From a fiery origin to its slow expansion and eventual formation of structure, the Universe's evolution has long fascinated scientists. A key chapter in this story unfolded a few hundred million years after the Big Bang, when the earliest galaxies and stars began coalescing from vast reservoirs of primordial gas. Astronomers have detected galaxies dating back to less than 300 million years post- by Robert Schreiber
Berlin, Germany (SPX) Apr 01, 2025
From a fiery origin to its slow expansion and eventual formation of structure, the Universe's evolution has long fascinated scientists. A key chapter in this story unfolded a few hundred million years after the Big Bang, when the earliest galaxies and stars began coalescing from vast reservoirs of primordial gas. Astronomers have detected galaxies dating back to less than 300 million years post-Big Bang, but the precise timing and mechanisms of their emergence remain hotly debated.
One early galaxy has now illuminated a critical shift in the young cosmos: the first signs of the Universe becoming transparent.
A foggy challenge
The birth of the first galaxies was veiled by dense clouds of neutral hydrogen gas. These early galaxies emitted powerful ultraviolet (UV) radiation, but neutral hydrogen absorbs UV light efficiently. As a result, much of the light from these young objects was obscured, particularly in the UV spectrum, making them nearly impossible to detect directly at those wavelengths.The light that does escape comes in less energetic forms, presenting a serious obstacle to astronomers trying to observe this formative period in cosmic history.
Clearing the cosmic haze
Over time, UV photons from the earliest stars and galaxies began to ionize the surrounding hydrogen atoms, splitting them and rendering the gas transparent to radiation. This transition, known as the Epoch of Reionization, represents a major milestone in the Universe's evolution.Researchers continue to investigate when and how this process began and what sources triggered it. The prevailing theory held that reionization commenced around 500 million years after the Big Bang and concluded another half-billion years later.
A new finding by astronomers at the Cosmic Dawn Center (DAWN), part of the Niels Bohr Institute and DTU Space, now suggests this process may have started significantly earlier. Their study focused on one of the most ancient galaxies yet discovered and offers compelling evidence of an ionized region surrounding it.
"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."
Bubble of light
This status quo has now been upended by the detection of Lyman alpha emissions from galaxy JADES-GS-z13-1, one of the most distant known.How could such light pass through a Universe still saturated with opaque hydrogen?
"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."
This isolated Lyman alpha signal indicates that a transparent, ionized zone surrounds the galaxy, permitting the light to escape and reach our instruments.
A view only Webb could provide
The unprecedented sensitivity and spectral capabilities of the James Webb Space Telescope enabled this landmark detection."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."
Although extremely bright stars are one potential source of such ionizing radiation, another hypothesis remains open:
"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.
Research Report:Witnessing the onset of reionization through Lyman-a emission at redshift 13
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