Webb's Discoveries Question Reionization Models

Reionization is a pivotal phase in the universe’s history when the first stars and galaxies transformed their surroundings and the cosmos itself. While established theories suggest this epoch ended about 1 billion years after the Big Bang, recent findings from the James Webb Space Telescope (JWST) indicate it may have actually concluded at least 350 million years earlier.

Initially, for the first 380,000 years, the universe was a hot plasma of protons and electrons. As it cooled, these particles combined to form neutral hydrogen atoms. Approximately 100 million years after the Big Bang, the first stars and galaxies formed, marking the beginning of the reionization era.

These early stars were incredibly massive, some predicted to be 30 to 300 times the mass of the Sun, emitting substantial extreme ultraviolet light. This radiation ionized nearby hydrogen atoms, splitting them into protons and electrons. Eventually, when most hydrogen became ionized, the reionization epoch came to an end.

Since about 75% of all matter is hydrogen, this transformation was monumental. Julian Muñoz, a lead author from The University of Texas at Austin, explained that this shift changed the universe from a neutral state to an ionized and hot environment, affecting the entire cosmos, not just individual galaxies.

John Chisholm, a co-author, noted that this process regulated the growth and evolution of galaxies. However, because reionization cannot be directly observed, astronomers rely on models based on indirect evidence, such as the Cosmic Microwave Background (CMB) and the Lyman-alpha Forest, to estimate when it ended.

The JWST challenges these established models by providing deeper observations of the universe. It has revealed an unexpectedly high number of galaxies emitting extreme ultraviolet light, suggesting these bright galaxies alone could ionize the universe, which counters prior expectations.

This has led to discrepancies in current models. If relying solely on JWST findings, it suggests reionization ended between 550 million to 650 million years after the Big Bang, rather than the established 1 billion years. Muñoz emphasizes the need for more detailed observations and a better understanding of the recombination process to resolve these tensions and improve our understanding of this crucial epoch.