Having a chronology for this massive transition would go a long way toward helping us comprehend the development of the Universe, but our best efforts so far have been hazy guesses based on low-quality evidence.
The light from dozens of distant objects known as quasars was used by an international team of astronomers led by the Max Planck Institute for Astronomy in Germany to remove uncertainties, revealing that the last major wisps of hydrogen 'fog' burned away far later than we first thought, more than a billion years after the Big Bang.
The first 380,000 years were characterized by a static hiss of subatomic particles congealing out of the expanding space-cooling time's vacuum.
When the temperature dropped, hydrogen atoms — simple structures made up of solitary protons and single electrons – formed.
Soon, the entire Universe was a sea of uncharged atoms swaying back and forth in the limitless blackness.
Gravity took control when crowds of neutral hydrogen atoms gathered due to the unpredictability of quantum principles, drawing more and more gas into balls where nuclear fusion may explode.
This first sunrise – the dawn of cosmic dawn – drenched the surrounding hydrogen fog in radiation, separating their protons from their electrons and reverting the atoms to ions.
It's never been clear how long it took from the first illumination of those early stars to the reionization of the final pockets of primordial hydrogen.
Light from wildly active galactic centres (called quasars) was absorbed by interceding gas floating about in the neighboring intergalactic medium in studies done more than 50 years ago. You may effectively visualize a chronology of neutral hydrogen gas becoming ionized if you find a sequence of quasars stretching into the distant.
It's one thing to understand the theory. In practice, deducing an exact timeframe from a few of quasars is difficult. Their light is bent not just by the expansion of the Universe, but also by pockets of neutral hydrogen that developed long after the cosmic start.
Researchers double the previous quantity of high-quality spectral data by studying light from a total of 67 quasars to gain a clearer feel of this stutter of ionized hydrogen across the sky.
The objective was to gain a better understanding of the influence of these fresher pockets of hydrogen atoms, which would help the researchers to locate more distant bursts of ionization.
According to their calculations, some 1.1 billion years after the Big Bang, the final dregs of primordial hydrogen were exposed to the rays of first-generation starlight.
"Until a few years ago, the conventional thinking was that reionization had occurred about 200 million years ago," says astronomer Frederick Davies of Germany's Max Planck Institute for Astronomy.
"We now have the best evidence yet that the process stopped far later, during a cosmic epoch more easily observed by current generation observational capabilities," says the study's lead author.
Future technology capable of directly detecting the spectral lines released by hydrogen reionization should be able to reveal vital insights on not just when this epoch ended, but also how it unfolded.
"This new dataset will be used to evaluate numerical models of the Universe's initial billion years for years to come," Davies adds.
