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New record-breaking simulator sheds light on “Space Dawn”

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New record-breaking simulator sheds light on

Much remains a mystery about the first billion years of the universe’s history, an era in which the cosmos emerged from its dark ages with the dawn of the earliest stars and galaxies. Now scientists have developed the largest and most detailed computer model of this period to date to help shed light on how the children’s universe evolved. Named THESEafter the Etruscan goddess of dawn, predictions of this new project about the primitive past will soon be verified by data from the recently launched NASA James Webb Space Telescope (JWST) and other next-generation observatories.

Immediately after the Big Bang, about 13.8 billion years ago, the universe was filled with cosmic fog. The heat of creation was so great that electrons could not combine with protons and neutrons to form atoms, and instead the space was filled with a dense soup of plasma – electrically charged (or ionized) particles that scattered rather than transmitted light. This cosmic fog briefly rose about 380,000 years later, during the so-called recombination era, when the universe cooled enough for atoms to freeze out of plasma in the form of clouds of optically transparent electrically neutral gaseous hydrogen. Suddenly released, the light after the glow of the Big Bang flashed across the universe, which then disappeared back into darkness because the stars had not yet formed.

Darkness reigned for the next few hundred million years until gravity began to connect matter into stars and galaxies. Even then, the darkness only gradually dissipated when intense ultraviolet radiation from the first luminous objects of the universe reionated the surrounding neutral hydrogen, eventually quenching gaseous darkness. This “era of reionization” lasted more than half a billion years, but scientists know little about its details. They know for sure that its end marked a cosmic moment when light from the entire electromagnetic spectrum – rather than the simple part that could pierce the neutral hydrogen curtain – began to travel freely in space. Simply put, this was when the universe finally became clear for the study of curious astronomers who sought to know exactly how the cosmic dawn took place.

This is not to say that such research is easy. To see light from such ancient times, researchers must use the largest and most sensitive telescopes to look for objects that are as far away as possible. This is because the greater the distance to the object, the longer it takes light to reach the Earth, and the weaker this light will be.

Computational space dawn

Another way to get an idea of ​​this bygone era is to simulate it on a computer. The early stages of reionization were relatively easy to rebuild because the universe was then relatively dark and monotonous, explains Aaron Smith, an astrophysicist at the Massachusetts Institute of Technology who helped develop THESAN. However, as primary matter breaks down into galaxies and stars, it becomes increasingly difficult to model the complex interactions between gravity, light, gas, and dust.

“Because modeling light is quite complex and computationally expensive, there are only a few cosmological simulations that focus on studying this era,” says astrophysicist Rahul Kannan of the Harvard-Smithsonian Center for Astrophysics, which helped develop THESAN. “Each of these cosmological simulations has its advantages and disadvantages.”

THESE designed to model the early universe to an unprecedented degree. Some cosmological simulations such as Space Dawn (CoDa) simulation and Space reionization on the computer (STEP), simulated large volumes at relatively low resolution, while others, e.g. Renaissance and SPHINX simulations, more detailed but not covering long distances. In contrast, THESAN “combines high resolution with large simulated volumes,” Canan says.

“There’s usually a trade-off between a detailed study of galaxy formation and space reionization, but THESAN can do both,” said astrophysicist John Wise of the Georgia Institute of Technology, who has not worked on THESAN.

The developers of THESAN built it based on an old simulation series called Illustris-TNG, which has been shown to accurately simulate many properties and populations of evolving galaxies. They then developed a new algorithm to simulate how the light of stars and galaxies interacted with surrounding gas and reionized them during the first billion years of the universe – details that previous simulations had not successfully incorporated on a large scale. Finally, the THESAN team included a model of how space dust in the early universe could have affected galaxy formation.

“They’ve combined the two most modern models and added a little more – it looks really interesting,” says Risa Wexler, a cosmologist at Stanford University and director of the Cowley Institute of Astrophysics and Cosmology, who didn’t. take part in THESAN.

Scale up

THESAN can track the birth and evolution of hundreds of thousands of galaxies in a cubic volume larger than 300 million light-years. Starting about 400,000 years after the Big Bang – before the first stars are thought to have appeared – the simulation extrapolates through the first billion years of space history. To do all this, THESAN is working on one of the largest supercomputers in the world, SuperMUC-NG, which used nearly 60,000 computer processing cores to perform simulation calculations over the equivalent of 30 million CPU hours. (For the future, this same computational feat will require 3,500 years of allocating numbers on a typical desktop computer.)

