Lumina Explained: The Simulation That Rebuilt the Universe's First 2 Billion Years

Scientists have built the largest and most detailed simulation of the early universe ever created — a virtual cosmos that follows the first 2 billion years of cosmic history, from a universe filled with neutral gas to one lit up by stars, galaxies and black holes. Called Lumina, the simulation was published on Monday in the Monthly Notices of the Royal Astronomical Society, and it is designed to do something telescopes alone cannot: show us the universe as it actually evolved, not just as it looks in a snapshot.

What Lumina actually simulates

Lumina is a cosmological radiation-hydrodynamics simulation — a model of how the universe's fundamental ingredients interact. It tracks ordinary matter, dark matter and radiation together, across a cosmic region spanning 500 megaparsecs, or about 1.6 billion light-years on a side. According to MIT News, it contains approximately 432 billion resolution elements representing dark matter particles and gas cells, and offers about 30 times the mass resolution of the well-known IllustrisTNG simulation.

The model begins roughly 45 million years after the Big Bang, when the universe was still dominated by neutral gas, and follows structure as gravity gathers matter into filaments and knots — the "cosmic web" — while radiation from the first stars reshapes the gas between galaxies.

The two transformations it captures for the first time

Over the 2 billion years Lumina simulates, it captures two huge transformations. The first is hydrogen reionization: early stars produced enough ultraviolet light to strip electrons from neutral hydrogen, letting light travel freely and lifting the veil that had hidden the young universe. The second is helium reionization, driven by much more energetic light from early supermassive black holes in their active, accreting phases.

Lumina is the first simulation to capture both processes together, while letting each one immediately influence the evolving universe inside the model. Its headline finding is a division of labour: hydrogen reionization was likely driven by light from early galaxies and stars, while helium reionization was fuelled by those early black holes. That distinction matters because it tells astronomers which objects to look for when they interpret what the James Webb Space Telescope sees.

Why the scale changes the science

Previous simulations faced a trade-off: model a large volume coarsely, or a small volume in detail. Lumina breaks it. Its volume is 100 times larger than previous best comparable simulations, according to MIT professor Mark Vogelsberger, an author of the study — yet it still resolves individual galaxies. The Smithsonian's account puts it the same way: the largest cosmological radiation-hydrodynamics simulation ever created, covering more than 100 times the volume of earlier comparable work.

Building it took serious computing power: more than 15,000 graphics processing units running on Frontier, one of the world's fastest supercomputers, at Oak Ridge National Laboratory in Tennessee. The project brings together researchers from MIT, the Harvard and Smithsonian Center for Astrophysics, the University of Texas at Dallas, York University and the Max Planck Institute for Astrophysics in Germany.

What it means for what telescopes will find next

Lumina is not just a pretty picture of the cosmic web. The team is making it available to the scientific community so researchers can compare its detailed predictions against snapshots from Webb and the upcoming Nancy Grace Roman Space Telescope. Where the simulation and the telescopes agree, the underlying physics is confirmed. Where they disagree, as the Smithsonian notes, scientists gain opportunities to uncover new insights about how the universe works.

That is the real point of a simulation like this. Telescopes show us the early universe in scattered, frozen moments. Lumina connects those moments into a continuous story — the first 2 billion years, played forward, with the physics included. For a field that has just begun to see the cosmic dawn clearly, that is less a luxury than a necessity.