Astronomers find Webb data conflicts with reionization models

Reionization is a critical period during which the first stars and galaxies changed the physical structure of their surroundings, and ultimately of the entire universe. Established theories claim that this era ended around 1 billion years after the Big Bang. However, if one calculated this step using observations from the James Webb Space Telescope (JWST), reionization would have ended at least 350 million years earlier than expected. This is what a new article published in Monthly notices of the Royal Astronomical Society: letters.

Throughout its history, the universe has undergone several major changes. For the first 380,000 years after the Big Bang, it was a hot, dense plasma of protons and electrons. Eventually, things cooled enough for those protons and electrons to combine and form neutral hydrogen atoms. Then, about 100 million years after the Big Bang, the first stars and galaxies began to form, ushering in the epoch of reionization.

These early stars were huge and hot – some predicted they would be 30 to 300 times more massive than our sun – and they emitted lots of energy in the form of extreme ultraviolet light. This energy was so intense that when it hit nearby hydrogen atoms, it split them into protons and electrons in a process called ionization. After hundreds of millions of years, when almost all the hydrogen in the universe was ionized, the epoch of reionization ended.

Considering that about 75% of all matter is hydrogen, this represents an immense transformation. “This is the last major change to happen,” said Julian Muñoz, assistant professor of astronomy at the University of Texas at Austin and lead author of the paper. “You went from neutral and cold and boring to ionized and hot. And that’s not something that only happened to one or two galaxies. It happened to the entire universe.”

“The process heated and ionized gas in the universe, which regulated the rate at which galaxies grew and evolved,” added John Chisholm, assistant professor of astronomy at UT Austin and co-author of the article. “These first stars established the overall structure of galaxies in the universe.”

Because astronomers are unable to directly observe the reionization process, they must use models to predict when it will end. These models are based on indirect evidence, including measurements of the amount of light that has reached us from the afterglow of the Big Bang, called the cosmic microwave background.

Further evidence is an early wavelength abundance associated with hydrogen energy changes, called the Lyman-alpha forest. These two elements help astronomers calculate how much hydrogen is transformed during reionization and, by extension, how much energy is required to do so.

“It’s an accounting game,” Muñoz said. “We know that all hydrogen was neutral before reionization. From there, you need enough extreme ultraviolet to split each atom. So ultimately you can do the math to determine when reionization ended.”

Today, the James Webb Space Telescope is challenging established models. Thanks to it, astronomers can peer further into the cosmos than ever before, deep into this critical era. This leads to many unexpected observations in the early universe, including a greater abundance of extreme ultraviolet emitting galaxies than expected. “JWST revealed that bright galaxies are enough to ionize the universe on their own,” Chisholm said. “It goes against what a lot of people expected.”

So, with these new observations, the accounting is now off. “If you were to blindly trust James Webb, it would tell you that reionization ended 550 to 650 million years after the Big Bang, instead of current estimates of a billion years,” Muñoz explained. “If that were true, the cosmic microwave background would be different, and the Lyman-alpha forest would be different. So there is a tension.”

In other words, it is unlikely that reionization occurred hundreds of millions of years earlier than previously thought. So what’s going on? One explanation could be that established models lack certain key information. For example, sometimes ionized protons and electrons come together to reform neutral hydrogen atoms. This process is called recombination. If this happened more often than current models assume, it could increase the amount of extreme ultraviolet light needed to ionize the entire universe.

“We need more detailed and deeper observations of galaxies, as well as a better understanding of the recombination process,” Munoz said. “Resolving this tension over reionization is a key step in finally understanding this pivotal period. I can’t wait to see what the years to come bring.”

Other authors of the study are Jordan Mirocha of NASA’s Jet Propulsion Laboratory and the California Institute of Technology; Steven Furlanetto of the University of California, Los Angeles; and Charlotte Mason of the University of Copenhagen.