The 1960s saw the discovery of two astrophysical phenomena which quickly accredited the theory of big Bang by Georges Gamow and Georges Lemaitremaking it increasingly untenable standard cosmological model precedent where the observable cosmos was only a fraction of an infinite universe. An eternally expanding universe without beginning or end in which processes of creation of matter gave rise to new galaxies to maintain a constant material density despite the diluting effect of expansion.
These phenomena were quasarswhich are now known to be most likely supermassive black holes of Kerr in rotation accreting a lot of matter, and the cosmic radiation.
The quasars have strong lines ofepisode Lyman-alpha, i.e. an emission of photons in the field ofultraviolet well described by Bohr model of the hydrogen atom which de-excites in a certain way. These emission lines are also produced in the same way by matter heated by the birth of young stars in galaxies.
the red shift from spectrum quasars that are measured with a quantity noted “z” which is all the higher as a quasar is observed far away, therefore early in the history of the observable cosmos, indicates to us according to the law of Hubble-Lemaître that they are mostly located billions oflight years of the Milky Way. We also observe a series of lines ofabsorption in the quasar spectrum. This is the same Lyman-alpha emission line absorbed by matter between a quasar and an instrument on Earth. But as the distances of the quasars vary, we also see lines shifted according to the distance and which ultimately form what are called Lyman-alpha forests.
It is paradoxical, as evidenced by the taking into account at a given moment of an effect discovered by astrophysicists James Gunn and Bruce Peterson in 1965. Indeed, neutral hydrogen known to exist between galaxies should quite quickly block measurable Lyman-alpha radiation by absorbing it. Unless you imagine that part of the hydrogen present is ionized.
For 13.8 billion years, the Universe has continued to evolve. Contrary to what our eyes tell us when we contemplate the sky, what composes it is far from being static. Physicists have observations at different ages of the Universe and carry out simulations in which they replay its formation and its evolution. It would seem that dark matter has played a big role since the beginning of the Universe until the formation of the large structures observed today. © CEA Research
A cosmic reionization
However, the discovery and interpretation of the existence of fossil radiation within the framework of the Big Bang theory and its study based on measurements made by the Planck satellitein particular, tell us that about 380,000 years after the Big Bang, the emission of fossil radiation is precisely due to the formation of hydrogen atoms andhelium neutral, the temperature of the plasma formed fromions and D’electrons leading them by its fall due to the expansion of the cosmos to combine.
Cosmologists therefore came to the conclusion in the few hundred million years after the emission of the cosmic radiation, that something happened that led to the reionization ordinary matter of the observable cosmos.
It is thought to be simply the formation of the first stars in the first galaxies and also theaccretion of matter by the first giant black holes which would have produced the radiation not only leading to what is called the end of the dark ages (at the beginning of which there were no stars yet) but also, at the same time, at reionization (Epoch of Reionization or EoR in English), more than 13 billion years ago.
Cosmologists would like to understand in detail the timing of reionization, because it carries information about the birth of stars and galaxies. Until now, we had only a timid and restricted beginning of access to the end of reionization with telescopes like Hubble, but everything should change once the James Webb telescope will be fully operational in a few months.
There is another radiation that can give us information not only about what happened during the dark ages, but also during reionization. The clouds of neutral hydrogen could in fact emit radiation radio via the famous parting at 21 cm. One should be able to observe, map and study a sort of equivalent of background radiation diffuse fossil but produced this time by the neutral hydrogen clouds of these two periods. Much is expected in this regard from the commissioning of the radio telescopes Square Kilometer Array (SKA).
An excerpt from the Thesan simulation with a core in the past that begins with observations at a spectral shift measured with z which is high and which decreases with the passage of time. We can see both the reionization which progresses in the neutral hydrogen content and the collapse of this hydrogen in galaxies and filaments of clusters of galaxies caused by the collapse of dark matter and which the simulation also takes into account. © Thesan simulations
Simulations to reproduce the history of the early Universe
However, in all cases, a model is needed to interpret the observations, observations which in turn serve to test the hypotheses behind a model. However, it turns out that if we can understand up to a certain point what happened during the few tens of millions of years after the Big Bang by simple analytical calculations with linear approximations of the equations used, this is no longer possible thereafter because it is necessary to deal with the nonlinear regime of these equations and numerical simulations are then essential.
For decades, these simulations have been performed to understand the birth of galaxies and how they coalesce over time into large filamentous structures. Initially, the aim was to describe the effect of gravitation on distributions of black matter alone, because it represents the essential of the mass in the form of matter. But over time, we realized – as well as the rise in power of computers ended up allowing it to be done – that it was also necessary to take into account the fine effects of the behavior of baryonic matter. Thus, a burst of star formation in a young galaxy leads to a burst of supernovae whose blast of explosions can expel from a galaxy the gas baryonic, modifying the distributions of normal matter and therefore by repercussion under the effect of the gravity the distribution of dark matter and also the way in which clouds of matter will accrete on galaxies and cause them to grow.
One of the latest such simulations is called Thesan and it was developed by scientists from MIT, Harvard University and the Max-Planck Institute for Astrophysics. She was named after the Etruscan goddess ofdawn, Thesan, as it is designed to specifically simulate cosmic reionization. As one can be convinced by reading an article on this subject published in Monthly Notices of the Royal Astronomical Societyand of which an open-access version can be found at arXiv, it breaks records of complexity in this respect by finely modeling the production of radiation by stars, the explosions of supernovae and the radiation of supermassive black holes as well as the effects of this radiation on galaxies and the intergalactic medium which also conditions by its material inputs, for example in the form of cold dark matter filamentsthe evolution of galaxies.
The Thesan Simulation was implemented on SuperMUC-NG – one of the largest supercomputers in the world – which simultaneously exploited 60,000 computing cores to perform Thesan calculations on an equivalent of 30 million CPU. Based on a previous simulation called Illustris-TNG which it extends, it breaks a record not only with regard to the extensive consideration of various astrophysical phenomena that have occurred since the emission of the fossil radiation but also with regard to the volume of ‘space-timewhere we finely describe the epoch of reionization (that is to say between 380,000 years and a billion years after the Big Bang approximately), namely a cubic volume of the observable Universe extending over 300 million light-years and in which we follow the appearance and evolution of hundreds of thousands of galaxies.
Another excerpt from the Thesan simulation showing, on the left, the decrease in the volume of neutral hydrogen clouds with the passage of time and which corresponds to observations at decreasing z. On the right, the formation of the number of stars in galaxies increasing, the quantity of ionizing photons in the intergalactic medium also increases with time and decreasing z. © Thesan simulations
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