Astronomers using the James Webb Space Telescope to look over 99.99% of the way back in time have discovered the earliest signs of the universe becoming transparent just 330 million years after the Big Bang.
Previous studies placed the beginning of the universe becoming transparent, a period astronomers call the Epoch of Reionization, at roughly 500 million years after the Big Bang. The new date discovered by astronomers at the Cosmic Dawn Center (DAWN) at the Niels Bohr Institute and DTU Space could force cosmologists to reevaluate existing models.
The Earliest Signs of the Universe Becoming Transparent Impossible Without JWST
Spotting signs of the universe becoming transparent so soon after the Big Bang would have been impossible before the JWST was put into service. That’s because simply capturing the light from a galaxy this far away requires JWST’s unique placement and specialized instruments.
Since its first observations, the JWST has made several discoveries by capturing the light from the early universe, including discoveries that challenge currently accepted ideas and others that offer support for alternative theories. Some examples include the discovery of the earliest known galaxy ever spotted, an inside-out galaxy, a dead galaxy, an ancient quasar, drifting quasars, “red monsters,” a lounging black hole, and even a primordial galaxy that shouldn’t have been able to form.
“We knew that we would find some of the most distant galaxies when we built Webb,” says Peter Jakobsen, affiliated professor at DAWN, project scientist behind James Webb’s spectrograph NIRSpec, and second-author of the study detailing the discovery “But we could only dream of one day being able to probe them in such detail that we can now see directly how they affect the whole Universe.”
According to the astronomers behind the study, JWST’s latest revelations may mean that the Epoch of Reionization began hundreds of millions of years before previously believed.
Reionization and a “Foggy Universe”
As the capabilities of telescopes continue to advance, astronomers have gained the ability to peer farther and farther back in time. Because hot, newborn galaxies are brightest in the ultraviolet (UV) spectrum, detections are made mainly by observing light outside the visible range. Unfortunately, even the ability of JWST and other modern observatories to peer back to the dawn of the universe has reached a limit at around 300 million years after the Big Bang.
According to a statement announcing the team’s discovery, the universe’s earliest galaxies were “enshrouded” in a dense neutral gas cloud. When the UV light from the galaxies collides with this gas, the light is absorbed and never makes it to humanity’s telescopes.
The collision between UV rays and gas also splits these gas atoms, a process called reionization. After enough time, the clouds surrounding the first galaxies dissipated, and the “foggy” universe started becoming transparent. Estimates typically place the start of this Epoch of Reionization around 500 million years after the Big Bang and its end about 500 million years later. Before JWST, spotting light from galaxies that formed before this time was considered impossible.
“Young galaxies shine brightest at a very specific wavelength of light, originating from hydrogen,” explained Joris Witstok, a postdoc at DAWN and the lead study leader. “To astronomers, this light goes under the name “Lyman alpha”. Because of its short UV wavelength, it is easily absorbed by the surrounding medium, and therefore no galaxy from when the Universe was less than half a billion years old has showed us this particular kind of light.”
universe becoming transparent
Observed through seven different filters, the galaxy JADES-GS-z13-1 is shown with more ultraviolet light (left) compared to longer wavelength images in the four furthest to the right. The galaxy is completely invisible in the shortest wavelengths images on the left (Credit: Witstok et al. (2025)).
The discovery occurred when the team used JWST to study JADES-GS-z13-1, a galaxy that formed just 300 million years after the Big Bang, or well before previous estimates for the Epoch. When examining their data, the researchers found unmistakable evidence of Lyman alpha light over 200 million years earlier than they thought possible.
The Universe’s Earliest “Bubbles”
Researchers are currently exploring possible explanations for the discrepancy. One theory involves black holes superheating the surrounding gas until it is destroyed.
solar wind
“Most galaxies are known to host a central, supermassive black hole,” Witstok said. “As these monsters engulf surrounding gas, the gas is heated to millions of degrees, making it shine brightly in X-rays and UV before disappearing forever.”
Computer simulations show that the reionization of gas formed individual “bubbles” of transparency within the early universe. Whitsok said these bubbles “percolate the Universe” until around a billion years after the Big Bang. By then, the bubbles all overlap, bringing the Epoch of Reionization to an end. Although this process likely took several hundred million years to complete, the study leader believes they may have simply spotted the earliest evidence of the universe becoming transparent.
“We believe that we have discovered one of the first such bubbles,” Witstok said
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him onX,learn about his books atplainfiction.com, or email him directly atchristopher@thedebrief.org.