A 100-Million-Year-Old Clue Found: Scientists Trace the Signal of Star Birth After the Big Bang

The Cosmic Dawn: When the Universe Woke Up, The 21-Centimeter Signal: A Fossil from the Universe’s Infancy, The Hunt for the First Stars: REACH and the Square Kilometre Array, X-Ray Binaries: The Overlooked Cosmic Players, A New Cosmic Clock: Timing the Universe’s First Light, What’s Next? The Future of Early Universe Astronomy

Astronomers have been trying to fit the cosmic puzzle of how the universe changed from a dark, featureless void into a glittering expanse of stars and galaxies for decades. Now, a faint whisper from the dawn of time, a radio signal emitted just 100 million years after the Big Bang has given scientists an unheard-of window into the first starlight in the universe. Under the direction of University of Cambridge researchers, an international team has cracked hints buried in this ancient signal, exposing secrets about the mass, formation, and explosive deaths of the very first stars of the universe.

The discovery depends on the 21-centimeter signal, a ghostly fingerprint left by hydrogen atoms formerly filling the early universe. Scientists are now closer than ever to grasp Cosmic Dawn, the turning point when darkness gave way to light by studying how the radiation of the first stars shaped this signal.

The Cosmic Dawn: When the Universe Woke Up

The Cosmic Dawn: When the Universe Woke Up, The 21-Centimeter Signal: A Fossil from the Universe’s Infancy, The Hunt for the First Stars: REACH and the Square Kilometre Array, X-Ray Binaries: The Overlooked Cosmic Players, A New Cosmic Clock: Timing the Universe’s First Light, What’s Next? The Future of Early Universe Astronomy

The universe was a chilly, hazy sea of hydrogen gas before stars. Then, about 200 million years following the Big Bang, gravity gathered the first clusters of matter, sparking Population III stars, massive, short-lived giants composed only of hydrogen and helium. Their ultraviolet light started to reionize electrons from nearby hydrogen atoms, changing the universe.

No telescope, though, has ever seen these stars directly since they lived quickly and died young. Instead, astronomers rely on subtle distortions in the 21-centimeter signal, caused by the way these stars and their remnants heated and ionized the surrounding gas.

"This is a special chance to learn how the first light of the universe emerged from the darkness," states project lead researcher Professor Anastasia Fialkov. "We are only starting to understand the tale from a cold, dark universe to one filled with stars."

The 21-Centimeter Signal: A Fossil from the Universe’s Infancy

The Cosmic Dawn: When the Universe Woke Up, The 21-Centimeter Signal: A Fossil from the Universe’s Infancy, The Hunt for the First Stars: REACH and the Square Kilometre Array, X-Ray Binaries: The Overlooked Cosmic Players, A New Cosmic Clock: Timing the Universe’s First Light, What’s Next? The Future of Early Universe Astronomy

The most plentiful element in the early universe, hydrogen atoms, go through a "spin-flip" transition that generates a photon at a wavelength of 21 centimeters. Having travelled over 13 billion years to get to us, this signal is quite faint and buried under layers of cosmic noise.

This signal is so important because it carries traces of the impact of the first stars. Their ultraviolet light changed the spin states of hydrogen atoms as they burned, and their explosive deaths left X-ray binaries (pairs of a star and a collapsed stellar remnant) further heated the gas, so altering the signal's appearance now.

"We are the first group to routinely model how the 21-centimeter signal depends on the masses of the first stars, including the impact of X-ray binaries," says Fialkov. " Previous studies underestimated this connection."

The Hunt for the First Stars: REACH and the Square Kilometre Array

The Cosmic Dawn: When the Universe Woke Up, The 21-Centimeter Signal: A Fossil from the Universe’s Infancy, The Hunt for the First Stars: REACH and the Square Kilometre Array, X-Ray Binaries: The Overlooked Cosmic Players, A New Cosmic Clock: Timing the Universe’s First Light, What’s Next? The Future of Early Universe Astronomy

Finding this signal calls for next-generation radio telescopes able to sort modern radio pollution from interference coming from our own galaxy. Two important initiatives driving the change are:

  • Designed to separate the 21-centimeter signal from foreground noise using Bayesian statistical modeling, a specialized antenna array in the Karoo desert of South Africa
  • Set to be operational by 2027, the massive worldwide telescope network known as the Square Kilometer Array (SKA) will map cosmic hydrogen fluctuations over large swathes of the sky.

Unlike the James Webb Space Telescope, which records breathtaking images of early galaxies, these radio telescopes will not detect individual stars. Rather, they will offer statistical analysis of whole populations of the first stars and their leftovers.

X-Ray Binaries: The Overlooked Cosmic Players

The Cosmic Dawn: When the Universe Woke Up, The 21-Centimeter Signal: A Fossil from the Universe’s Infancy, The Hunt for the First Stars: REACH and the Square Kilometre Array, X-Ray Binaries: The Overlooked Cosmic Players, A New Cosmic Clock: Timing the Universe’s First Light, What’s Next? The Future of Early Universe Astronomy

The important part X-ray binary pairs of a living star and a collapsed stellar core (neutron star or black hole) play in the study is among its most unexpected results. Strong X-rays emitted by these systems heat the surrounding hydrogen gas, changing the 21-centimeter signal in ways past models overlooked.

"Before, we understated the number and brightness of these X-ray binaries among Population III stars," notes Fialkov. "Now we see they're key to understand how the first stars shaped the early universe."

Therefore, the first stars did not only illuminate the universe; their deaths still affected it long after they burst.

A New Cosmic Clock: Timing the Universe’s First Light

The Cosmic Dawn: When the Universe Woke Up, The 21-Centimeter Signal: A Fossil from the Universe’s Infancy, The Hunt for the First Stars: REACH and the Square Kilometre Array, X-Ray Binaries: The Overlooked Cosmic Players, A New Cosmic Clock: Timing the Universe’s First Light, What’s Next? The Future of Early Universe Astronomy

The 21-centimeter signal is cosmic clock rather than a snapshot. Scientists can ascertain, depending on whether the signal shows in absorption or emission:

  • When cosmic dawn's first stars emerged
  • How fast the universe ionized under Epoch of Reionization?
  • When X-ray heating started (set off by the first stellar remnants)

"It takes imagination to link radio data to the tale of the first stars, but the implications are profound," says Fialkov. "We're practically reading the baby album of the universe."

What’s Next? The Future of Early Universe Astronomy

The Cosmic Dawn: When the Universe Woke Up, The 21-Centimeter Signal: A Fossil from the Universe’s Infancy, The Hunt for the First Stars: REACH and the Square Kilometre Array, X-Ray Binaries: The Overlooked Cosmic Players, A New Cosmic Clock: Timing the Universe’s First Light, What’s Next? The Future of Early Universe Astronomy

As REACH is in calibration and SKA is under construction, the next decade may change our understanding of the first billion years of the cosmos. Still unresolved are certain important issues:

  • The first stars were of what size? ( Current models show they were ten to thousand times heavier than the Sun.)
  • Did they evolve alone or in clusters?
  • In what ways did their passing sow next galaxies?

"Radio telescopes like REACH are promising to unlock the mysteries of the infant universe," co-author and chief investigator of REACH Dr. Eloy de Lera Acedo said. " These projections are absolutely essential for guiding our Karoo observations."

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