Hubble Study Challenges Supernova Theory of How the Early Universe Cleared Its Fog

For decades, textbooks have told a tidy story about how the early universe became transparent. After the Big Bang, space was filled with a fog of neutral hydrogen that blocked ultraviolet light. Then, the story went, exploding stars — supernovae — blew powerful winds through young galaxies, carving channels through the fog so that light could escape and ionise the universe. A new study using NASA's Hubble Space Telescope says that story may be backwards: the supernova winds, it finds, actually trap the light instead of freeing it.

The fog, and the old explanation

Within the first billion years after the Big Bang, the universe was filled with neutral hydrogen gas that absorbed ultraviolet light. During the epoch of reionisation, which began about 13 billion years ago, energetic radiation from early galaxies collided with that hydrogen and broke it apart into charged protons and electrons, clearing the fog. Astronomers call reionisation the last major phase transition of the universe — and they have long debated exactly how the ionising light, known as Lyman continuum photons, escaped the dense gas and dust of the galaxies that produced it.

The leading theory gave supernovae the starring role. According to reports from Phys.org and Johns Hopkins University, cosmological simulations typically predicted that supernova-driven winds would clear pathways through interstellar gas, letting ionising radiation escape into intergalactic space. The new research, published October 15 in the journal Nature Astronomy by an international team led by researchers at the University of Michigan and Zhejiang University, tested that prediction directly — and found the opposite.

What Hubble actually saw

The team observed six compact starburst galaxies about 3.5 billion light-years away — galaxies smaller than about 6,500 light-years across, forming roughly 10 solar masses of stars per year — chosen because they resemble the ancient galaxies thought to have driven reionisation. By analysing the spectra of these galaxies, the researchers measured both the speeds of their galactic winds and the fraction of ionising radiation escaping. Gas in the winds absorbs light at specific wavelengths, leaving dips in the spectra whose positions reveal how fast the gas is moving.

The result was a clear negative correlation: galaxies with the fastest winds allowed the smallest fraction of ionising radiation to escape, while the strongest emitters of ionising light lacked fast galactic winds. The team identified a two-phase process. In the first phase, during the first 3 million years of a starburst, radiation pressure and stellar winds from young, massive stars carve through neutral gas and dust, letting ionising photons escape. In the second phase, after about 3 million years, the most massive stars die as supernovae — and their expelled material stirs up surrounding gas and dust, increasing cloud coverage and suppressing the escaping radiation rather than clearing its path. "Our observations show that these photons are escaping before galactic winds form," said Cody Carr, a University of Michigan postdoctoral researcher who co-authored the study. Senior author Stephan McCandliss added that supernova energy "has stirred up the surroundings and increased, rather than decreased, the coverage of neutral gas and dust that suppresses ionizing radiation escape."

Why six galaxies matter — and why they don't settle it

The finding strengthens an alternative theory: that young stars themselves, not their explosive deaths, drove reionisation. If local starburst galaxies are accurate proxies for the early universe, cosmologists will need to update their simulations to include early radiative feedback and local stellar winds before supernova activity begins. Co-author Renyue Cen of Zhejiang University made exactly that point.

Caution is warranted. Six galaxies are a small sample, and the team itself notes that they do not represent the full diversity of galactic evolution. The researchers are seeking more Hubble time to gather further evidence. But the consistency of the result across their targets — faster winds, less escaping light, every time — is hard to dismiss. Sometimes a textbook story survives because it is elegant. This one, Hubble suggests, may survive only until the light gets out another way: not blown free by explosions, but slipped out quietly, in the first few million years, before the explosions begin.