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The largest virtual catalogue of the Universe ever created has just been released by the Euclid Consortium. Containing 3.4 billion simulated galaxies, each with hundreds of properties, the catalogue reproduces the large-scale structure of the cosmos in great detail. It will help by interpreting the observations of Euclid, the European Space Agency satellite launched in 2023 to explore the “dark universe.”

Over the next six years, Euclid will map one third of the sky with unprecedented precision. Its aim is to investigate dark matter and dark energy, the invisible components that together make up about 95% of the energy content of the Universe. By studying the shapes and distribution of billions of galaxies, astronomers hope to uncover how cosmic structures have grown over time and what drives the accelerated expansion of the Universe.

Mock universe

Before Euclid’s real data arrive, researchers need a virtual laboratory to test their methods and models. That is the purpose of the so-called Flagship simulation: a “mock Universe” where galaxies are placed inside a vast web of cosmic matter, following the laws of physics as we know them. With its 3.4 billion galaxies and more than 400 assigned properties, the new Flagship catalogue is the most complete synthetic Universe of its kind, now freely available to the scientific community via the  platform.

Aligned galaxies

Utrecht ľ�ϸ���Ӱ�� researchers have played a role in making this simulation more realistic, focusing on galaxy alignments. “Galaxies do not always appear randomly oriented,” explains Elisa Chisari, associate professor of physics at Utrecht ľ�ϸ���Ӱ��. “They can align with each other due to processes such as mergers, or through tidal interactions on cosmological scales. These alignments affect the measurements that Euclid will use to probe dark matter and dark energy. If we want to interpret the data correctly, our simulations must account for them.”

Galaxies aren't always randomly oriented. Mergers and tidal interactions on cosmological scales can cause them to align differently. We need to account for this effect in our simulations.

Drawing on years of numerical studies, Chisari helped developed a model to ‘paint’ aligned galaxies into the Flagship simulation. This ensures that the mock catalogue not only contains the right number and types of galaxies but also captures the subtle patterns in how they point toward one another. PhD candidate Marloes van Heukelum is continuing this work by helping to implement new alignment information in regions where current observations are scarce.

Public resource

The new Flagship catalogue will be used within the Euclid Consortium to test analysis pipelines, prepare cosmological studies, and refine the strategies for extracting information from Euclid’s observations. As a public resource, it can serve as a reference for many other galaxy surveys, allowing scientists worldwide to study how galaxies form, evolve, and cluster into the cosmic web. Chisari: “We look forward to continuing this collaboration with the Flagship developers, making the mock Universe even more realistic and helping to interpret future Euclid data releases.”