An ancient radio signal, generated just 100 million years after the Big Bang, may hold the key to understanding the Cosmic Dawn—the period when the first stars, black holes, and galaxies formed—and the Epoch of Reionization, when early stars reionized neutral hydrogen. A new international study led by the University of Cambridge shows how the earliest stars influenced this elusive signal.
While the finite speed of light and the vast distances in space allow astronomers to peer back in time, no telescope can directly observe the Cosmic Dawn. During this early epoch, the signal was formed as hydrogen atoms flowed into the voids between star-forming regions. Today, that signal may offer insights into the masses of the universe’s first stars.
The 21-Centimeter Signal
The signal at the center of the study is known as the “21-centimeter signal,” named for the wavelength of photon emissions produced when neutral hydrogen atoms undergo a spin-flip transition—reversing the orientation of an electron’s spin relative to its proton. The new research highlights how future advances in radio astronomy could help scientists use this signal to understand how the modern universe emerged from an early cosmic environment dominated by hydrogen.
“This is a unique opportunity to learn how the universe’s first light emerged from the darkness,” said co-author Professor Anastasia Fialkov of Cambridge’s Institute of Astronomy and leader of REACH’s theory group. “The transition from a cold, dark universe to one filled with stars is a story we’re only beginning to understand.”
Advancing the Radio Signal Search
The signal is faint and subtle, reflecting more than 13 billion years of influence from stars and black holes. Two major upcoming projects are poised to provide astronomers with the tools needed to detect and analyze these signals with unprecedented precision.
The Radio Experiment for the Analysis of Cosmic Hydrogen (REACH) is a multi-antenna radio array still in its calibration phase. A collaboration between the University of Cambridge and Stellenbosch University in South Africa, REACH could play a key role in unlocking the potential of the 21-centimeter signal.
Another major initiative, the Square Kilometre Array (SKA), is a large-scale international radio telescope project currently under construction. A joint effort between the governments of Australia and South Africa, SKA will feature an expansive array of antennas designed to map radio signal fluctuations across vast areas of the sky. The project is expected to become operational in 2027.
Recognizing a New Data Source
“We are the first group to consistently model the dependence of the 21-centimeter signal of the masses of the first stars, including the impact of ultraviolet starlight and X-ray emissions from X-ray binaries produced when the first stars die,” said Fialkov. “These insights are derived from simulations that integrate the primordial conditions of the universe, such as the hydrogen-helium composition produced by the Big Bang.”
The study examined how the mass distribution of the earliest stars affected the signal, finding that earlier models underestimated the correlation. The difference lies in the new study’s consideration of the amount and brightness of binary systems consisting of a living star paired with a collapsed one—so-called X-ray binaries. The team’s findings suggest that, rather than relying solely on imaging data from observatories like the James Webb Space Telescope, researchers can use statistical analysis of REACH and SKA data to better understand entire populations of early stars.
“It takes a bit of imagination to connect radio data to the story of the first stars, but the implications are profound,” said Fialkov.
“The predictions we are reporting have huge implications for our understanding of the nature of the very first stars in the Universe,” said co-author Dr. Eloy de Lera Acedo, Principal Investigator of the REACH telescope and Cambridge lead for the SKA project. “We show evidence that our radio telescopes can tell us details about the mass of those first stars and how these early lights may have been very different from today’s stars.
“Radio telescopes like REACH are promising to unlock the mysteries of the infant Universe, and these predictions are essential to guide the radio observations we are doing from the Karoo, in South Africa.”
The paper “Determination of the Mass Distribution of the First Stars from the 21-cm Signal” appeared on June 20, 2025, in Nature Astronomy.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.