For tens of millions of years, our newborn universe was shrouded in hydrogen. Gradually, this vast mist was torn apart by the light of the very first stars in a dawn that defined the shape of the emerging cosmos.
Having a timeline for this colossal shift would go a long way in helping us understand the evolution of the Universe, but so far our best attempts have been fuzzy estimates based on low-quality data.
An international team of astronomers led by the Max Planck Institute for Astronomy in Germany has used light from dozens of distant objects called quasars to dispel uncertainties, determining the last great streaks of scorched hydrogen “fog” far more later than we first thought, more than a billion years after the big Bang.
The first 380,000 years were a static hiss of subatomic particles freezing out of the cooling vacuum of expanding spacetime.
Once the temperature dropped, hydrogen atoms were formed – simple structures made up of lone protons teaming up with lone electrons.
Soon, the entire universe was filled with uncharged atoms, a sea of them swaying back and forth in endless darkness.
Where crowds of neutral hydrogen atoms gathered under the unpredictable nudge of quantum laws, gravity took over, pulling more and more gas into balls where nuclear fusion could burst.
That first sunrise – the cosmic dawn – bathed the surrounding hydrogen fog in radiation, knocking their electrons out of their protons and turning atoms into the ions they once were.
The exact duration of this dawn, from the first light of these first stars to the reionization of the last remaining pockets of primordial hydrogen, has never been clear.
Studies over 50 years ago used how light from violently active galactic nuclei (called quasars) was absorbed by interceding gas floating in the nearby intergalactic medium. Find a series of quasars extending into the distance, you can actually see a timeline of ionized neutral hydrogen gas.
Knowing the theory is one thing. Concretely, it is difficult to interpret a precise chronology from a handful of quasars. Not only is their light distorted by the expansion of the Universe, but it also passes through pockets of neutral hydrogen formed long after the cosmic dawn.
To get a better idea of this stutter of ionized hydrogen across the sky, the researchers oversized their sample, tripling the previous number of high-quality spectral data by analyzing light from a total of 67 quasars.
The goal was to better understand the impact of these new pockets of hydrogen atoms, allowing researchers to better identify more distant ionization bursts.
According to their own figures, the last remnants of original hydrogen fell into the rays of first-generation starlight about 1.1 billion years after the Big Bang.
“Until a few years ago, the prevailing wisdom was that reionization had been completed nearly 200 million years earlier,” said Frederick Davies, astronomer at the Max Planck Institute for Astronomy in Germany.
“We now have the strongest evidence yet that the process ended much later, in a cosmic time more easily observable by current-generation observing facilities.”
Future technology capable of directly detecting the spectral lines emitted by hydrogen reionization should be able to further clarify not only the end of this epoch, but provide critical details of how it happened.
“This new dataset provides a crucial benchmark against which numerical simulations of the Universe’s first billion years will be tested for years to come,” said Davies.
This research was published in the Royal Astronomical Society Monthly Notices.
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