Astronomers using NASA’s Hubble Space Telescope have uncovered evidence that could change scientists’ understanding of how the early universe emerged from darkness. A new study suggests that powerful winds created by exploding stars may have blocked, rather than helped, the ultraviolet radiation needed to clear the ancient cosmic fog.

The discovery challenges a long-standing explanation of a major transformation in the universe’s history known as cosmic reionization. During this period, which began approximately 13 billion years ago, energetic radiation gradually transformed the neutral hydrogen gas filling space, allowing light to travel more freely across the cosmos.

The international research team found that galaxies with the fastest-moving stellar winds allowed the smallest amount of ionising radiation to escape. This suggests that supernova explosions may have made it more difficult for galaxies to release the radiation responsible for clearing the cosmic fog.

Understanding the universe’s ancient darkness

During the first billion years after the Big Bang, the universe contained enormous quantities of neutral hydrogen. This gas absorbed much of the ultraviolet light travelling through space, leaving the cosmos opaque to certain forms of radiation.

Astronomers call this period the Cosmic Dark Age.

The situation gradually changed when energetic ultraviolet photons began stripping electrons from neutral hydrogen atoms. This process, known as reionization, transformed the gas into charged particles and allowed light to travel across increasingly large regions of the universe.

However, scientists have continued to investigate which objects produced the radiation and how it managed to escape from early galaxies.

Supernova explosions may have blocked ultraviolet light

One leading explanation suggested that supernova explosions helped drive the reionization process.

When massive stars reach the end of their lives, they can explode with enormous energy. These explosions generate powerful winds that push gas and dust away from their surroundings.

Scientists previously considered that these winds might create openings in galaxies, allowing ultraviolet radiation from young stars to escape into surrounding space.

The new Hubble observations challenge this idea.

Researchers found that galaxies with stronger and faster winds generally allowed less ionising radiation to escape. The material swept up by supernova-driven winds may increase the amount of gas and dust blocking the radiation.

In other words, the explosions that were expected to clear pathways for light may sometimes have made those pathways more difficult to cross.

Hubble studies nearby galaxies to understand the distant past

Directly observing galaxies from the earliest stages of the universe remains challenging. To investigate the process, astronomers used Hubble to examine nearby starburst galaxies, which experience exceptionally intense periods of star formation.

These galaxies provide useful modern examples of some of the physical conditions believed to have existed in the early universe.

The researchers studied the light coming from six compact star-forming galaxies and examined their spectra. Spectral patterns allowed the team to measure the movement of gas and estimate how much ionising ultraviolet radiation escaped from each galaxy.

The comparison revealed an important relationship: the strongest galactic winds were associated with lower levels of escaping radiation.

The findings indicate that the timing of stellar activity may be just as important as the energy released by exploding stars.

Young stars could have played a more important role

The research supports an alternative explanation in which radiation from young, massive stars escaped their galaxies before powerful supernova-driven winds developed.

During the earliest stages of star formation, intense radiation and smaller-scale stellar winds may create favourable conditions for ultraviolet photons to travel beyond their galaxies.

As the stars age and begin exploding as supernovae, the resulting large-scale winds can redistribute gas and dust, potentially reducing the amount of radiation that escapes.

This suggests that the earliest phase of star formation could have been especially important in helping the universe transition from an opaque environment into one through which light could travel more freely.

Findings could change computer models of the early universe

The discovery has implications for cosmological simulations, which scientists use to recreate how galaxies formed and evolved after the Big Bang.

Many existing models give supernova-driven winds a central role in helping ionising radiation escape from galaxies.

The new observations suggest that researchers may need to examine more carefully how radiation, young stellar winds, gas and dust interact before the first supernova explosions occur.

An improved understanding of these processes could help astronomers build more accurate models of early galaxy formation and cosmic reionization.

Scientists seek further observations

The research team is planning additional observations to determine whether the same relationship between galactic winds and escaping radiation exists in a wider population of galaxies.

More data will be important because the current findings are based on a limited sample of nearby star-forming galaxies rather than direct observations of every galaxy that existed during the early universe.

The study does not completely solve the mystery of cosmic reionization, but it provides a significant new clue about how the process may have unfolded.

The research, published in Nature Astronomy, demonstrates that the forces created by dying stars may have had a more complicated influence on the early cosmos than previously understood.

By studying how young stars, radiation and powerful explosions interact, astronomers are moving closer to explaining how the universe emerged from its ancient darkness and became the transparent, star-filled cosmos observed today.