What did the universe look like when the first stars switched on? A new computer simulation called Lumina offers the most detailed answer so far. Physicists from MIT, Harvard and several other institutions presented it on October 26 in the Monthly Notices of the Royal Astronomical Society.
Lumina follows the young cosmos from its so-called dark age through roughly the next two billion years. About 380,000 years after the Big Bang, the universe was filled with neutral hydrogen that blocked light like a thick fog. The simulation shows how gas and dark matter clumped together, how the first galaxies and stars formed inside these clumps, and how their radiation slowly cleared the fog.
For the first time, a single simulation captures two great turning points: hydrogen reionization, driven by light from early galaxies and stars, and helium reionization, which needed far more energetic radiation from growing supermassive black holes.
The numbers are impressive. Lumina models a patch of space about 1.6 billion light-years wide, a volume 100 times larger than previous best simulations, with around half a trillion elements such as dark matter particles and gas cells. It ran on Frontier, one of the world’s most powerful supercomputers, at Oak Ridge National Laboratory, using the equivalent of more than 100,000 CPU cores and about 15,000 GPUs at peak. The code builds on IllustrisTNG, an earlier galaxy simulation, refined to keep the same level of detail across a far larger volume and combined with a model of how radiation travels through space.
The team is sharing Lumina with other scientists so they can compare its virtual galaxies with real observations from new telescopes. It may also help predict the gravitational-wave signals that early black holes could have produced. In short, the cosmic dark age is getting a lot brighter.
