The Epoch of Reionization, the cosmos’s last major transition, may have occurred earlier than previously believed

JWST is renowned for discovering young, bright galaxies in the very early cosmos. How such areas, bursting with stars, formed so very quickly and survived is enticing scientists to rethink cosmic progression. A recent study published in MNRAS Letters uses JWST data to probe a long-standing question: When did the cosmos transition from cold and neutral to hot and ionized? Neutral atoms lack charge, as the number of electrons they have matches the number of protons in their nucleus. Ionized particles, or ions, have lost one or more electrons, and are therefore electrically charged. The study proposes that this change, termed as the Epoch of Reionization, ended far earlier than formerly thought. And if that’s the case, the find would have significant implications for understanding the cosmos’s early years.   

Reionizing the universe

The cosmos was ionized for the first time immediately following the Big Bang. A blend of electrons, protons, and neutrons permeated the cosmos, until the ambient temperature cooled enough for the charged particles to unite into neutral hydrogen. As gravity further drew the gas together, the first stars and galaxies came into being. Then, another extraordinary change took place: Photons from stars of these galaxies escaped and collided with the hydrogen in intergalactic space, causing those atoms to again lose their electrons. Over time, these “bubbles” of ionized gas grew bigger and merged together to fill the cosmos. “[This] was the last major phase transition and set the stage for the rest of cosmic history,” says lead author Julian Muñoz (theoretical cosmologist from the University of Texas at Austin).

Probing reionization

To probe when the Epoch of Reionization transpired, Muñoz and his team used James Webb Space Telescope to estimate how many photons were formed by initial galaxies. The team also estimated the fraction of such light that may have escaped into intergalactic space, where it may have ionized neutral hydrogen. By comparing the latter to the number of hydrogen atoms in the early cosmos, they found that reionization would have ended around 650 million years post the Big Bang.

Scientists formerly dated the end of reionization based on when the light they observe from early galaxies no more shows emission from hydrogen at a specific wavelength, termed as Lyman-alpha (α), due to absorption of this light by neutral hydrogen. This, in conjunction with satellite measurements of the fraction of the cosmos’s first light, termed as the Cosmic Microwave Background (CMB), that is scattered by electrons freed during reionization, suggest the epoch ended roughly 1 billion years post the Big Bang. Erica Nelson (an astrophysicist at the University of Colorado Boulder not involved in the study) says, “the paper brings together a lot of this information to show that these [two estimates] are inconsistent and that we have a problem, which is a really powerful thing to do.”