scientists interpret and analyze the massive datasets generated by Euclid, ESA's space telescope that has been surveying the cosmos since June 2023 with unprecedented resolution.

The simulation is built on an algorithm developed by UZH astrophysicist Joachim Stadel. The calculation was carried out in 2019 on the supercomputer Piz Daint at the Swiss National Supercomputing Center (CSCS) in Lugano. At the time, Piz Daint was the third most powerful supercomputer in the world—and more than 80% of its total capacity was dedicated to the project.

"It was a huge challenge to simulate such a large portion of the universe at this resolution in a single calculation," recalls Stadel.

The computation tracked the gravitational interactions of four trillion particles. In a second step, these structures were populated with galaxies that lie within Euclid's field of view, producing a realistic blueprint of what Euclid will actually observe.

Preparing automated data analysis

"These simulations are crucial for preparing the analysis of Euclid's data," explains Julian Adamek of UZH's Department of Astrophysics, who collaborated with Stadel and Aurel Schneider on the project. Euclid produces data in such sheer volume and speed that they must first be processed automatically. The methodology for interpreting these data had to be developed in advance—using simulations.

The Flagship 2 galaxy mock is based on the standard cosmological model and incorporates the current state of knowledge about the composition and evolution of the universe. Stadel and Adamek expect Euclid's observations to broadly confirm the matter distribution predicted in the simulation.

Cracks in the standard model

At the same time, the researchers anticipate surprises and unexpected discoveries. "We already see indications of cracks in the standard model," says Stadel. Euclid may reveal further phenomena that cannot be explained within the current theoretical framework.

"It will be exciting to see whether the model holds up against Euclid's high-precision data—or whether we uncover signs of new shortcomings," adds Adamek.

The mission also aims to shed light on the nature of dark energy—the mysterious force driving cosmic expansion. "In the model, dark energy is just a constant that explains the expansion of the universe," says Stadel.

Euclid's data now enable astronomers to look back up to 10 billion years in cosmic history. "We can see how the universe expanded at that time and measure whether this constant really remained constant," notes Adamek.

While Euclid may not yet provide definitive answers, Stadel is convinced: "Euclid will bring us a step closer to understanding the mysterious realm of dark energy."

Unprecedented resolution

Euclid is the most comprehensive survey of the cosmos ever undertaken—not only in scale but also in precision. Its high resolution allows researchers to detect even minute distortions in galaxy images caused by gravitational lensing. These effects, produced by regions of high mass density bending light, reveal how invisible dark matter is distributed across the universe.

Spectroscopic measurements from Euclid also enable scientists to determine galaxy distances with great accuracy. Together, these techniques create a three-dimensional map of galaxies spanning a cosmic sphere with a radius of 10 billion light-years.

Glimpse of rare cosmic phenomena

Adamek expects that Euclid will also uncover rare cosmic phenomena. "Some events are extremely uncommon, but because Euclid covers such a vast region, the chances of finding unexpected or rare objects are high."

In March 2025, Euclid released its first observational data. This "Quick Data Release" represented only a small fraction of the mission's full dataset but already offered new insights into the cosmic web and galaxy clusters. Earlier, related research was also in the journal Astronomy & Astrophysics. The publication of further datasets is planned for spring 2026.