Could galaxies straight out of a computer fool even the most seasoned astronomers? That is the case with the COLIBRE simulation, whose virtual galaxies are so realistic they are indistinguishable from those photographed by the James Webb Space Telescope.
This computational feat relies on the standard model of cosmology, known as Lambda Cold Dark Matter (ΛCDM or LCDM). Researchers ran the COSMA8 supercomputer at Durham University to model galaxy formation from cold gas and dust, from the first billion years after the Big Bang to the present day. The result is an unprecedented reproduction of the observable universe.
On the left, the cosmic web where color encodes the projected density of gas and stars. On the right, two of the many galaxies formed in the simulations, seen face-on (top) and edge-on (bottom).
Credit: Schaye et al. (2026) COLIBRE's major innovation is its ability to simulate cold gas and dust, key elements in star formation. Previous simulations ignored this essential component. Today, scientists accurately reproduce the number, brightness, color, and size of real galaxies, thus validating our understanding of cosmic evolution.
"It's thrilling to see galaxies emerge from our computer and look strikingly like the real ones," enthuses Carlos Frenk, a member of the team. He likes to tease his observing colleagues by asking them to guess which image comes from the sky and which from the simulation. All of this, simply by solving the equations of physics applied to cosmic expansion.
However, even this highly advanced simulation stumbles upon an anomaly: the "little red dots" observed by James Webb. These objects appear in abundance 600 million years after the Big Bang and then vanish when the universe is about 1.5 billion years old. They may be seeds of supermassive black holes, but COLIBRE does not yet reproduce them.
The results of COLIBRE were published in the journal
Monthly Notices of the Royal Astronomical Society. Although most simulations were completed in 2025, some are ongoing. The data already collected will take years to analyze, paving the way for future discoveries about the birth and evolution of galaxies.
The five COLIBRE cubic boxes, with sides measuring from approximately 81.5 million to 1.304 billion light-years (25 to 400 cMpc). The color indicates the total surface density (on faces 16.3 million light-years (5 Mpc) thick).
Available volumes at high (m5), medium (m6), and low (m7) resolution are indicated. The standard model of cosmology (ΛCDM)
The ΛCDM model, for Lambda Cold Dark Matter, is the dominant theory describing the universe. It assumes the universe is composed of about 5% ordinary matter, 27% cold dark matter, and 68% dark energy (represented by the cosmological constant Lambda 'Λ'). This model successfully explains the accelerated expansion of the universe, the formation of large structures such as galaxies, and the cosmic microwave background radiation from the Big Bang. COLIBRE uses this framework to simulate cosmic evolution over 13 billion years.
Cold dark matter, invisible but detectable by its gravitational effects, plays a central role. It served as the "skeleton" for the formation of the first galaxies. Dark energy, on the other hand, accelerates the expansion of the universe. The ΛCDM model has been repeatedly confirmed by observations, but questions remain, particularly about the exact nature of dark matter and dark energy. Simulations like COLIBRE help test its predictions.
By faithfully reproducing the properties of observed galaxies, COLIBRE provides a new validation of the ΛCDM model. However, the inability to explain the "little red dots" could indicate gaps in our understanding, such as processes related to primordial black holes. Cosmologists therefore continue to refine the model using supercomputers.
The "little red dots" and black hole seeds
The "little red dots" are compact, red objects observed by the James Webb Space Telescope in the young universe, about 600 million years after the Big Bang. They appear in large numbers for a short period, then disappear completely when the universe reaches about 1.5 billion years old. Their exact nature is unknown, but one hypothesis suggests they are seeds of forming supermassive black holes.
These seeds would be primordial black holes, born from the direct collapse of massive gas clouds. They could then grow rapidly to become the supermassive black holes we observe at the centers of galaxies today. Their red color would come from dust obscuring their light, and their apparent small size would be due to their distance. But other explanations exist, such as very dusty dwarf galaxies.
The COLIBRE simulations cannot reproduce these objects, indicating that current models of galaxy and black hole formation are incomplete. To solve this puzzle, researchers may need to include additional physical processes, such as black hole feedback or more intense star formation.