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Thesan-Simulation

A record-breaking look at the early days of the universe – the largest and most accurate simulation of cosmic reionization has been created

Cosmic Time Travel: Astrophysicists have created the largest and most detailed simulation of the early universe to date. It shows for the first time in detail how radiation from the first galaxies changed the universe and led to the re-ionization of interstellar hydrogen gas. Thus, Thesan’s simulations illuminate a crucial phase of turbulence in the universe and at the same time prepare future astronomical observations for this time.

When the first atoms formed about 380,000 years after the Big Bang, the universe initially turned dark and cold. Because there were no stars that could light up the universe. Only when the first stars and galaxies formed after a few hundred million years did the dark phase end and another change was imminent: intense radiation from young stars ionized interstellar hydrogen clouds and thus created important prerequisites for further development.

The complex representation of six properties of the early universe reconstructed in the Thyssen- tian simulations. © Thesan Collaboration /

But when exactly cosmic reionization occurred and how it happened is not yet clear. Previous simulations of the early cosmic days were too harsh or showed sections too small to depict complex interactions and their consequences.

Galaxies, radiation, gas and dust

This has now changed. A team led by Aaron Smith of the Massachusetts Institute of Technology (MIT) has succeeded in creating the most comprehensive and accurate simulation of the early universe to date. The simulation, named “Thesan” after the Etruscan goddess of dawn, shows what happened in the 300 million light-year part of the universe in the first billion years after the Big Bang.

“Thesan acts as a bridge to the beginnings of the universe,” Smith says. The simulation was based on a high-resolution model of galaxy formation Illustris TNG, which traces the evolution of the first star clusters. The researchers supplemented this with a newly developed algorithm that uses physical laws to reconstruct how radiation from stars and galaxies affects the gases in their environment.

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This was supplemented by a reconstruction of the distribution of cosmic dust. Computing simulations in one of the world’s most powerful supercomputers required 30 million CPU hours on 60,000 computing cores at the same time—a typical desktop computer would have taken 3,500 years for this.

Back to the beginnings

The result is a virtual journey through time that shows the evolution of the early universe with unique precision. The time period from the formation of atoms and the release of the cosmic background radiation extends from about 380,000 years after the Big Bang to about a billion years later. For the first time, it became the processes that heralded reionization and thus paved the way for today’s universe.

“Thesan is the first simulation that quantitatively explains how the first galaxies changed the gas around them,” explains co-author Enrico Garaldi of the Max Planck Institute for Astrophysics in Garching. So far, the first results agree relatively well with current models, but they also reveal some previously unknown details.

This is a simulation of the early development of our universe.© Thesan Collaboration

News of light atomization and reionization hot spots

One such result: Previous models assumed that light and other radiation could not have spread very far in the early days of the universe. Only at the end of reionization did the radiation range increase. The Thesan simulation verifies this by measuring the radiation of individual galaxies and groups of galaxies in the respective ultraviolet range, the so-called Lyman alpha radiation similar to.

The simulations reveal that the changes in the radiation range during reionization were more dramatic than previously assumed: “In the early days of the universe, the distances were still very small, but then increased by ten times at the end of reionization within a few hundred million years. Written by author Rahul Kanan of the Harvard-Smithsonian Center for Astrophysics.

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The first results also indicate that the most massive and brightest galaxies of that time were also the impetus for reionization: “The Thesan simulations confirm that large ionized bubbles formed around the brightest relevant galaxies of their age,” the research team wrote.

Addition to future telescope observations

Simulations are also important for future astronomical observations, for example with James Webb Space Telescope. Thanks to its sharp infrared optics, this supposedly dates back to the era of re-ionization for the first time. “This is where our simulations come in: They will help us interpret observations and understand what we’re seeing,” Kinan explains.

And that’s exactly where things get exciting: “If our Thesan simulations and models match what the James Webb Telescope found, this will confirm our ideas about the universe,” says co-author Mark Vogelsberger of MIT. “But when there are significant discrepancies, it shows that our understanding of the early universe is incorrect.” (Monthly Notices of the Royal Astronomical Society, 2022; doi: 10.1093/mnras/stac713)

Source: Harvard-Smithsonian Center for Astrophysics, Massachusetts Institute of Technology (MIT), Max Planck Institute for Astrophysics