For about 100 million years in the first universe, from about 380,000 years after the great eruption, the universe was completely black. Then, stars and galaxies began to form, emitting light and releasing ionizing intergalactic hydrogen gas through a process called reionization, or cosmic dawn. It ended, with all its ionized hydrogen, 1.1 billion years after the big bang.
Sarah Bosman at the Max Planck Institute for Astronomy in Germany calculated the day using light from 67 quasars – extremely light objects powered by large black holes – and observed using the Very Large Telescope in Chile and W. M. Keck Observatory in Hawaii. All of these quasars are far enough away that we know they must have formed within one billion years of the big bang.
As light from quasars moved toward Earth, different wavelengths could be captured by neutral hydrogen and ionized hydrogen. Considering the ever-increasing expansion of the universe, Bosman and his team analyzed the absorption lines in the light field to determine when they stopped moving in neutral hydrogen and began to encounter only ioned hydrogen in the space between galaxies instead.
“Reionization has this bubble-like structure where galaxies release these large bumps around them and reappear first,” Bosman said. The recycling is not completed until all of those bubbles are reassembled and hydrogen gas is released into the atmosphere, in places of all quasars. “We can say that the end of the repetition when all the quasars agree – is everywhere.”
The date they found was 1.1 billion years after the big bang, which is 200 million years later than previously estimated. That means that the first generation of stars and galaxies, which are the source of reionization, may be closer to us and thus easier to see than the cosmologists thought.
“The history of the universe has gone through many stages between the big bang and now, and now we are starting to trace all these stages,” Bosman said. “The next step is to go back in time and link the details of the regenerative galaxies that cause it, so that we can actually see galaxies destroying gas.”
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