Recent findings from the Hubble Space Telescope challenge theories regarding the early universe following the Big Bang.
Approximately 380,000 years after the Big Bang, the universe cooled enough for electrons and protons to combine into neutral hydrogen atoms. This hydrogen absorbed most ultraviolet (UV) photons with enough energy to ionize it, causing the universe to be largely opaque to these wavelengths. Most of this hydrogen became ionized over time due to a period beginning roughly 13 billion years ago known as reionization. Reionization was the consequence of energetic radiation released from galaxies; energetic UV photons, known as Lyman continuum (LyC) radiation, ionized hydrogen atoms, separating them into protons and electrons.
A leading theory in the field suggests reionization was caused by winds, or outflows of gas and charged particles, expelled by supernovae–highly energetic and explosive deaths of massive stars–in early galaxies. For years, many scientists proposed that these winds could create openings in the gas surrounding a galaxy, hence allowing ionizing photons released by young stars to escape and ionize the hydrogen gas within the universe. However, recent findings from the Hubble Space Telescope contradict this belief. Observations of six nearby, compact star-forming galaxies that serve as local analogs for early galaxies1 provided evidence suggesting an inverse relationship between supernova-driven winds and the escape fraction of LyC radiation–the proportion of LyC radiation which successfully escapes into space. This suggests that supernova-driven winds may actually inhibit the release of LyC radiation.
This discovery has sparked renewed debate among cosmologists. One alternate theory proposes that stellar winds from young, massive stars facilitated the escape of ionizing photons within a roughly 3-million-year window following periods of rapid star formation. However, as these stars evolved and exploded, supernova-driven winds may have increased the amount of surrounding gas and dust, hence inhibiting the release of LyC radiation and potentially suppressing the rate of reionization. Renyue Cen, director of the Center for Cosmology and Computational Astrophysics at Zhejiang University, notes, “Our findings highlight the need to examine how radiation and winds from young stars shape the surrounding gas before the first supernovae explode.” Cody Carr, a postdoctoral researcher from the University of Michigan and the lead author of the study, similarly emphasizes this perspective.
These findings have significant implications within cosmological simulations used by scientists to model the formation and evolution of galaxies in the early universe. Many current models emphasize the role of supernova winds in allowing the release of ionizing radiation; however, these observations suggest that such winds may actually do the opposite and suppress the escape of ionizing photons. If confirmed, this research will likely prompt scientists to reconsider modern models of processes responsible for reionization in simulations.
Cosmologists recognize that they do not yet have the answers; however, the research team is currently requesting further access to Hubble to gather more evidence, and future observations may help resolve these questions (Carr, 2026).2
Work Cited:
1 Carr, C., Cen, R., McCandliss, S. et al. Lyman continuum escape during the onset of galactic winds in compact starbursts. Nat Astron (2026). https://doi.org/10.1038/s41550-026-02999-z
2 Carr, C. (2026, October 22). Hubble observations reveal supernovae may not have ushered in the reionization of the early universe | physics & astronomy. Physics & Astronomy. https://physics-astronomy.jhu.edu/2026/10/22/hubble-observations-reveal-supernovae-may-not-have-ushered-in-the-reionization-of-the-early-universe/