The neutral hydrogen gas that filled the universe during the first few hundred million years after the Big Bang absorbed ambient light, leading to what cosmologists and science fiction authors poetically call the “dark ages,” although the cosmos was filled with a diffuse ambient light from the cosmic microwave background (CMB) — the “afterglow” of the Big Bang.
During reionization, when the universe transitioned out of the ‘dark ages’, the space between galaxies was filled with a plasma of ionized hydrogen with a temperature of about 10,000˚C. This is puzzling because fifty million years after the big bang, the universe was cold and dark, with temperatures only a few degrees above absolute zero, and no luminous stars and galaxies.
How is it then, astronomers ask, that today, about 13.6 billion years later, the universe is bathed in light from stars in a variety of galaxies, and the gas is a thousand times hotter?
Answering this question has been an important goal of cosmological research over the last two decades, leading to the conclusion of a new study suggests that reionization occurred 12.7 billion years ago, quite a bit later than previously thought.
Large differences in the ‘fogginess’ of the early universe were caused by islands of cold gas left behind when the universe heated up after the big bang, according to an international team of astronomers. The results, reported in the Monthly Notices of the Royal Astronomical Society, have enabled astronomers to zero in on the time when reionization ended and the universe emerged from a cold and dark state to become what it is today: full of hot and ionized hydrogen gas permeating the space between luminous galaxies.
Image below: Reionization as illustrated by data from the Hubble and Chandra space telescopes. NASA/CXC/M.Weiss
Hydrogen gas dims light from distant galaxies much like streetlights are dimmed by fog on a winter morning. By observing this dimming in the spectra of a special type of bright galaxies, called quasars, astronomers can study conditions in the early universe.
In the last few years, observations of this specific dimming pattern (called the Lyman-alpha Forest) suggested that the fogginess of the universe varies significantly from one part of the universe to another, but the reason behind these variations was unknown.
“We expected the light from quasars to vary from place to place at most by a factor of two at this time, but it is seen to vary by a factor of about 500,” said lead author Girish Kulkarni, who completed the research while a postdoctoral researcher at the University of Cambridge. “Some hypotheses were put forward for why this is so, but none were satisfactory.”
The new study concludes that these variations result from large regions full of cold hydrogen gas present in the universe when it was just one billion years old, a result which enables researchers to pinpoint when reionization ended.
The team of researchers from India, the UK, Canada, Germany, and France drew their conclusions with the help of state-of-the-art computer simulations performed on supercomputers based at the Universities of Cambridge, Durham, and Paris, funded by the UK Science and Technology Facilities Council (STFC) and the Partnership for Advanced Computing in Europe (PRACE).
“When the universe was 1.1 billion years old there were still large pockets of the cosmos where the gas between galaxies was still cold and it is these neutral islands of cold gas that explain the puzzling 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 pinpoint the end of reionization much more accurately than before,” said Laura Keating of the Canadian Institute of Theoretical Astrophysics.
The study suggests that the universe was reionized by light from young stars in the first galaxies to form. “Late reionization is also good news for future experiments that aim to detect the neutral hydrogen from 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.”
One such experiment is the ten-nation Square Kilometer Array (SKA) of which Canada, France, India, and the UK are members.
Image at the top of the page shows clutter of both distant galaxies and foregrounded stars in the Milky Way. Photo: CFHT, Pierre-Alain Duc (Obs. De Strasbourg) & Jean-Charles Cuillandre (CEA Saclay/Obs. De Paris).
The Daily Galaxy, Sam Cabot via C. Crewes and University of Cambridge