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Posted on Apr. 16, 2019
The hydrogen neutral gas that filled the universe in the first several hundred million years after the Big Bang tended to absorb the ambient light, leading cosmologists and science fiction writers to poetically call the "dark ages" of the universe. Ambient light from the Cosmic Microwave Background (CMB) – what is called the Big Bang remanence – this neutral gas absorbed at specific wavelengths.
During reionization, when the universe came out of the "dark age," the space between the galaxies was filled with an ionized hydrogen plasma at a temperature of about 10,000 ° C. is amazing, because fifty million years after the big bang, the universe was cold and dark. It contained gases whose temperature was only a few degrees above absolute zero, without stars or luminous galaxies. How is it that today, about 13.6 billion years later, the universe is bathed in the starlight of various galaxies and the gas is a thousand times hotter?
Answering this question has been an important goal of cosmological research over the past two decades. The findings of the new study suggest that reionization took place 1.1 billion years after the big bang (ie, 12.7 billion years ago), a little later than expected.
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According to an international team of astronomers, large differences in the "fog" of the primitive universe were caused by islands of cold gas left when the universe warmed up after the big bang. The findings, reported in the Royal Astronomical Society's monthly reports, allowed astronomers to focus on when reionization ended and where the universe emerged from a cold, dark state to become what it was. It is today: full of hot and ionized hydrogen gas permeating the space between the bright galaxies.
Image at the top of the page: Reionization illustrated by Hubble and Chandra space telescope data. NASA / CXC / M.Weiss
Hydrogen gas attenuates the light of distant galaxies, much like streetlights are mitigated by the fog of winter mornings. By observing this attenuation in the spectra of a special type of bright galaxy, called quasars, astronomers can study the conditions that prevail in the primitive universe.
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In recent years, observations of this specific gradation model (called the Lyman-alpha forest) have suggested that the nebulosity of the universe varied significantly from part of the universe to another, but the reason for these variations was unknown.
"We expected the light emitted by the quasars to vary from place to place by a factor of two at a time, but it is assumed to vary by about a factor of 500," said lead author Girish Kulkarni, who completed the research during postdoctoral researcher at the University of Cambridge. "Some hypotheses have been put forward to explain why, but none was satisfactory."
The new study concludes that these variations result from vast regions filled with cold gaseous hydrogen present in the universe at the age of one billion years, a result that allows researchers to accurately determine the end of the reionization.
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The team of researchers from India, the United Kingdom, Canada, Germany and France drew their conclusions using advanced computer simulations made on supercomputers based at the Universities of Cambridge, Durham and Paris, funded by The British Council for Scientific and Technological Facilities (STFC) and the Partnership for Advanced Informatics in Europe (PRACE).
"When the Universe was 1.1 billion years old, there were still large pockets of the cosmos where the gas between the galaxies was still cold and it is these neutral cold gas islands that explain these confusing observations." said Martin Haehnelt of the University of Cambridge. who led the group that conducted this research, supported by funding from the European Research Council (ERC).
"This allows us to finally determine the end of reionization with much more precision than previously," said Laura Keating of the Canadian Institute for Theoretical Astrophysics.
The new study suggests that the universe has been re-ionized by the light of the young stars of the first galaxies to form. "The late re-ionization is also good news for future experiments aimed at detecting neutral hydrogen in the early universe," said Kulkarni, who is now based at the Tata Institute of Fundamental Research in India. "The later the reionization, the easier it will be for these experiments to succeed."
The Kilometer Square (SKA), composed of ten countries, including Canada, France, India and the United Kingdom are members, is one of these experiences.
The image at the top of the page shows the clutter of distant galaxies and stars in the foreground of the Milky Way. Photo: CFHT, Pierre-Alain Duc (Strasbourg Observer) and Jean-Charles Cuillandre (CEA Saclay / Obs de Paris).
The Daily Galaxy via the University of Cambridge
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