Hubble reveals a stunning twist in how the universe emerged from darkness

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. However, new observations from Hubble suggest that this explanation may have the process backward. Rather than opening routes for radiation to escape, the researchers found that stronger galactic winds were associated with less escaping ionizing radiation. The observations indicate that the winds generated by supernovae can actually make it harder for the radiation to leave a galaxy. "Basically, how these galaxies were leaking ionizing radiation isn't what we thought," Carr said. The results lend support to a different possibility. Radiation from young, massive stars may escape their galaxies before those stars explode as supernovae and generate large winds that reshape the surrounding environment. "Our observations show that these photons are escaping before galactic winds form," Carr said. Photons are individual packets of light energy. In this case, the researchers are interested in ultraviolet photons energetic enough to ionize hydrogen, making them essential to understanding how the early universe became transparent. What Hubble Discovered in Nearby Starburst Galaxies To investigate how radiation escapes, the international team used the Hubble Space Telescope to examine nearby starburst galaxies. These are galaxies experiencing exceptionally intense periods of star formation, making them useful modern examples of the kinds of galaxies believed to have contributed to reionization. Hubble, a NASA mission launched with contributions from the European Space Agency, allowed researchers to study the light coming from these galaxies in detail. The team examined their spectra, which reveal how much light is present at different wavelengths. By studying the patterns in those spectra, astronomers can learn about the gas surrounding stars and determine how quickly it is moving. Gas absorbs particular wavelengths of light, creating recognizable dips in a galaxy's spectrum. The locations and shapes of these dips provide clues about the speed and movement of galactic winds. When the researchers compared wind speeds with the amount of ionizing radiation escaping from each galaxy, they uncovered a surprising relationship. Galaxies with the fastest winds allowed the smallest fraction of ionizing radiation to escape. That result directly challenges the expectation that stronger winds should clear away more gas and make it easier for radiation to reach intergalactic space. "It appears that energy input from supernovae has stirred up the surroundings and increased, rather than decreased, the coverage of neutral gas and dust that suppresses ionizing radiation escape," said Stephan McCandliss, a senior author of the study and director of the Center for Astrophysical Sciences at Johns Hopkins University. In other words, instead of clearing a path for ultraviolet light, energy from exploding stars appears to redistribute gas and dust in ways that can block more of it. Young Stars May Hold the Key to Cosmic Reionization Although large galactic winds appear to suppress escaping radiation, smaller winds and other activity around young stars could still help create openings in the surrounding gas. These processes may occur before the first supernova explosions, giving radiation a chance to escape while conditions are more favorable. The findings suggest that astronomers may need to pay greater attention to what happens during the earliest stages of star formation, rather than focusing primarily on the effects of exploding stars. Carr and his colleagues are seeking additional observing time with Hubble to examine more galaxies and investigate whether the same patterns appear elsewhere. More observations could help establish how important the activity of young stars is in allowing ionizing radiation to escape. The results also have implications for computer simulations of the early universe. These simulations attempt to recreate how galaxies formed, how gas moved through them, and how radiation eventually transformed the hydrogen filling intergalactic space. "Supernovae play a central role in cosmological hydrodynamic simulations that explain how ionizing radiation escapes from galaxies," said Renyue Cen, a senior author of the study and professor and director of the Center for Cosmology and Computational Astrophysics at Zhejiang University. "Our findings highlight the need to examine how radiation and winds from young stars shape the surrounding gas before the first supernovae explode." By accounting more accurately for these early interactions between stars, radiation, and gas, researchers may be able to build better models of how reionization unfolded. The discovery does not yet settle the question of what caused the universe's last major phase transition. However, it provides an important new clue, suggesting that the radiation that helped lift the universe's ancient fog may have escaped galaxies before powerful stellar explosions began to alter their surroundings. International Research Collaboration and Funding The research brought together nearly 30 scientists from more than a dozen institutions around the world. Sally Oey, a professor of astronomy at the University of Michigan, also contributed to the study. The project received additional financial support from Zhejiang University, the Zhejiang Provincial Top-Level Research Support Program, the National Natural Science Foundation of China, the Ministry of Science, Innovation and Universities in Spain, the European Regional Development Fund, and the European Union's Horizon 2020 Research and Innovation Program. The research was also supported in part by federal funding from NASA and the U.S. National Science Foundation. Additional support for the Hubble Space Telescope observations came through a grant administered by the Space Telescope Science Institute.