How the universe lit up after the Big Bang: Lumina simulation

Astronomers have built the biggest simulation yet of the universe’s first two billion years, the time when the first galaxies switched on their lights and cleared away a cosmic fog. It is called Lumina, it ran on Frontier, one of the world’s most powerful supercomputers, and it is described in a paper published on 26 October in the Monthly Notices of the Royal Astronomical Society.

Why it matters: the James Webb Space Telescope is sending back pictures of galaxies from that early era in stunning detail, and wide-field telescopes such as NASA’s Nancy Grace Roman are mapping it on a much larger scale. A simulation this large gives astronomers a “control universe” to compare those pictures with, and to check whether our ideas about how galaxies were born actually hold up.

When the universe cleared its fog

A few hundred thousand years after the Big Bang, the universe had cooled into a dark sea of neutral hydrogen and helium gas. When the first stars, galaxies and hungry black holes lit up, their strong ultraviolet light began to strip electrons from the atoms around them, turning the gas transparent bubble by bubble. Astronomers call this “reionisation”. Think of a foggy morning as the sun comes out: clear patches appear, grow and finally join up.

Hydrogen cleared first. Helium holds on to its last electron much more tightly, so it needed even harder light, mostly from quasars (very bright, feeding black holes), and cleared later. Lumina is the first simulation to follow helium’s clean-up while letting its effects feed straight back into the growing universe, according to the Center for Astrophysics | Harvard & Smithsonian.

A clear glass cube on a white table holds a glowing model of the young universe: blue filaments, white galaxies and pink-rimmed bubbles, next to a yellow lamp and a green plant. — The first galaxies cleared the cosmic fog bubble by bubble, shown here as a glass-cube model · Illustration by DIGITAL WOXTER TECHNEWS.

How big is “biggest”?

The numbers are a little dizzying, so here they are in human terms:

The box: a cube of space about one and a half billion light-years on each side.
The pieces: 2 × 6,000³ resolution elements, which is about 432 billion small parcels of gas and dark matter, each tracked as it moves, heats up and lights up, according to the paper.
The volume: more than 100 times larger than earlier simulations of this kind, says the Center for Astrophysics.
The machine: Frontier at Oak Ridge National Laboratory in the United States, using more than 15,000 graphics processors, about a fifth of the whole supercomputer.

Graphics processors (GPUs) are the same family of chips that draw video games and train AI models. The team rewrote the part of their code that tracks light so it runs on them, and on Frontier that part became about 20 times faster than the earlier version written for ordinary processors, the paper reports.

What Lumina shows

In the simulation, hydrogen reionisation happens fairly late and is driven mostly by starlight: the typical region is fully cleared at a redshift of about 5.2, roughly a billion years after the Big Bang, with a few stubborn neutral patches lingering a little longer. Helium’s second clean-up is powered by the growing black holes and is nearly complete by about two billion years after the Big Bang.

Crucially, the virtual universe passes some real-world tests. Its overall “fogginess”, measured through the faint afterglow of the Big Bang, agrees very well with measurements by the European Space Agency’s Planck satellite, and the temperature history of the gas between galaxies broadly matches observations, the authors write.

“With JWST, the early universe has become an exciting new frontier in astrophysics,” says lead author Oliver Zier of the Center for Astrophysics. His co-author Aaron Smith, of the University of Texas at Dallas, adds that for the first time the team can follow hydrogen and helium as they turn into their modern form.

What happens next

Because the box is so big, it holds rare objects that smaller simulations miss, such as rare bright quasars and the very largest clear bubbles. The team plans to use Lumina to study how gas flows between galaxies, how the Big Bang’s leftover radiation shaped what came after and how regions of intense star birth developed. Companion studies are already using it to look at early black holes and the gas between galaxies. Each new Webb image can now be checked against a universe built from the laws of physics, one parcel of gas at a time.