Scientists using the James Webb Space Telescope have discovered an ancient and distant galaxy that provides clues to a crucial transition period that may have allowed the early universe to emerge from its "dark ages" earlier than previously thought.
According to the researchers, the telescope—which peers into the past by peering into the depths of the universe—detected a galaxy named JADES-GS-z13-1, as it appeared about 330 million years after the Big Bang, which occurred about 13,8 billion years ago. For comparison, Earth is about 4,5 billion years old.
The universe is thought to have expanded rapidly within a fraction of a second of the Big Bang. Once sufficiently cooled, it went through a phase known as the cosmic dark ages, during which it was immersed in a dense fog of neutral hydrogen. This was followed by the Epoch of Reionization, when light began to penetrate this fog. The Webb telescope found evidence that the galaxy JADES-GS-z13-1 had already reached this phase.
Joris Witstok, astrophysicist at the Center for the Origin of the Universe from the University of Copenhagen and the Niels Bohr Institute, said:
« The Webb telescope has confirmed that JADES-GS-z13-1 is one of the most distant galaxies known to date. »
Co-lead author of the study published in the journal Nature, Witstok adds:
« Unlike other distant galaxies, this one shows a very clear signature of a powerful source of high-energy ultraviolet radiation, indicating that it began the process of reionization much earlier than expected. »
A still unsolved enigma
The period when the first stars, black holes, and galaxies formed is called the cosmic dawn. During this phase, the emitted UV rays altered the chemistry of neutral hydrogen, initiating a process of reionization that allowed light to circulate freely, illuminating the universe.
Kevin Hainline, an astrophysicist at the University of Arizona's Steward Observatory and co-author of the study, explains:
« After the Big Bang, the universe was a mixture of hydrogen and helium gases and dark matter, which gradually cooled. The universe then became completely opaque to UV rays. Hydrogen floated in neutral form, each atom bound to an electron. »
He adds :
« As the first stars and galaxies formed from this gas, UV rays emitted by young stars and growing massive black holes began to ionize the hydrogen. "Over hundreds of millions of years, the universe transitioned from dark to light.»
The researchers say the light detected by Webb could come from intense star formation in the galaxy's core, or from the presence of a growing supermassive black hole, or even a combination of both.
The JADES-GS-z13-1 galaxy is about 230 light-years across—hundreds of times smaller than the Milky Way. (One light-year is 9,5 trillion kilometers.)
Launched by NASA in 2022 and operational since 2023, the James Webb telescope provides a deeper understanding of the early universe.
So far, only four older galaxies have been observed, including the current record holder, observed 294 million years after the Big Bang – with no evidence of reionization.
The surprising discovery of signs of reionization in JADES-GS-z13-1, in the form of a bubble of ionized hydrogen, astonished scientists, as it was previously thought that this phenomenon began much later.
