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Not long after the Big Bang, all went dark. The hydrogen gas that pervaded the early universe would have snuffed out the light of the universe’s first stars and galaxies. For hundreds of millions of years, even a galaxy’s worth of stars — or unthinkably bright beacons such as those created by supermassive black holes — would have been rendered all but invisible.
Eventually this fog burned off as high-energy ultraviolet light broke the atoms apart in a process called reionization. But the questions of exactly how this happened — which celestial objects powered the process and how many of them were needed — have consumed astronomers for decades.
Now, in a series of studies, researchers have looked further into the early universe than ever before. They’ve used galaxies and dark matter as a giant cosmic lens to see some of the earliest galaxies known, illuminating how these galaxies could have dissipated the cosmic fog. In addition, an international team of astronomers has found dozens of supermassive black holes — each with the mass of millions of suns — lighting up the early universe. Another team has found evidence that supermassive black holes existed hundreds of millions of years before anyone thought possible. The new discoveries should make clear just how much black holes contributed to the reionization of the universe, even as they’ve opened up questions as to how such supermassive black holes were able to form so early in the universe’s history.
In the first years after the Big Bang, the universe was too hot to allow atoms to form. Protons and electrons flew about, scattering any light. Then after about 380,000 years, these protons and electrons cooled enough to form hydrogen atoms, which coalesced into stars and galaxies over the next few hundreds of millions of years.
Starlight from these galaxies would have been bright and energetic, with lots of it falling in the ultraviolet part of the spectrum. As this light flew out into the universe, it ran into more hydrogen gas. These photons of light would break apart the hydrogen gas, contributing to reionization, but as they did so, the gas snuffed out the light.
Quanta Magazine: Discoveries Fuel Fight Over Universe’s First LightAshley Yeager
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