Epoch of Reionization: Astronomers close in on signal from ‘one of the most unexplored periods in our universe’

Long before starlight filled the cosmos for the first time, the young universe may have been simmering, according to a new study.

The findings suggest that about 800 million years after the Big Bang, energy from newborn black holes and the fading embers of the first stars was already warming vast clouds of intergalactic hydrogen gas, offering a rare glimpse into a largely uncharted chapter of the universe’s youth.

“It’s one of the most unexplored periods in our universe,” study co-author Ridhima Nunhokee, a research scientist at the International Centre for Radio Astronomy Research in Perth, Australia, told Live Science. “There’s just so much to learn.”

Astronomers know that the universe began in an extremely hot, dense state, the Big Bang, about 13.8 billion years ago, and then cooled rapidly as it expanded. Roughly 400,000 years later, temperatures dropped enough for protons and electrons to merge into neutral hydrogen atoms, and the cosmos slipped into the “cosmic dark ages” — a long, lightless stretch when space was veiled by a dense fog of hydrogen gas.

Hundreds of millions of years later, the first generations of massive stars and faint young galaxies ignited, emitting intense ultraviolet light that slowly burned away this fog in a transformative period known as the Epoch of Reionization. That process, which ended about 1 billion years after the Big Bang, made the universe transparent and allowed starlight to travel freely through space for the first time, marking the dawn of the cosmos as we know it.

What the universe was like as it began to emerge from those dark ages remains one of astronomy’s biggest open questions.

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The new findings, detailed in a paper published Sept. 30 in The Astrophysical Journal, suggest that before the universe “lit up,” it may not have been as frigid as many models predict. By narrowing the possibilities for what the early cosmos was like, the results offer an important new clue to understanding how the first stars and galaxies began to reshape their environment, researchers say.

The radio sky image (background) represents the cleanest signal ever produced from data collected by the Murchison Widefield Array (foreground) in Western Australia. (Image credit: Nunhokee et al/ICRAR/Curtin University)The universe’s echoes

Detecting this ancient signal, however, is extraordinarily difficult. It is buried beneath layers of much stronger radio noise from the Milky Way, other nearby galaxies, Earth’s atmosphere and even the telescope itself. To uncover it, the team developed a new statistical filtering technique to strip away these foreground signals and isolate the most probable emission from hydrogen gas dating to roughly 800 million years after the Big Bang.

This new approach produced the cleanest radio map yet of the early universe and set the most stringent limits so far on the strength of the 21-centimeter signal, the team noted in the study.

Despite focusing on what Nunhokee described as “kind of a cold patch where we have just a few sources,” and using “the best data that we have,” the team found no evidence for the telltale signal. “Because it’s very faint, it’s very hard,” she said.

After cleaning the data, the researchers didn’t see the distinctive signature that would indicate a “cold start” to reionization. This feature would have been visible in their data if the universe, about 800 million years after the Big Bang, had remained frigid until the first stars ignited, so the result suggested the universe was warmer than expected, according to the study.

“As the universe evolved, the gas between galaxies expands and cools, so we would expect it to be very, very cold,” study lead author Cathryn Trott, a professor at the Curtin Institute of Radio Astronomy, said in a statement. “Our measurements show that it is at least heated by a certain amount. Not by a lot, but it tells us that very cold reionisation is ruled out — that’s really interesting.”

Cosmological models point to X-rays from early black holes and the remnants of massive stars as the likely culprits heating the intergalactic gas long before visible starlight filled the cosmos, Nunhokee said.

The team’s new data-cleaning technique also lays crucial groundwork for the upcoming Square Kilometre Array (SKA). Scientists say this next-generation radio telescope, which is now under construction in Australia and South Africa, will have the sensitivity to detect the elusive 21-centimeter signal directly.

“We know what we are looking for,” Nunhokee said. “We just need a few hours of [SKA’s] data that will allow us to go to the levels that we want to.”