A surprising discovery made with NASA's Hubble Space Telescope is challenging a leading explanation for how the early universe emerged from darkness. The findings suggest that powerful explosions from dying stars may have actually interfered with a process scientists once believed they helped drive. An international research team, including astronomers from the University of Michigan, found that powerful winds produced by exploding stars can prevent ultraviolet radiation from escaping galaxies.
This is the opposite of what a widely accepted theory had predicted, and it could change how scientists understand one of the most important transformations in cosmic history. The discovery, published in Nature Astronomy, offers new clues about how the universe became transparent to ultraviolet light roughly 13 billion years ago. How the Early Universe Emerged From Its Cosmic Fog During the first billion years after the Big Bang, vast amounts of neutral hydrogen filled the space between galaxies.
This hydrogen formed a kind of cosmic fog that absorbed ultraviolet radiation, preventing that light from traveling freely across the universe. That changed during a period known as reionization, which began approximately 13 billion years ago. Intense radiation from early galaxies gradually transformed the hydrogen by stripping electrons from its atoms, leaving behind positively charged protons and free electrons.
As more hydrogen became ionized, ultraviolet light could travel much farther through intergalactic space. This transformation helped establish the relatively transparent universe that astronomers observe today. "Eventually, when it ionized the majority of hydrogen filling intergalactic space, that was the end of reionization and we see the universe today," said Cody Carr, a postdoctoral researcher in the University of Michigan's Department of Astronomy.
"We call reionization the last major phase transition of the universe." Despite its enormous importance, astronomers still do not fully understand which objects supplied the radiation responsible for reionization or how enough of that radiation managed to escape from early galaxies. "We don't know exactly what objects caused reionization or how it evolved," said Carr, who conducted and led the research while working as a postdoctoral scholar at Zhejiang University in China, before joining the University of Michigan. "We know that it took place, but we don't have complete answers to those questions." Hubble Challenges a Leading Theory About Exploding Stars For years, one prominent explanation centered on supernovae, the powerful explosions that occur when certain stars reach the ends of their lives.
These explosions can generate enormous winds that sweep through galaxies, pushing gas and other material outward. Scientists proposed that these winds cleared pathways through the gas surrounding young stars, allowing ultraviolet radiation to escape into the space between galaxies. Once outside, that radiation could help break apart the neutral hydrogen responsible for the universe's ancient fog.
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