Supercomputer Simulation Recreates 2 Billion Years of the Early Universe

Researchers at MIT, Harvard University and other institutions have built a supercomputer simulation that follows how the first stars, galaxies and black holes pulled the universe out of its cosmic dark age. The model, called Lumina, covers 2 billion years of early cosmic history across a region about 1.6 billion light-years wide, according to a summary distributed by MIT and published by ScienceDaily.

The team says Lumina is the first simulation to capture two major transformations of the early universe together: the re-ionization of hydrogen and the re-ionization of helium. The work was published in the Monthly Notices of the Royal Astronomical Society.

How the universe slipped into a dark age

The story starts with the Big Bang, when the universe was extremely hot and dense and filled with electrons and other subatomic particles. As it expanded, it cooled. Eventually particles slowed enough for electrons to join atomic nuclei, producing electrically neutral hydrogen.

Around 380,000 years after the Big Bang, the universe entered what scientists call the cosmic dark age. Space had become transparent to much of the leftover radiation from the Big Bang, but no stars existed yet to light it up. Neutral hydrogen filled space and could absorb certain kinds of radiation, including the ultraviolet light that the first stars would later produce.

What Lumina shows about the first stars and black holes

Lumina tracks dark matter, gas, stars and radiation across a region of 500 megaparsecs, which the researchers equate to roughly 1.6 billion light-years. It captures both the behavior of matter over huge distances and the formation of individual galaxies.

In hydrogen re-ionization, ultraviolet light from the first stars and galaxies strips electrons from neutral hydrogen atoms. As more of the gas became ionized, the universe grew more transparent to ultraviolet light, which could then travel much farther. Lumina reproduces that gradual change in detail.

Helium is harder to ionize because it takes much more energetic radiation. The simulation points to different sources for the two changes. Early stars and galaxies appear to have been mainly responsible for ionizing hydrogen, while supermassive black holes that were actively feeding supplied the intense radiation needed for helium.

MIT physics professor Mark Vogelsberger, a study author, said Lumina handles everything from small-scale star and galaxy formation to large-scale re-ionization of hydrogen and helium, in a volume 100 times larger than the best earlier simulations.

How a virtual universe with half a trillion elements was built

The name Lumina comes from a Latin word meaning lights. The project builds on IllustrisTNG, an earlier simulation developed by scientists at MIT and other institutions, including members of the Lumina team. IllustrisTNG models how galaxies form and evolve from the Big Bang to the present day by following dark matter, gas and other physical processes.

To create Lumina, the researchers upgraded that software so it could reproduce galaxy formation in comparable detail across a much larger region. They also added a radiation transport model that calculates how light, including ultraviolet radiation, moves through the universe and interacts with matter. The resulting model contains about half a trillion individual elements, including dark matter particles and cells that represent gas. It offers roughly 30 times the resolution of IllustrisTNG while covering a volume 100 times larger.

Why it matters for understanding the first galaxies

Scientists have few direct ways to see the period when the first stars switched on, which unfolded billions of years before the solar system formed. A detailed simulation gives them predictions to test. The researchers are making Lumina available to other scientists so its predictions about early cosmic structures can be compared with observations from existing and future telescopes.

Such comparisons could help astronomers better understand the events that shaped the first galaxies and stars. For now, the findings come from a computer model, and the model's value will depend on how well its predictions hold up against what telescopes actually observe.

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Around Prague, based on reporting by ScienceDaily