One of the biggest open questions in cosmology is embarrassingly simple to state: how did the early universe become transparent? For decades, astronomers believed that winds blown by exploding stars — supernovae — cleared the fog that filled the young cosmos, letting ultraviolet light stream freely through space. New observations from the Hubble Space Telescope have just turned that story on its head. The winds didn't clear the way. In fact, they did the opposite: they blocked the light.
A team led by Cody Carr, a postdoctoral researcher at the University of Michigan, with senior authors Renyue Cen of Zhejiang University and Stephan McCandliss of Johns Hopkins University, reports in the journal Nature Astronomy (published October 15, 2026) that galaxies with the fastest supernova-driven winds allowed the least ionizing radiation to escape. The finding challenges one of the leading explanations of cosmic reionization — the epoch, within the first billion years after the Big Bang, when the universe's last major phase transition took place.
What was cosmic reionization — and why does it matter?
To understand why this matters, start with the fog. After the Big Bang, the universe cooled, and about 380,000 years later, protons and electrons combined into neutral hydrogen atoms. The cosmos went dark and hazy — a universe filled with neutral hydrogen that swallowed ultraviolet light whole. Astronomers call this the cosmic dark ages.
Then something changed. Within the first billion years, the first stars and galaxies ignited, and their ultraviolet light began knocking electrons back off hydrogen atoms — reionizing them. Slowly, bubble by bubble, the fog lifted, and the universe became transparent to ultraviolet light. That transformation is called the epoch of reionization, and it is often described as the last major phase transition the universe ever went through.
We know it happened. What we have never fully known is how. For ionizing radiation to do its work, it first had to escape the galaxies that produced it — and escaping a galaxy is not easy. Young galaxies are full of neutral hydrogen gas and dust that absorb ultraviolet photons before they can get out. So the central puzzle has been: what opened the exits?
The old idea: supernova winds blew the fog away
The leading theory sounded reasonable. Massive stars live fast and die young, ending their lives as supernovae — colossal explosions that drive powerful winds of gas outward from galaxies. Astronomers reasoned that these galactic winds would plow through the surrounding gas and dust, sweeping neutral hydrogen aside and punching low-density channels through which ionizing radiation could escape. Supernovae, in this picture, were the janitors of the early universe: violent, but essential.
The idea was so influential that it became baked into cosmological simulations — the computer models astronomers use to recreate the early universe. "Supernovae play a central role in cosmological hydrodynamic simulations that explain how ionizing radiation escapes from galaxies," said Renyue Cen, one of the study's senior authors and director of the Center for Cosmology and Computational Astrophysics at Zhejiang University. If the winds weren't the key, those simulations would need rethinking.
What Hubble actually found
Carr and his team, nearly 30 researchers from more than a dozen institutions worldwide, put the theory to a direct test — not by looking at the reionization-era galaxies themselves, which are too distant and faint for this kind of detailed work, but by studying nearby compact starburst galaxies that closely resemble them. These nearby galaxies form stars at furious rates, making them good stand-ins for the young galaxies of the early universe.
Using the Hubble Space Telescope, the researchers analyzed the spectra of these galaxies, measuring which wavelengths of ultraviolet light escaped and which were absorbed. Gas in a galaxy's outflowing wind absorbs light at specific wavelengths, leaving dips in the spectrum; the positions and widths of those dips reveal how fast the gas is moving. This gave the team two crucial measurements for each galaxy: the speed of its galactic wind, and the fraction of ionizing (Lyman continuum) radiation that managed to escape.
The result was the opposite of what the standard theory predicted. Galaxies with the fastest winds let the smallest fraction of ionizing radiation out. Where the old picture said strong winds should mean more escaping light, the data showed strong winds meant less.
"Basically, how these galaxies were leaking ionizing radiation isn't what we thought," Carr said. "Our observations show that these photons are escaping before galactic winds form."
