For millions of years, our nascent universe was submerged in hydrogen. Gradually, this vast haze separated from the light of the first stars at dawn, which defined the shape of the emerging universe.
Having a timeline for this massive change would go a long way in helping us understand the evolution of the universe, yet our best efforts so far have been vague guesses based on low-quality data.
An international team of astronomers led by the Max Planck Institute for Astronomy in Germany used light from dozens of distant objects called quasars to remove uncertainties, causing the last major burst of hydrogen ‘fog’ to occur much later than the first. burned up in the U.S., more than a billion years after the Big Bang.
The first 380,000 years were a steady hiss of subatomic particles ejecting from the cooling vacuum of the expanding space-time.
Once the temperature dropped, so did hydrogen atoms – simple structures consisting of solitary protons combined with single electrons.
Soon the whole universe was filled with unchanging atoms, a sea of them moving back and forth in infinite darkness.
Where crowds of neutral hydrogen atoms gathered under the unexpected nudge of quantum laws, gravity pulled more and more gas into balls, where nuclear fusion could flare up.
This first sun rise – the breaking of the cosmic dawn – bathed the surrounding hydrogen fog in radiation, stripping their electrons from their protons and turning the atoms into the ions they once were.
How long this dawn took, from the first light of those early stars to the re-ionization of the last remaining pockets of primary hydrogen, has never been clear.
Studies conducted more than 50 years ago used the way light from a violently active galactic core (called a quasar) was absorbed by floating gas in a nearby space medium. Find a chain of quasars spread out in the distance, you can see the timeline of the ionization of effectively neutral hydrogen gas.
Knowing the theory is one thing. In practice, it is hard to interpret the exact timeline 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 well after cosmic dawn.
To get a better understanding of this stuttering of ionized hydrogen across the sky, the researchers supersized their sample by analyzing light from a total of 67 quasars, tripling the previous number of high-quality spectral data.
The goal was to better understand the effect of these fresher pockets of hydrogen atoms, allowing researchers to better identify more distant bursts of ionization.
According to their own data, about 1.1 billion years after the Big Bang, the last remnants of the original hydrogen fell into the rays of the first generation of stars.
“Until a few years ago, the prevailing wisdom was that the reionization completed about 200 million years ago,” says astronomer Friedrich Davies of the Max Planck Institute for Astronomy in Germany.
“Here we now have the strongest evidence ever that the process ended much later, during a cosmic epoch more readily observable by current-generation observational facilities.”
Future technology capable of directly detecting the spectral lines emitted by re-ionization of hydrogen should be able to provide important details on not only when this era ended, but how it unfolded.
“This new dataset provides an important benchmark against which numerical simulations of the first billion years of the universe will be tested in the coming years,” says Davis.
This research was published in Monthly Notices of the Royal Astronomical Society,