A subtle radio glow from the early universe may hold hidden clues about the first stars. Scientists now think this signal could reveal unexpected details about how cosmic light first emerged from darkness. (Artist’s concept). Credit: SciTechDaily.com

Astronomers are uncovering new ways to study the universe’s first stars, objects too distant and faint to observe directly, by examining the ancient 21-centimeter radio signal left behind by hydrogen atoms shortly after the Big Bang.

Understanding how the universe shifted from complete darkness to the first glow of starlight marks a major milestone in cosmic history, a period known as the Cosmic Dawn. Yet even today’s most advanced telescopes cannot directly capture these earliest stars, which makes uncovering their basic properties one of astronomy’s most difficult tasks.

A team of international researchers led by the University of Cambridge has now demonstrated that clues about the masses of these first stars can be found in a particular radio signal. The signal is produced by hydrogen atoms that filled the space between early star-forming regions and originated only about a hundred million years after the Big Bang.

By examining how the earliest stars and their remnants altered this signal, known as the 21-centimeter signal, the scientists show that upcoming radio observatories have the potential to reveal how the young universe evolved from a nearly uniform cloud of hydrogen into the richly structured cosmos seen today. The findings appear in the journal Nature Astronomy.

“This is a unique opportunity to learn how the universe’s first light emerged from the darkness,” said co-author Professor Anastasia Fialkov from Cambridge’s Institute of Astronomy. “The transition from a cold, dark universe to one filled with stars is a story we’re only beginning to understand.”

Research into the universe’s first generation of stars depends on the extremely faint 21-centimeter glow, an ancient form of energy that has traveled for more than 13 billion years. Because this signal is shaped by radiation from early stars and black holes, it offers one of the few ways scientists can explore the universe during its earliest stages.

REACH, SKA, and the Search for Ancient Starlight

Fialkov leads the theory group of REACH (the Radio Experiment for the Analysis of Cosmic Hydrogen). REACH is a radio antenna and is one of two major projects that could help us learn about the Cosmic Dawn and the Epoch of Deionization, when the first stars reionized neutral hydrogen atoms in the universe.

Although REACH, which captures radio signals, is still in its calibration stage, it promises to reveal data about the early universe. Meanwhile, the Square Kilometer Array (SKA)—a massive array of antennas under construction—will map fluctuations in cosmic signals across vast regions of the sky.

Both projects are vital in probing the masses, luminosities, and distribution of the universe’s earliest stars. In the current study, Fialkov – who is also a member of the SKA – and her collaborators developed a model that makes predictions for the 21-centimeter signal for both REACH and SKA, and found that the signal is sensitive to the masses of first stars.

“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, who is also a member of Cambridge’s Kavli Institute for Cosmology. “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.”

In developing their theoretical model, the researchers studied how the 21-centimeter signal reacts to the mass distribution of the first stars, known as Population III stars. They found that previous studies have underestimated this connection as they did not account for the number and brightness of X-ray binaries – binary systems made of a normal star and a collapsed star – among Population III stars, and how they affect the 21-centimeter signal.

A Statistical View of the First Stars

Unlike optical telescopes like the James Webb Space Telescope, which capture vivid images, radio astronomy relies on statistical analysis of faint signals. REACH and SKA will not be able to image individual stars, but will instead provide information about entire populations of stars, X-ray binary systems, and galaxies.

“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 PI at Cambridge of the SKA development activities. “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.”

Reference: “Determination of the mass distribution of the first stars from the 21-cm signal” by T. Gessey-Jones, N. S. Sartorio, H. T. J. Bevins, A. Fialkov, W. J. Handley, E. de Lera Acedo, G. M. Mirouh, R. G. Izzard and R. Barkana, 20 June 2025, Nature Astronomy. DOI: 10.1038/s41550-025-02575-x

The research was supported in part by the Science and Technology Facilities Council (STFC), part of UK Research and Innovation (UKRI). Anastasia Fialkov is a Fellow of Magdalene College, Cambridge. Eloy de Lera Acedo is an STFC Ernest Rutherford Fellow and a Fellow of Selwyn College, Cambridge.

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