Small clip of the Thesan simulation. See video in article below.
Named after a goddess of the dawn, the Thesan simulation of the first billion years helps explain how radiation shaped the early universe.
It all started around 13.8 billion years ago with a big, cosmological “bang” that brought the universe suddenly and spectacularly into existence. Shortly after, the infant universe cooled dramatically and went completely dark.
Then, within a couple hundred million years after the
Evolution of simulated properties in the main Thesan run. Time progresses from left to right. The dark matter (top panel) collapse in the cosmic web structure, composed of clumps (haloes) connected by filaments, and the gas (second panel from the top) follows, collapsing to create galaxies. These produce ionizing photons that drive cosmic reionization (third panel from the top), heating up the gas in the process (bottom panel). Credit: Courtesy of THESAN Simulations
With Thesan, the researchers can simulate a cubic volume of the universe spanning 300 million light years across. They run the simulation forward in time to track the first appearance and evolution of hundreds of thousands of galaxies within this space, beginning around 400,000 years after the Big Bang, and through the first billion years.
So far, the simulations align with what few observations astronomers have of the early universe. As more observations are made of this period, for instance with the newly launched
Thesan simulation of gas and radiation evolution shows rendering of the neutral hydrogen gas. Colors represent density and the brightness, revealing the patchy reionization structure within a network of high-density neutral-gas filaments.
Cosmic bridge
With the simulation’s ingredients in place, the team set its initial conditions for around 400,000 years after the Big Bang, based on precision measurements of relic light from the Big Bang. They then evolved these conditions forward in time to simulate a patch of the universe, using the SuperMUC-NG machine — one of the largest supercomputers in the world — which simultaneously harnessed 60,000 computing cores to carry out Thesan’s calculations over an equivalent of 30 million CPU hours (an effort that would have taken 3,500 years to run on a single desktop).
The simulations have produced the most detailed view of cosmic reionization, across the largest volume of space, of any existing simulation. While some simulations model across large distances, they do so at relatively low resolution, while other, more detailed simulations do not span large volumes.
“We are bridging these two approaches: We have both large volume and high resolution,” Vogelsberger emphasizes.
Early analyses of the simulations suggest that towards the end of cosmic reionization, the distance light was able to travel increased more dramatically than scientists had previously assumed.
“Thesan found that light doesn’t travel large distances early in the universe,” Kannan says. “In fact, this distance is very small, and only becomes large at the very end of reionization, increasing by a factor of 10 over just a few hundred million years.”
The researchers also see hints of the type of galaxies responsible for driving reionization. A galaxy’s mass appears to influence reionization, though the team says more observations, taken by James Webb and other observatories, will help to pin down these predominant galaxies.
“There are a lot of moving parts in [modeling cosmic reionization],” Vogelsberger concludes. “When we can put this all together in some kind of machinery and start running it and it produces a dynamic universe, that’s for all of us a pretty rewarding moment.”
Reference: “The thesan project: Lyman-a emission and transmission during the Epoch of Reionization” by A Smith, R Kannan, E Garaldi, M Vogelsberger, R Pakmor, V Springel and L Hernquist, 24 March 2022, Monthly Notices of the Royal Astronomical Society. DOI: 10.1093/mnras/stac713
This research was supported in part by NASA, the National Science Foundation, and the Gauss Center for Supercomputing.
