Reconstructing an age telescopes are beginning to see

After the Big Bang, the Universe contained hydrogen and helium that blocked part of the light. The first stars, galaxies and active galactic nuclei gradually ionised that medium. This “reionisation” is difficult to model because it connects very local processes with structures spanning hundreds of millions of light-years.

Oliver Zier and colleagues at the Center for Astrophysics | Harvard & Smithsonian, MIT and partner institutions simulated a cube 500 comoving megaparsecs wide. Lumina uses twice 6,000 cubed resolution elements and six radiation frequency bands to follow gravity, gas, galaxy formation, stars and black holes together.

A numerical experiment, not a photograph

Lumina predicts relatively late hydrogen reionisation mainly driven by stars, with neutral pockets persisting. It also models helium ionisation by active galactic nuclei. Its parameters match several existing constraints, including opacity measured by Planck, but that does not prove every detail is correct.

A simulation begins with assumptions about dark matter, star formation, black-hole growth and radiation transfer. Its power is to turn those assumptions into synthetic observations. If James Webb and later the Roman telescope find incompatible populations, the models will have to change.

Why scale matters

A small volume can resolve a galaxy but misses rare objects and large-scale variation. A huge volume often loses small structures. Lumina uses GPUs and adapted hydrodynamic software to reduce that compromise. More than 15,000 GPUs also underline the energy and hardware cost of this science, which future comparisons should document.

The data can guide observations, estimate cosmic variance and distinguish a genuine anomaly from a small sample. They replace neither telescopes nor theory; they connect both.

Conclusion

Lumina is digital scientific infrastructure. Its value is not a final picture of the young Universe, but a catalogue of falsifiable predictions. The possibility of being contradicted by the sky is precisely what makes it powerful.