The Lumina simulation is the largest computer model yet of the universe’s first 2 billion years, according to researchers at the Center for Astrophysics | Harvard & Smithsonian (CfA). Described in a study in the Monthly Notices of the Royal Astronomical Society and announced by the Smithsonian on 25 October 2026, the Lumina simulation recreates how ordinary matter, dark matter and radiation interacted as the early cosmos moved from darkness to light. The team says it is the largest cosmological radiation-hydrodynamics simulation ever created.
What the Lumina simulation models
A radiation-hydrodynamics simulation tracks how gas, dark matter and light interact to form stars and galaxies. Lumina follows the first 2 billion years of cosmic history, tracing how the earliest galaxies emerged and how their light transformed the vast clouds of gas between them.
That transformation is called reionization. Shortly after the Big Bang, the gas filling space was neutral, and the universe was dark. As the first stars, galaxies and black holes switched on, their radiation stripped electrons from atoms. Hydrogen changed first. Helium followed later, driven mainly by bright quasars powered by growing black holes.
According to the Smithsonian release, Lumina is the first simulation to capture helium reionization as it happened while letting its effects immediately influence the evolving universe inside the model.
How big is the Lumina simulation?
The simulated region is a cube roughly 1.5 billion light-years on each side. The team says that is more than 100 times the volume of space modelled by previous comparable simulations, and comparable to the scale astronomers can observe with today’s most powerful telescopes.
The project’s technical paper describes a box 500 comoving megaparsecs across, filled with 2 × 6,000³ resolution elements, or nearly half a trillion. It combines the IllustrisTNG galaxy-formation model with a GPU-accelerated radiation-transport solver inside the moving-mesh code AREPO.
Running it took more than 15,000 graphics processing units on Frontier, one of the world’s fastest supercomputers, at Oak Ridge National Laboratory in Tennessee. The paper lists 1,884 compute nodes, with 105,504 CPU cores and 7,536 AMD Instinct MI250X accelerators, about 20% of the machine. Peak memory use reached roughly 600 terabytes, and each full checkpoint took up about 250 terabytes.
Why it matters for JWST and Roman
The James Webb Space Telescope (JWST) and the Nancy Grace Roman Space Telescope are revealing the universe’s earliest chapters in detail. Lumina is meant to help astronomers interpret those observations. By comparing what the telescopes see with what the simulation predicts, scientists can check whether their understanding of how the cosmos evolved still holds, or whether new findings call for a rethink.
“With JWST, the early universe has become an exciting new frontier in astrophysics,” said lead author Oliver Zier, an ITC Fellow at the CfA. “To understand what we are seeing with this telescope, we need to translate our theories into detailed predictions. After several years of work, with plenty of ups and downs, it is incredibly rewarding to see the simulation results and watch our predictions come to life in video.”
What Lumina predicts
According to the project paper, Lumina points to a late hydrogen reionization driven mostly by stars. In the simulation, a typical region is fully ionized by a redshift of about 5.2, with leftover neutral patches lasting until about 4.75. Helium reionization, driven by active galactic nuclei, is nearly complete by a redshift of 3.
The authors report that the results agree well with measurements from the Planck satellite and are broadly consistent with observations of the gas between galaxies. They add that the much larger volume improves statistics for rare objects such as quasars and for large-scale environments.
The authors include researchers from the CfA, MIT, the Max Planck Institute for Astrophysics, the University of Texas at Dallas and York University.
My take
I find it striking that a project like this is now as much an engineering feat as a scientific one. Its real test, I feel, will come as JWST and Roman data pile up and either match Lumina’s predictions or break them.
FAQ
What is the Lumina simulation?
Lumina is a CfA-led radiation-hydrodynamics simulation of the universe’s first 2 billion years. It models how gas, dark matter and light interacted as the first galaxies formed and reionized the cosmos.
How much computing power did Lumina use?
It ran on more than 15,000 GPUs on the Frontier supercomputer at Oak Ridge National Laboratory. That was about 20% of the machine, using 1,884 nodes.
What makes Lumina different from earlier simulations?
It covers more than 100 times the volume of previous comparable simulations, a cube about 1.5 billion light-years wide. It is also the first to capture helium reionization as it happened, with its effects fed back into the model.