Faster winds, darker galaxies: the counterintuitive physics
Why would stronger winds block more light? The researchers' explanation is a lesson in cosmic unintended consequences. When supernovae start exploding in large numbers, they do push gas outward — but they also churn and stir up the surrounding material. That stirring increases the coverage of neutral hydrogen gas and dust, exactly the material that absorbs ionizing radiation. Instead of clearing clean escape channels, the supernova energy creates a messier, more turbulent environment that is harder for ultraviolet photons to get through.
"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 McCandliss, senior author and director of the Center for Astrophysical Sciences at Johns Hopkins University.
The team describes an evolving picture. At first, in a young starburst, intense radiation and the winds of hot young stars drive a relatively slow outflow that clears gas and dust, while clumping of gas creates dense pockets surrounded by clearer channels — and ionizing radiation escapes efficiently. But as supernovae begin detonating, they drive neutral gas and dust back into the galaxy's surroundings, lowering the average escape of ionizing light even as they carve out some low-density channels. Later, large-scale supernova-driven winds shut the escape down almost entirely.
In other words, supernovae may have helped a little at the very beginning, but once they really got going, they got in the way. For readers following the deep-space story, this fits a wider pattern of the early universe refusing to behave as expected — much like the radio glow of hydrogen from billions of years ago picked up by CHIME, which keeps pushing astronomers to rethink how cosmic structure formed.
A new timeline: light escaped before the winds came
The finding strengthens an alternate theory of reionization — one in which the work was done early, by the first generation of young stars themselves, before supernovae had time to reshape their galaxies. In this scenario, the radiation and stellar winds of massive young stars cleared the way and ionized the surroundings before the first supernovae in a galaxy ever exploded. The photons got out during a window of opportunity that closed once the explosions began.
"Our findings highlight the need to examine how radiation and winds from young stars shape the surrounding gas before the first supernovae explode," Cen said.
This matters because the universe had a deadline. Reionization was complete within roughly the first billion years after the Big Bang. If supernova winds were helping throughout, the timing worked one way; if instead the escape happened early and supernovae then suppressed it, the chronology of the cosmic dawn shifts — and our models of how quickly the fog lifted need to be rewritten.
Why simulations now have to change
The implications go beyond one paper's result. Cosmological simulations are the laboratories in which astronomers test their understanding of the universe's evolution, and supernova feedback — the modeled effect of stellar explosions on their surroundings — is one of their most important knobs. If real galaxies show that stronger winds suppress rather than enhance the escape of ionizing radiation, then simulations that treat supernovae as the great escape-hatch of the early universe are getting a key piece of physics wrong.
The team is not claiming the case is closed. Because the research examined nearby galaxies as analogs rather than the reionization-era galaxies directly, it tests the physical mechanism rather than observing the ancient epoch itself. More observations are needed — and Carr and his colleagues are already working to secure more Hubble time to gather more evidence about exactly how ionizing radiation slips out of galaxies.
That follow-up work will matter for more than cosmology. Understanding how light escapes young galaxies also bears on how the first galaxies grew, how heavy elements spread through the intergalactic medium, and how the environments around early star clusters developed. The reionization era is when the universe took its modern shape, and every correction to the story ripples outward.
A universe full of surprises
There is something poetic in the result. For years, astronomers pictured supernovae as the heroes of the cosmic dawn — the titans whose winds blew the fog away. Hubble's new data suggest they were more complicated characters: helpful at first, obstructive later, and ultimately not the heroes of the story at all. The real heroes may have been the young stars themselves, whose light slipped out quietly before the explosions began.
That theme — the early universe doing the unexpected — keeps repeating. From a possible dark galaxy spotted by China's FAST telescope that may contain no stars at all, to Euclid's newly discovered comet and Webb's views of the first billion years, the cosmos keeps refusing to match our first guesses. Each correction brings us closer to the true history of how darkness became light.
The study — "Lyman continuum escape during the onset of galactic winds in compact starbursts," by Carr, Cen, McCandliss, Sally Oey of the University of Michigan, and colleagues — is published in Nature Astronomy (DOI: 10.1038/s41550-026-02999-z). More Hubble observations are on the way, and with them, a sharper picture of the billion-year dawn that made the universe transparent.