At Last, We Understand What Switched the Lights On at the Birth of Time
A study published in February 2024 in Nature examines how light first filled the cosmos. The research shows that tiny dwarf galaxies helped clear a region of neutral hydrogen that filled intergalactic space, allowing light to travel more freely.
Dwarf galaxies: the unlikely heroes
The study’s main finding is that it was not the large galaxies or massive black holes that did most of the work. Instead, ultra-faint, low-mass dwarf galaxies appear to have been the primary sources of the ionising photons that drove cosmic reionization. The team found that these small galaxies emitted enough energetic radiation to turn neutral hydrogen into ionised plasma, a major step in the universe’s evolution.
Hakim Atek from the Institut d’Astrophysique de Paris, who led the work, said, “These cosmic powerhouses collectively emit more than enough energy to get the job done.” He was joined by Iryna Chemerynska (also at the Institut d’Astrophysique de Paris) and Themiya Nanayakkara from Swinburne University of Technology, Australia. Iryna commented, “This discovery shows the important role played by ultra-faint galaxies in the early Universe’s evolution.”
How they looked at the early universe
To probe the cosmic dawn the team used the James Webb Space Telescope (JWST) alongside the Hubble Space Telescope. The JWST, designed to observe those early times, provided detailed spectra of the dwarf galaxies. The study used gravitational lensing, with the dense galaxy cluster Abell 2744 acting as a magnifier to bring these faint objects into view. Combining JWST and Hubble data allowed an analysis supporting both the numbers and the influence of dwarf galaxies at that epoch.
A short timeline of what happened
The move from darkness to light began soon after the Big Bang, when the universe was a hot, dense ionised plasma. After roughly 300,000 years, the universe cooled enough for particles to combine into neutral hydrogen (the recombination epoch). Later, the first stars, made mostly of hydrogen and helium, began to shine. Their radiation reionised the gas, and by about 1 billion years after the Big Bang the whole universe was reionised.
What the study can’t yet tell us (and what comes next)
The researchers note the limits of their work: the observations come from “one small patch of the sky.” More gravitational-lens fields need to be studied to determine whether dwarf galaxies were generally dominant in reionization rather than only in this region. As Nanayakkara put it, “We have now entered uncharted territory with the JWST.”
This investigation into ultra-faint galaxies challenges earlier assumptions that larger galaxies and black holes were the main contributors and points to further study as new data arrive.