Since humanity first looked to the stars, we have dreamed of understanding the grand structure of the universe. That dream has taken an extraordinary leap forward with the European Space Agency’s Euclid mission, a space telescope launched in June 2023. Euclid’s task is monumental: to map billions of galaxies across a cosmic sphere spanning 10 billion light-years and, in doing so, help us unravel one of the greatest mysteries in science—the true nature of dark energy and dark matter.
But to prepare for this mission, scientists had to create something equally astonishing here on Earth: a digital universe of unprecedented scale. This effort has now produced the largest synthetic simulation of the cosmos ever built, known as the Flagship 2 galaxy mock. It is a breathtaking achievement that combines supercomputing power, theoretical physics, and human imagination to anticipate what Euclid will see as it gazes deep into the past of our universe.
The Birth of a Synthetic Universe
At the heart of this achievement is an algorithm developed by Joachim Stadel, an astrophysicist at the University of Zurich (UZH). With this tool, scientists used one of the world’s most powerful supercomputers, Piz Daint, to simulate nothing less than the universe itself.
In 2019, more than 80 percent of Piz Daint’s computing capacity was devoted to the task. For weeks, the machine churned through calculations, tracking the gravitational interactions of four trillion virtual particles. From these interactions, cosmic structures—like the vast filaments of the cosmic web—emerged, just as they do in the real universe.
Then, in a second step, these structures were populated with galaxies. Not just a handful, or even millions, but a staggering 3.4 billion galaxies. Each was assigned around 400 properties, including brightness, shape, position, and velocity. The result is not a random collection of dots on a screen, but a living, evolving blueprint of the cosmos—an artificial sky that mirrors what Euclid itself will soon observe with its cameras and instruments.
“It was a huge challenge to simulate such a large portion of the universe at this resolution in a single calculation,” Stadel recalls. The scale is mind-bending: every galaxy in the simulation is modeled in enough detail to reflect the subtle variations Euclid will measure in reality.
Why Simulations Matter
The importance of this effort goes far beyond curiosity. Euclid is designed to gather an ocean of data, so vast and complex that no human could analyze it manually. Automated methods must be ready in advance to process and interpret the incoming flood of information.
Julian Adamek, another UZH astrophysicist involved in the project, explains: “These simulations are crucial for preparing the analysis of Euclid’s data. They let us develop the methodology we need before the telescope even begins its work.” In other words, the Flagship 2 mock universe acts as a rehearsal, teaching scientists how to read Euclid’s signals and distinguish between what is expected and what might be a sign of new physics.
The simulation is based on the standard cosmological model, which describes the universe as a mixture of ordinary matter, dark matter, and dark energy, evolving since the Big Bang. But it also leaves room for surprises. If Euclid’s actual observations deviate from the simulation’s predictions, those differences could point to cracks in our current understanding of the cosmos.
Cracks in the Standard Model
For decades, cosmology has relied on the so-called ΛCDM model (Lambda Cold Dark Matter). It is remarkably successful at explaining the large-scale distribution of galaxies and the expansion of the universe. Yet there are signs that it may not tell the whole story.
“We already see indications of cracks in the standard model,” says Stadel. Small tensions have appeared in measurements of cosmic expansion, galaxy clustering, and the behavior of dark energy. Euclid, with its high-precision data, has the potential either to reinforce the standard model or to expose its weaknesses more clearly.
Adamek adds: “It will be exciting to see whether the model holds up against Euclid’s data—or whether we uncover new shortcomings.” These shortcomings could be the seeds of a new paradigm, just as the discovery of quantum mechanics and relativity transformed physics a century ago.
Peering into the Mystery of Dark Energy
Perhaps the most tantalizing goal of Euclid is to probe dark energy, the mysterious force that is accelerating the expansion of the universe. In the standard model, dark energy is represented by a simple constant, a placeholder for something we do not yet understand.
Euclid offers a way to test whether that constant truly remained the same across billions of years. Its observations can trace the universe’s expansion up to 10 billion years into the past, allowing astronomers to measure whether dark energy behaved differently at different times.
“If this constant has varied, even slightly, it would open the door to an entirely new physics,” explains Adamek. While Euclid may not deliver final answers, it promises to bring us closer to grasping the elusive nature of dark energy.
Mapping the Invisible Universe
Euclid’s strength lies not only in its scale but also in its resolution. It can detect the faint distortions in the shapes of galaxies caused by gravitational lensing—tiny warps in light produced by massive concentrations of dark matter. These distortions act as a cosmic fingerprint, revealing the invisible scaffolding that underlies the universe.
By combining lensing with spectroscopic measurements of galaxy distances, Euclid creates a three-dimensional map of the cosmos. This map will stretch across a sphere with a radius of 10 billion light-years, offering the most complete and detailed survey of the universe ever achieved.
Through this map, scientists will not only trace the hidden web of dark matter but also uncover the rarest of cosmic phenomena—events so uncommon that only by surveying such a vast volume of space can they be found.
The First Glimpse and What Lies Ahead
In March 2025, Euclid shared its first small set of observational data, known as the Quick Data Release. Though only a fraction of what the mission will ultimately deliver, it already revealed fresh details about the cosmic web and galaxy clusters. Researchers now eagerly await the next major release, planned for spring 2026, which promises to expand our view even further.
For the scientists behind the Flagship 2 simulation, the early results are both a test and a thrill. Each new observation from Euclid can be compared against the artificial sky they created years earlier. Agreement between the two strengthens our confidence in the standard model, while differences hint at discoveries waiting to be made.
A Cosmic Blueprint and a Human Triumph
The Flagship 2 simulation and Euclid’s mission together represent the pinnacle of human ingenuity. They show what is possible when imagination meets technology, when questions as ancient as “What is the universe made of?” are pursued with tools of staggering sophistication.
In the artificial galaxies spun from equations and computer code, we glimpse not only the fabric of the cosmos but also the creativity of the human spirit. Every particle simulated, every galaxy modeled, and every ray of light bent in a computer’s memory reflects our determination to understand the world beyond our own.
As Euclid peers into the vastness of space and time, we stand on the threshold of discoveries that could redefine our picture of reality. Whether it confirms our models or exposes their flaws, one truth is certain: we are closer than ever to unlocking the mysteries of the universe.
And in that pursuit—through simulations, telescopes, and the relentless drive of science—we are reminded that humanity itself is part of this grand cosmic story.
More information: Euclid. V. The Flagship galaxy mock catalogue: A comprehensive simulation for the Euclid mission, Astronomy & Astrophysics (2025). DOI: 10.1051/0004-6361/202450853
