Billions of years ago, our universe was much smaller and hotter than it is today. It was so small and hot that it existed in a state of plasma, where electrons were separated from atomic nuclei. But when the universe was about 380,000 years old, it cooled to the point where electrons rejoined their nuclei, forming a soup of neutral atoms.
However, observations of the present universe reveal that almost all matter in the universe is not neutral at all. Matter is ionized and still present in a plasma state. Something, in the billions of years since, has induced the reionization of the universe ‘s neutral gas, and thanks to the James Webb Space Telescope, we are close to the solution. Astronomers call this event the Epoch of Reionization and suspect that it occurred in the first few hundred million years after the Big Bang.
Hypotheses for the reionization of the universe
One of the big debates in cosmology is discovering the source of reionization. One hypothesis is that quasars are the main culprits. Quasars are ultra-luminous nuclei that surround supermassive black holes that emit huge amounts of high-energy radiation. This radiation could easily ionize the cosmos. But the problem with this hypothesis is that quasars are very rare.
Another hypothesis predicts that young galaxies in full star formation are responsible. In this scenario, the process of ionizing neutral gas is more widespread throughout the universe. Each individual galaxy can only ionize gas in its immediate vicinity. Since the distribution of galaxies is massive in the cosmos, it is likely that reionization starts from these objects.
A team of astronomers used James Webb to investigate this hypothesis. Obviously, they could not directly investigate the radiation coming from the galaxies. Part of this radiation is absorbed by the billions of light-years of matter that separates us from the galaxies in question. Using James Webb’s ability to study distant galaxies, they measured how compact and rich they were in the process of star formation. By comparing these galaxies with similar galaxies found in the present universe, they generated an estimate of the amount of radiation that emanates from them.
The research estimates that, on average, galaxies in the primordial universe lost about 12% of their available high-energy photons. This share is potentially sufficient to reionize the entire cosmos in a relatively short period. The findings, obtained from James Webb ‘s observations, were released in the journal Astronomy & Astrophysics, showcasing the credibility of reionization through galaxies in the universe (ref.).
However, the results are not conclusive due to the numerous assumptions that the researchers had to make. But finally, we are pointing in an intriguing direction to definitively solve this old cosmic puzzle.