This artist’s concept shows the first galaxies forming in the young cosmos. Credits: NASA, ESA, CSA, Joseph Olmsted (STScI)
The James Webb Space Telescope (JWST) is known for discovering young, bright galaxies in the early universe. How such regions, filled with stars, formed so quickly and survived is pushing researchers to rethink cosmic evolution.
A recent study published in Monthly notices of the Royal Astronomical Society: letters uses JWST data to probe a long-standing question: When did the universe’s transition from cold and neutral to hot and ionized occur? (Neutral atoms have no charge, since the number of electrons they contain corresponds to the number of protons in their nucleus. Ionized particles, or ions, have lost one or more electrons and are therefore electrically charged.)
The study suggests that this change, known as the epoch of reionization, ended much earlier than previously thought. And if so, the discovery would have important implications for understanding the early years of the universe.
Related: When did the lights in the universe turn on?
Reionize the cosmos
The universe was first ionized soon after the Big Bang. A mixture of electrons, protons and neutrons permeated the cosmos, until the ambient temperature cooled enough for the charged particles to combine into neutral hydrogen. As gravity pulled the gas further, the first stars and galaxies formed.
Then another notable change occurred: Photons from the stars in these galaxies escaped and collided with hydrogen in intergalactic space, causing those atoms to lose their electrons. Over time, these “bubbles” of ionized gas grew larger and merged together until they filled the universe.
“[This] it was the last major phase transition and set the stage for the rest of cosmic history,” says lead author and theoretical cosmologist Julian Muñoz of the University of Texas at Austin.
Reionization survey
To probe when the epoch of reionization occurred, Muñoz and his team used JWST to estimate how many photons were produced by early galaxies. The team also estimated the fraction of that light that would escape into intergalactic space, where it would ionize neutral hydrogen. Comparing the latter with the number of hydrogen atoms in the early universe, they found that reionization would have ended about 650 million years after the Big Bang.
Scientists had previously dated the end of reionization by the point at which light observed from primordial galaxies no longer shows hydrogen emission at a specific wavelength, known as Lyman-alpha (α), due to absorption of this light by neutral hydrogen. This, along with satellite measurements of the fraction of the universe’s first light, known as the Cosmic Microwave Background (CMB), that is scattered by electrons released during reionization, suggest that the epoch ended about 1 billion years after Big Bang.
Erica Nelson, an astrophysicist at the University of Colorado Boulder who was not involved in the study, says, “the paper brings together a lot of this information to show that these [the latter and the JWST observations] are inconsistent and that we have a problem, which is a really powerful thing to do.”
Voltage or not?
According to the study, early JWST results suggest an excess of ionizing photons. The surplus does not include light sources from black hole accretion disks, which were also detected using JWST. “If you add these photons, it creates even more tension,” between the two determinations to end reionization, Muñoz says.
The researchers note that perhaps their estimates of the efficiency of early galaxies in producing ionizing photons are too high, as JWST is used to target mostly the brightest starburst galaxies. And the fraction of photons that would have escaped galaxies to ionize hydrogen cannot be measured. To get around this problem, the team used escape fractions measured from local ionizing galaxies, which vary between 10 and 20%.
Connect observations and models
The new study highlights the importance of using different probes to study the epoch of reionization. In particular, JWST provides a way to measure photon production by primordial galaxies – the “ant’s eye view,” as Muñoz puts it – while models based on Lyman-alpha and CMB observations offer a “bird’s eye view “of reionization.
Christopher Cain, a theoretical astrophysicist at the University of Arizona who was not involved in the study, adds: “We need to put JWST together with these other probes to get the full picture of what was happening.” According to him, such a synergy could help understand the physical processes that took place in the early universe.
For example, perhaps the reionization was driven by small galaxies that were initially full of stars and eventually quieted down. Or perhaps massive, luminous galaxies drove the epoch, or the accumulation of black holes played a bigger role than previously thought.
For now, the new study offers an interesting look at the timing of the universe’s final, monumental change and is inspiring researchers to think about how to reconcile the differences.
“Something has to be wrong,” Nelson says, “and inherently, if you say that, you become controversial. But this is how we advance our understanding of the universe.”
Editor’s note: This story has been updated to state that that Epoch of Reionization was the first time hydrogen atoms lost their electrons, as well as clarifying the two values Nelson was referring to.
