For tens of millions of years, our newborn Universe was surrounded by hydrogen. Gradually, this great fog was broken by the light of the first stars at a dawn that defined the shape of the emerging cosmos.
Having a timeline for this colossal change would help us a lot in understanding 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 used the light of dozens of distant objects called quasars to remove uncertainties, causing the last large drops of hydrogen “fog” to go away. burn much later than we thought, more. more than a billion years after the Big Bang.
The first 380,000 years were a static whisper of subatomic particles that froze out of the cooling vacuum of expanding space-time.
Once the temperature dropped, hydrogen atoms formed: simple structures made up of solitary protons that join with individual electrons.
Soon, the entire Universe was filled with uncharged atoms, a sea of them swaying back and forth in infinite darkness.
Where crowds of neutral hydrogen atoms gathered under the unpredictable thrust of quantum laws, gravity took over, dragging more and more gas into balls where nuclear fusion could explode.
This first sunrise, the cosmic dawn, bathed the hydrogen fog surrounding it in radiation, expelling its electrons from its protons and converting the atoms into the ions they once were.
It has never been clear how long this dawn took, from the first light of those first stars to the reionization of the last bags of primordial hydrogen.
Studies conducted more than 50 years ago made use of the way light from violently active galactic nuclei (called quasars) was absorbed by the intercessory gas floating around the nearby intergalactic medium. Find a series of quasars extending into the distance, you can actually see a neutral hydrogen timeline ionizing.
Knowing the theory is one thing. In practical terms, it is difficult to interpret a precise timeline from a handful of quasars. Its light is not only distorted by the expansion of the Universe, but also passes through bags of neutral hydrogen formed long after the cosmic dawn.
To get a better idea of this ionized hydrogen stuttering in the sky, the researchers oversized their sample, tripling the previous number of high-quality spectral data by analyzing the light from a total of 67 quasars.
The goal was to better understand the impact of these cooler pockets of hydrogen atoms, allowing researchers to better identify farther ionization bursts.
According to its own figures, the last remnants of the original hydrogen fell into the rays of first-generation stellar light about 1.1 billion years after the Big Bang.
“Until a few years ago, the predominant wisdom was that reionization was completed almost 200 million years earlier,” says astronomer Frederick Davies of the Max Planck Institute for Astronomy in Germany.
“Here we have the strongest evidence to date that the process ended much later, during a cosmic epoch more easily observable by the observation facilities of the current generation.”
Future technology capable of directly detecting the spectral lines emitted by hydrogen reionization should be able to further clarify not only when this epoch ended, but also provide critical details about how it developed.
“This new dataset provides a crucial benchmark for testing numerical simulations of the first billion years of the universe over the next few years,” says Davies.
This research was published in the Monthly Notices of the Royal Astronomical Society.