Variations in the "fog" of the universe mark a turning point in cosmic history

According to an international team of astronomers, large differences in the "fog" of the primitive universe were caused by islands of cold gas left behind when the universe warmed up after the big Bang.

The results, reported in the Monthly Notices from the Royal Astronomical Societyallowed astronomers to focus on the moment when reionization ended and the universe emerged from a cold, dark state to what it is today: full of warm, ionized hydrogen gas permeating space between the bright galaxies.

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.

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 quasar light to vary from place to place by a factor of two at a time, but it is expected to vary by about a factor of 500," said lead author Girish Kulkarni. who completed the search for a while. postdoctoral researcher at the University of Cambridge. "Some hypotheses have been advanced to explain this, but none was satisfactory."

The new study concludes that these variations result from vast regions full of 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.

During the reionization, when the universe came out of the cosmic "dark ages", the space between the galaxies was filled with an ionized hydrogen plasma at a temperature of about 10,000 ° C. It 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.

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 finally allows us to determine the end of reionization much more accurately than before," 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 young stars in 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 Basic 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.