When the first stars and galaxies formed, they didn’t just illuminate the cosmos. These bright structures also fundamentally changed the chemistry of the universe.
During that time, the hydrogen gas that makes up most of the material in the space between galaxies today became electrically charged. That epoch of reionization, as it’s called, was “one of the last major changes in the universe,” says Brant Robertson, who leads the Computational Astrophysics Research Group at the University of California, Santa Cruz. It was the dawn of the universe as we know it.
But scientists haven’t been able to observe in detail what occurred during the epoch of reionization—until now. NASA’s newly active James Webb Space Telescope offers eyes that can pierce the veil on this formative time. Astrophysicists like Robertson are already poring over JWST data looking for answers to fundamental questions about that electric cosmic dawn, and what it can tell us about the dynamics that shape the universe today.
The epoch of reionization wasn’t the first time that the universe was filled with electricity. Right after the big bang, the cosmos were dark and hot; there were no stars, galaxies, and planets. Instead, electrons and protons roamed free, as it was too steamy for them to pair up.
But as the universe cooled down, the protons began to capture the electrons to form the first atoms—hydrogen, specifically—in a period called “recombination,” explains Anne Hutter, a postdoctoral researcher at the Cosmic Dawn Center, a research collaboration between the University of Copenhagen and the National Space Institute at the Technical University of Denmark. That process neutralized the charged material.
Any material held in the universe was spread out relatively evenly at that time, and there was very little structure. But there were small fluctuations in density, and over millions of years, the changes drew early atoms together to eventually form stars. The gravity of early stars drew more gases, particles, and other components to coalesce into more stars and then galaxies.
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Once the beginnings of galaxies lit up, the cosmic dark age, as astrophysicists call it, was over. These stellar bodies were especially bright, Robertson says: They were more massive than our sun and burned hot, shining in the ultraviolet spectrum.
“Ultraviolet light, if it’s energetic enough, can actually ionize hydrogen,” Robertson says. All it takes is a single, especially energetic particle of light, called a photon, to strip away the electron on a hydrogen atom and leave it with a positive electrical charge.
As the galaxies started coming together, they would first ionize the regions around them, leaving bubbles of charged hydrogen gas across the universe. As the light-emitting clusters grew, more stars formed to make them even brighter and full of photons. Additional new galaxies began to develop, too. As they became luminous, the ionized bubbles began to overlap. That allowed a photon from one galaxy to “travel a much larger distance because it didn’t run into a hydrogen atom as it crossed through this network,” Robertson explains.
At that point, the rest of the intergalactic medium in the universe—even in regions far from galaxies—quickly becomes ionized. That’s when the epoch of reionization ended and the universe as we know it began.
“This was the last time whole properties of the universe were changed,” Robertson says. “It also was the first time that galaxies actually had an impact beyond their local region.”
With all of the hydrogen between galaxies charged the universe entered a new phase of formation. This ionization had a ripple effect on galaxy formation: Any star-studded structures that formed after the cosmic dawn were likely affected.
“If you ionize a gas, you also heat it up,” explains Hutter. Remember, high temperatures it difficult for material to coalesce and form new stars and planets—and can even destroy gases that are already present. As a result, small galaxies forming in an ionized region might have trouble gaining enough gas to make more stars. “That really has an impact on how many stars the galaxies are forming,” Hutter says. “It affects their entire history.”
Although scientists have a sense of the broad strokes of the story of reionization, some big questions remain. For instance, while they know roughly that the epoch ended about a billion years after the big bang, they’re not quite sure when reionization—and therefore the first galaxy formation—began.
That’s where JWST comes in. The new space telescope is designed to be able to search out the oldest bits of the universe that are invisible to human eyes, and gather data on the first glimmers of starlight that ionized the intergalactic medium. Astronomers largely detect celestial objects by the radiation they emit. The ones farther away from us tend to appear in the infrared, as the distance distorts their wavelengths to be longer. With the universe expanding, the light can take billions of years to reach JWST’s detectors.
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That, in a nutshell, is how scientists are using JWST to peer at the first galaxies in the process of ionizing the universe. While older tools like the Hubble Space Telescope could spot the occasional early galaxy, the new space observatory can gather finer details to place the groups of stars in time.
“Now, we can very precisely work out how many galaxies were around, you know, 900 million years after the big bang, 800, 700, 600, all the way back to 300 million years after the big bang,” Robertson says. Using that information, astrophysicists can calculate how many ionizing photons were around at each age, and how the particles might have affected their surroundings.
Painting a picture of the cosmic dawn isn’t just about understanding the large-scale structure in the universe: It also explains when the elements that made us, like carbon and oxygen, became available as they formed inside the first stars. “[The question] really is,” Hutter says, “where do we come from?”
Correction (September 21, 2022): The fluctuations in the early universe’s density took place over millions of years, not billions as previously written. This was an editing error.