Astronomers have built the largest simulation of its kind of the universe's first 2 billion years, a virtual cube about 1.5 billion light-years on each side that follows matter, dark matter and radiation together. Called Lumina, it covers more than 100 times the volume of comparable earlier simulations and is the first of its type to track both hydrogen and helium reionization as they unfold, according to the Center for Astrophysics | Harvard & Smithsonian.

The work, described in Monthly Notices of the Royal Astronomical Society, ran on Frontier, the Department of Energy supercomputer at Oak Ridge National Laboratory in Tennessee. At its peak, the calculation used about 15,000 graphics processors and the equivalent of more than 100,000 computer cores.

The timing matters because the James Webb Space Telescope keeps finding early galaxies that look brighter than many astronomers expected. Lumina's sheer size gives researchers huge virtual galaxy populations to compare against those observations, which could help settle whether the bright galaxies fit standard theory or point to something new.

A Virtual Cosmos From the Dark Ages Onward

The simulation begins about 45 million years after the Big Bang, in a dark era before the first stars, and runs through the first 2 billion years. It tracks about half a trillion elements, including dark matter particles and gas cells, at roughly 30 times the resolution of IllustrisTNG, the well-known simulation it builds on, according to MIT News. MIT describes the box as about 500 megaparsecs, or roughly 1.6 billion light-years, across.

That range of scales is the point. Mark Vogelsberger, an MIT physics professor and study author, said "other simulations cannot zoom in as far as this" while also covering such a large volume. The team can follow individual galaxies forming stars while watching radiation reshape gas across the whole box.

Lead author Oliver Zier, a fellow at the Center for Astrophysics, tied the project directly to Webb. "With JWST, the early universe has become an exciting new frontier in astrophysics," he said.

Hydrogen and Helium Reionization in One Run

Reionization is the period when radiation from early sources stripped electrons from atoms that had been neutral since the universe cooled after the Big Bang. Hydrogen reionized first, and helium needed more energetic light. Lumina is the first radiation-hydrodynamics simulation of its kind to follow helium reionization while also tracking its effects on surrounding gas.

In the simulation, light from early galaxies and stars drove hydrogen reionization, while helium reionization was fueled largely by supermassive black holes in their active, feeding phases, MIT reported. That picture matches leading theories, and Lumina now provides a detailed, testable version of it.

Striking Visuals with a Scientific Job

The team released renderings and animations that show the process unfolding. One image spans a region about 650 million light-years across, with fine filaments tracing dark matter in the cosmic web and warm orange and yellow tones marking where gas moves fastest between galaxy clusters.

Other releases include a time-lapse of hydrogen reionization bubbles growing over 2 billion years, a fly-through of a massive halo, and a zoom from the full volume down to a single disk-shaped galaxy. The visuals are more than decoration. They show where ionized bubbles form first and how gas flows between galaxies, which observers can compare with telescope maps.

Size Matters for Rare Galaxies

Volume is what sets Lumina apart for the Webb debate. The brightest early galaxies are rare, and a small simulated box may contain only a handful of them, or none, making it hard to tell whether theory falls short or the sample is simply too small. A box more than 100 times larger yields many more candidates to compare.

MIT co-author Xuejian Shen called the simulation's size one of its major advantages, because it produces very large galaxy populations that can be matched against upcoming sky surveys, MIT News reported. That includes surveys planned with Webb and with future observatories such as NASA's Nancy Grace Roman Space Telescope, which is designed to map wide areas of sky.

The Bright-Galaxy Question Is Still Open

Since 2022, Webb has spotted galaxies in the first few hundred million years after the Big Bang that appear surprisingly bright for their age. Some early studies suggested they might be too massive to fit the standard cosmological model, a claim that drew wide attention.

Other teams have proposed explanations that do not require new physics. A 2023 Northwestern University study using separate simulations found that "bursty" star formation, brief intense bursts followed by quiet periods, can make smaller galaxies glow very brightly. Astronomer Claude-André Faucher-Giguère explained the puzzle simply: "Typically, a galaxy is bright because it's big," he told Astronomy magazine.

Lumina has not yet been compared with Webb's observations, so it does not answer the question. Its starting conditions were set from theoretical expectations, not measurements, and claims that it is the largest simulation of its type come from the research team. The value lies in testing — with enough simulated galaxies to see whether rare, very bright ones appear naturally — and that work is still ahead.

For stargazers and science fans, there is no action to take, but the simulation shapes what the next telescopes will look for. The team plans to compare Lumina with Webb data and upcoming surveys, map early large-scale structure, and predict gravitational-wave signals from early galaxies and black holes. The bottom line is that astronomers now have their most detailed virtual early universe, and its first real test will be whether Webb's bright galaxies show up in it without rewriting the rules.

What Readers Want to Know

What is Lumina?

It is a large cosmological simulation that recreates the universe's first 2 billion years, following matter, dark matter, and radiation together.

How big is it?

It models a cube roughly 1.5 to 1.6 billion light-years on a side, more than 100 times the volume of comparable earlier simulations.

What is reionization?

It is the era when radiation from early stars, galaxies, and black holes stripped electrons from hydrogen and helium atoms across the universe.

Does Lumina explain Webb's bright early galaxies?

Not yet. It has not been compared with Webb's observations, but its large galaxy samples are designed for that test.

Where did the simulation run?

On Frontier, a supercomputer at Oak Ridge National Laboratory in Tennessee, using about 15,000 graphics processors at peak.

What comes next?

The team plans comparisons with Webb and future surveys, plus maps of early structure and predicted gravitational-wave signals.