How did galaxies form after the Big Bang? JWST will see.


When the first stars and galaxies formed, it not only illuminated the universe. 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 today’s intergalactic space became electrically charged. This 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 happened during the era of reionization — until now. NASA’s newly active James Webb Space Telescope provides eyes that can pierce the veil at this formative time. Astrophysicists like Robertson are already studying JWST data in search of answers to fundamental questions about this electric cosmic dawn, and what it can tell us about the dynamics that shape the universe today.

What happened after the big bang?

The era of reionization wasn’t the first time the universe was filled with electricity. Immediately after the Big Bang, the universe was dark and hot; There were no stars, galaxies, or planets. Instead, the electrons and protons were free-roaming, being too steamy for them to pair up.

But as the universe began to cool, the protons began capturing 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. This process neutralizes the charged material.

Any matter in the universe was spread out relatively evenly at that time, and there were very few structures. But there were slight fluctuations in density, and over billions of years, changes attracted early atoms together to eventually form stars. The gravity of early stars attracted more gases, particles, and other components to combine into more stars then galaxies.

[Related: How old is the universe? Our answer keeps getting better.]

Once the beginnings of galaxies were lit up, the cosmic dark age, as astrophysicists call it, ended. These stellar objects were especially bright, says Robertson: They were more massive than our sun and scorched hot, bright in the ultraviolet spectrum.

“UV rays, if they are active enough, can ionize hydrogen,” Robertson says. All it takes is one particle of light, and especially energy, called a photon, to strip an electron from a hydrogen atom and leave it with a positive electric charge.

When galaxies began to cluster, they first ionized their surroundings, leaving bubbles of charged hydrogen gas across the universe. As the light-emitting groups grow, more stars form to make them brighter and full of photons. Additional new galaxies began to develop as well. When it became luminous, the ionized bubbles began to overlap. This allowed a photon from a galaxy to “travel a much greater distance because it did not collide with a hydrogen atom as it passed through this lattice,” Robertson explains.

At this point, the rest of the intergalactic medium in the universe quickly ionizes—even in regions far from galaxies. Then the era of reionization ended and the universe as we know it began.

“This was the last time the properties of the universe were completely altered,” says Robertson. “It was also the first time that galaxies have had an impact beyond their local area.”

The James Webb Space Telescope searches for ionizing evidence

With all the hydrogen found between galaxies charged, the universe entered a new stage of formation. This ionization had a ripple effect on the formation of galaxies: any starry structures that formed after the cosmic dawn were likely affected.

“If you ionize a gas, you are also heating it up,” Hutter explains. Remember, high temperatures make it difficult for matter to assemble and form new stars and planets—and can even destroy existing gases. As a result, small galaxies forming in an ionized region may have trouble getting enough gas to form more stars. “This really does have an impact on the number of stars that galaxies form,” Hutter says. “It affects their entire history.”

Although scientists have a sense of the broad strokes of the reionization story, some big questions remain. For example, while they roughly know that the era ended about a billion years after the Big Bang, they aren’t entirely sure when reionization—and thus the formation of the first galaxy—began.

This is where JWST comes in. The new space telescope is designed to be able to search for the oldest parts of the universe invisible to the human eye, collecting data on the first rays of starlight that ionized the intergalactic medium. Astronomers largely discover celestial bodies through the radiation they emit. Objects further away from us tend to appear in infrared light, as distance distorts their wavelengths to become longer. As the universe expands, it may take billions of years for light to reach JWST detectors.

[Related: Astronomers are already using James Webb Space Telescope data to hunt down cryptic galaxies]

This, in a nutshell, is how scientists use the James Webb Telescope (JWST) to look at the first galaxies in the universe’s ionization process. While older instruments like the Hubble Space Telescope can spot the episodic early galaxy, a new space observatory can gather minute details to put star clusters back in time.

“Now, we can accurately work out how many galaxies are around us, you know, 900 million years after the Big Bang, 800, 700, 600, all the way to 300 million years after the Big Bang,” Robertson says. Using this information, astrophysicists can calculate how many ionizing photons were present at each age, and how the particles might affect their surroundings.

Painting a picture of the cosmic dawn is not 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 when they formed inside the first stars. “[The question] Hutter says, “Where did we come from?”


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