Astronomers have found early evidence of the Universe’s transformation from a foggy, opaque state to a transparent one, thanks to a galaxy blazing with UV light nearly 13.6 billion years ago.
The first galaxies in the Universe were born hidden within a thick “fog” of neutral gas, making them difficult to observe. This fog didn’t clear until galaxies themselves began to ionize the gas around them. Now, using the James Webb Space Telescope, a team of astronomers from the Cosmic Dawn Center in Copenhagen has detected the earliest known sign of this dramatic transformation. They observed a galaxy that existed just 330 million years after the Big Bang, surrounded by a bubble of transparent, ionized gas—evidence that the Universe’s transition from opaque to transparent began earlier than previously believed. The discovery was published in Nature.
When Galaxies First Lit Up
Forged in fire, cooled by expansion, shaped by gravity.
That’s the Universe, in brief.
A few hundred million years after the Big Bang, the first stars and galaxies began to form, condensing out of massive clouds of gas. The exact timing is still under active investigation, but astronomers have already found galaxies dating back to less than 300 million years after the Universe’s origin.
One of these early galaxies now offers new insight into how the young Universe evolved.
Fog of the Early Universe
Detecting the first galaxies isn’t easy, mainly because of the very gas they formed from.
Newborn galaxies emit most of their light in the energetic ultraviolet (UV) range. However, during the Universe’s first 500 million years, much of the gas surrounding and filling the space between galaxies was neutral, meaning it hadn’t yet been ionized by starlight. Neutral gas is highly effective at absorbing UV light, so the brightest emissions from these galaxies were blocked.
As a result, galaxies from this early era are practically invisible in UV wavelengths, hidden behind a thick cosmic fog.
The Rise of Reionization
As UV radiation is emitted from the first sources of light, it slowly begins to transform the Universe. The neutral atoms that hide the galaxies are split apart by the UV light, eventually rendering the Universe transparent.
This process is known as the Epoch of Reionization, and its detailed circumstances are a subject of intense research in astronomy: When did it start, how long did it take, how did it proceed, and which sources were responsible?
Until recently, the consensus was that reionization did not begin until the Universe was around half a billion years old, completing another half billion years later.
But this notion is now challenged by a new study, led by astronomers at the Cosmic Dawn Center (DAWN) at the Niels Bohr Institute and DTU Space. Investigating one of the most distant galaxies, the researchers have discovered a clear sign of reionization beginning significantly earlier than hitherto thought.
Joris Witstok, postdoc at DAWN who led the study, explains:
“Young galaxies shine brightest at a very specific wavelength of light, originating from hydrogen. 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.”
Cosmic Bubbles of Clarity
That is, until now.
What Witstok and his team found was that one of the very most distant galaxies, known as JADES-GS-z13-1, is gleaming brilliantly with Lyman alpha light.
But how can Lyman alpha escape a galaxy cloaked in dense, neutral gas?
“We know from our theories and computer simulations, as well as from observations at later epochs, that the most energetic UV light from the galaxies »fries« the surrounding neutral gas, creating bubbles of ionized, transparent gas around them,” Witstok elaborates. “These bubbles percolate the Universe, and after around a billion years, they eventually overlap, completing the epoch of reionization. We believe that we have discovered one of the first such bubbles.”
In other words, the detection of the Lyman alpha light is a telltale signature of an ionized bubble, because it would not be able to escape otherwise.
Webb Unlocks the Fog
The observations would not have been possible without the sensitivity of James Webb and its ability to explore the light of galaxies wavelength by wavelength.
“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. “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.”
Black Holes or Starfire?
The question remains what exactly is the cause of the ionized bubble. Although the first stars are thought to be very hot and extremely UV bright, there is another possibility:
“Most galaxies are known to host a central, supermassive black hole. 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. This is another viable cause of the bubbles, which we will now investigate,” Witstok concludes.
Explore Further: Webb Telescope Detects “Impossible” Light From the Dawn of Time
Reference: “Witnessing the onset of reionization through Lyman-α emission at redshift 13” by Joris Witstok, Peter Jakobsen, Roberto Maiolino, Jakob M. Helton, Benjamin D. Johnson, Brant E. Robertson, Sandro Tacchella, Alex J. Cameron, Renske Smit, Andrew J. Bunker, Aayush Saxena, Fengwu Sun, Stacey Alberts, Santiago Arribas, William M. Baker, Rachana Bhatawdekar, Kristan Boyett, Phillip A. Cargile, Stefano Carniani, Stéphane Charlot, Jacopo Chevallard, Mirko Curti, Emma Curtis-Lake, Francesco D’Eugenio, Daniel J. Eisenstein, Kevin N. Hainline, Gareth C. Jones, Nimisha Kumari, Michael V. Maseda, Pablo G. Pérez-González, Pierluigi Rinaldi, Jan Scholtz, Hannah Übler, Christina C. Williams, Christopher N. A. Willmer, Chris Willott and Yongda Zhu, 26 March 2025, Nature.
DOI: 10.1038/s41586-025-08779-5
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