Simulation Traces How Stars and Black Holes Cleared the Fog Between Galaxies

Oliver Zier of the Center for Astrophysics | Harvard & Smithsonian and colleagues have published Lumina, a simulation that follows the reionization of hydrogen and helium together in one calculation across a cube 500 comoving megaparsecs on a side. Their paper appeared online Oct. 26, 2026, in Monthly Notices of the Royal Astronomical Society.

Reionization is the stretch of cosmic history in which radiation from the first galaxies stripped electrons off the gas between them. The size of the box is the point, according to the authors. It holds more of the rare, bright objects that sky surveys turn up, and more variety of cosmic environments, so the model's predictions can be set against what telescopes actually find.

The paper reports that in the model hydrogen reionization is late and driven mainly by stars, with the median sub-volume fully ionized by redshift 5.2 and pockets of neutral hydrogen surviving until redshift 4.75. Higher redshifts mean earlier times. Helium reionization, which the model attributes to active galactic nuclei (galaxies with a feeding black hole at the center), is nearly complete by the end of the run at redshift 3.

The run follows more than 400 billion resolution elements on Arepo, a moving-mesh code, coupled to a radiation-transport solver accelerated on graphics processors.

The authors check the run against existing measurements themselves. They report that Lumina's Thomson-scattering optical depth, which measures how strongly free electrons scattered light in the early universe, matches the value from the Planck satellite, and that the intergalactic temperature history is broadly consistent with observational constraints. The galaxy population stays consistent with the earlier IllustrisTNG simulations, whose galaxy-formation model Lumina reuses.

The paper is open access under a Creative Commons Attribution license. The journal record lists the U.S. Department of Energy, NASA, Canada's NSERC and the Simons Foundation among the funders.