A THESAN simulation rendering that shows how stars and galaxies in the early universe interact with surrounding gas clouds and reionize them to create the familiar space structures we see today.

“One of the most interesting things about THESAN modeling for me is increasing the resolution,” says astrophysicist Brian Welch of Johns Hopkins University, who hasn’t worked on THESAN. “They seem to be able to connect small-scale structures within galaxies that create ionizing photons with a larger-scale intergalactic environment where these photons drive the era of reionization. Simulations can help determine how ionizing photons erupt from galaxies and thus how these galaxies stimulate reionization. ”

Using the Hubble Space Telescope, Welch and colleagues recently discovered the most distant single star ever discovered, Earendel, which dates back to a time when the universe was only 900 million years old. Although THESAN cannot model individual stars such as Earendel, “because it will require excessive computing power,” it can still shed light on the conditions in the galaxies in which Earendel and his compatriots formed, he says.

Researchers say THESAN is already making predictions about the early universe. For example, this suggests that the distance traveled by light increased more sharply at the end of zionization than previously thought – 10 times in several hundred million years – probably because dense gas pockets, which took longer to ionize, were missed. previous lower -resolution simulation.

However, one of the disadvantages of THESAN is that it uses a relatively simplified model of cold dense gas in galaxies, Cannan says. The THESAN team is currently working on a next project called THESAN-ZOOMS to replace this much more complex model, which takes into account many additional physical processes that affect the properties of this dense gas, ”he said.

Another drawback of THESAN is that the volume it simulates may be too small to properly identify key details about how the early universe evolved, such as the size and number of pockets of ionized transparent gas, Canan says. Scientists now plan to increase the amount of simulation by 64 times through a diverse set of optimizations designed to improve its overall performance, he says.

Expectations against reality

Whether any of these shortcomings will significantly affect THESAN’s predictions will soon be revealed through recent observations from JWST, which is designed to observe the first stars and galaxies. Will the stars and galaxies that unite in THESAN virtual space reflect populations of ancient objects visible to JWST optics? Researchers are eager to find out. Models of weak galaxies in the early universe are very sensitive to the uncertainty of phenomena such as star formation, “which remain highly debated,” said Aaron Jung, an astrophysicist-theorist at NASA’s Goddard Space Flight Center who has not worked on THESAN. Simulations that can successfully model known galaxies, “can give different predictions in weak populations. [JWST] will discover these galaxies for the first time and provide limitations for the physics that drives the formation of these galaxies. ”

By the end of this year, JWST will be able to gather enough data to test THESAN when it comes to numerous predictions of galaxy properties, Smith says. “We are already working with astronomers involved in JWST to interpret the data that will be available this year.”

“My intuition tells me that JWST will fit the statistics of bright galaxies modeled in CoDa, CROC and THESAN,” says Wise, who helped develop Renaissance simulations. “However, they do not have enough separation to simulate small-mass galaxies and small galaxies where Renaissance and SPHINX will fit better.” Astrophysicists, he argues, are likely to use a combination of both types of modeling to interpret JWST observations of ancient galaxies.

No one expects THESAN or any other Zionist-era simulation to get it all right. “Most, if not all, of the simulations made in this era lack a piece of physics – although THESAN has a fairly high resolution, it is still low resolution compared to the physical processes that take place on its own. in fact, ”says Wexler. “Progress occurs when data from observatories and simulation ideas work in concert. This interaction is fascinating. “

Ultimately, “we will need more than JWST to confirm the full picture of space evolution in the early universe,” Smith says. “Understanding different aspects of this era requires different tools that cover a wide range of wavelengths.” These include the Hydrogen Reionization Array (HERA), the Square Kilometer Array (SKA), the Fred Young Submillimeter Telescope (FYST), the Spectrophotometer for the History of the Universe, the Rheonization Age and Ice Explorer (SPHEREx), and the next flagship telephony Astra Grace. Ambitious computer models, such as THESAN, can ultimately help scientists understand the data flow that these projects will bring.

“THESAN seeks to make predictions for as many of these observations as possible,” Smith notes. “Discrepancies with data are often just as fascinating because it tells us about the lack of our models, forcing us to rethink the basic physics of these complex processes.”

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