The Big Bang created a hot, dense soup of electrons and sub‑atomic particles that expanded and cooled rapidly. About $3.8\times10^5$ years later neutral hydrogen formed, shielding light and ushering in the cosmic dark age. Recently, physicists from MIT, Harvard and others published Lumina in the Monthly Notices of the Royal Astronomical Society. Lumina is a large‑volume, high‑resolution simulation covering 500 Mpc (≈1.6 billion light‑years) that follows the universe from the dark ages through the next two billion years. It tracks dark matter, gas, stars and photons, linking the largest cosmic scales to the details of individual galaxies.
During the simulated two‑billion‑year span, Lumina captures two pivotal re‑ionization events. Hydrogen re‑ionization is driven by ultraviolet radiation from early galaxies and stars, stripping electrons from neutral hydrogen and allowing light to travel freely. Helium re‑ionization requires higher‑energy photons, primarily supplied by accreting super‑massive black holes. This is the first simulation to resolve both hydrogen and helium re‑ionization in a single framework.
Lumina builds on the IllustrisTNG code base, which was upgraded to retain galaxy‑formation detail across a 500 Mpc volume. The run contains roughly $5\times10^{14}$ dark‑matter particles and gas cells, giving a resolution about thirty times finer than IllustrisTNG. Calculations were performed on the Frontier supercomputer at Oak Ridge National Laboratory, peaking at over 100 000 CPU cores and 15 000 GPUs, a remarkable feat of computer engineering.
The simulation starts around 45 million years after the Big Bang, when neutral hydrogen dominates. Initial conditions follow theoretical expectations for that epoch. As the run progresses, gas collapses into dark‑matter haloes, forming the first galaxies; stars ignite, emit radiation, and gradually ionize surrounding hydrogen. The project website hosts several videos that zoom from the full cosmological volume down to a single disk galaxy and show a time‑lapse of hydrogen re‑ionization across two billion years. Compared with previous efforts, Lumina spans an unprecedented range of scales.
Looking ahead, Lumina will be used to map the large‑scale structure of the early universe, predict gravitational‑wave signatures from early galaxies and black holes, and compare its outputs with observations from upcoming telescopes. Its massive synthetic galaxy population should enable direct comparisons with the huge samples expected from future surveys.
Review