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The Big Bang captured our imagination like no other scientific theory: the magnificent and explosive birth of our universe. But do you know what happened next?
About 100 million years of darkness.
When the cosmos finally lit up its very first stars, they were bigger and brighter than the ones that followed. They shone with a UV light so intense that it transformed the surrounding atoms into ions. The cosmic dawn – from the first star to the completion of this "cosmic reionization" lasted about a billion years.
"Where do these stars come from? And how did they become galaxies – the Universe teeming with radiation and plasma – that we see today? These are our driving issues," says the Professor Michael Norman, director of the San Diego Supercomputer Center and lead author of a new journal published in Frontiers in Astronomy and Space Science.
The universe in a box
Researchers like Professor Norman solve mathematical equations in a cubic virtual universe.
"We have spent over 20 years using and perfecting this software to better understand the Cosmic Dawn."
To start, a code was created that allowed to model the formation of the first stars of the universe. These equations describe the movement and chemical reactions inside gas clouds in a universe before the light, as well as the immense gravitational attraction of a much larger but invisible mass, mysterious dark matter.
"These pure hydrogen and helium clouds have collapsed under the effect of gravity to ignite single, massive stars, hundreds of times heavier than our Sun," says Norman. .
The very first heavy elements were formed in the pressure cooker cores of the first stars: a little lithium and beryllium. But with the death of these ephemeral giants – collapsing and exploding dazzling supernova – metals as heavy as iron were created in abundance and projected into space.
Equations have been added to the virtual universe to model the enrichment of gas clouds with these newly formed metals, which resulted in the formation of a new type of star.
"The transition was fast: in 30 million years, virtually all new stars were fortified with metal."
This despite the fact that the chemical enrichment was local and slow, leaving more than 80% of the virtual universe without metal at the end of the simulation.
"The formation of giant, metal-free stars has not entirely stopped – small galaxies of these stars should exist where there is enough dark matter to cool immaculate hydrogen clouds." and helium.
"But without this enormous gravitational appeal, the intense radiation of existing stars warms the gas clouds and tears their molecules, so in most cases, the metal-free gas collapses completely to form a single black hole. supermassive. "
From stars to galaxies
"The new generations of stars that have formed in galaxies are smaller and much more numerous, because of the chemical reactions made possible by metals," observes Norman.
The increased number of reactions in the gas clouds has allowed them to fragment and form multiple stars via "metal line cooling": reduced gas density ranges, where the combined elements gain room for radiate their energy into space – instead of the other.
At this point, we have the first objects in the universe that we can legitimately call galaxies: a combination of dark matter, metal-enriched gas and stars.
"The first galaxies are smaller than expected because the intense radiation emitted by massive young stars drives dense gases from star-forming regions.
"In turn, the radiation of the smaller galaxies has contributed significantly to cosmic reionization."
These galaxies, difficult to detect but numerous, can therefore explain the expected end date of cosmic dawn – that is, when the cosmic reionization was over.
Go off the beaten track
Norman and his colleagues explain how some groups overcome the computational limitations of these numerical simulations by either importing their results directly or by simplifying parts of a model less relevant to the results we are interested in.
"These semi-analytical methods have been used to more accurately determine the length of time that early metal-free stars have been created, how many have yet to be observed, and the contribution of these – as well as black holes and stars enriched in metal – – to cosmic reionization ".
The authors also highlight the areas of uncertainty that will drive a new generation of simulations, using new codes, on future high-performance computing platforms.
"This will help us understand the role of magnetic fields, X-rays and space dust in gas cooling, as well as the identity and behavior of the mysterious dark matter at the origin of the formation of # 39; stars ".
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