Listening to the Cosmic Past: Hydrogen Signals Detected by MeerKAT

When you stare at the night sky, you’re really looking back in time. In a recent study, a group of astronomers actually managed to hear that ancient whisper – the faint radio signal of hydrogen that was traveling across the cosmos four to five billion years ago.

The feat was pulled off with MeerKAT, a sprawling radio telescope in South Africa’s Northern Cape that strings together 64 dish antennas. It’s not the kind of instrument you’d normally use for this job, but the researchers squeezed out a surprising amount of information from data that were originally collected for other purposes.

Hydrogen, the lightest and most plentiful element in the universe, emits a characteristic radio glow known as the 21‑centimeter line. As the universe expands, that signal gets stretched – or “red‑shifted” – and the amount of stretch tells us how long the photons have been on their long trek to Earth. By measuring the red‑shifted 21‑cm line across many points in the sky, scientists can build a three‑dimensional map of where hydrogen sits, a technique called hydrogen intensity mapping.

“This is a very exciting milestone,” said project leader Sourabh Paul. “Hydrogen intensity mapping has long been seen as a promising way to map the universe efficiently, but the signal is extremely faint and hard to pull out from foreground noise, human‑made interference, and instrumental quirks.”

After combing through about 96 hours of MeerKAT observations, the team identified a clear 21‑cm signature that dates back roughly four to five billion years – a period when the cosmos was still a youngster compared to its current 13.8‑billion‑year age. The hydrogen they detected stretches across swaths of space that span millions of light‑years, comparable to the gap between the Milky Way and our neighbour, Andromeda.

“Neutral hydrogen is one of the key ingredients for understanding how galaxies form and evolve,” explained Zhaoting Chen from the University of Edinburgh. “With intensity mapping we don’t have to pick out each galaxy one by one; we can just measure the collective glow of hydrogen across huge volumes and learn about both galaxy growth and the underlying matter distribution.”

The breakthrough arrives just as the next‑generation Square Kilometre Array Observatory (SKAO) is taking shape in Western Australia and the Karoo region of South Africa. “MeerKAT continues to open new windows for cosmology,” noted Laura Wolz of the University of Manchester. “Extracting this signal from data that weren’t originally meant for hydrogen mapping is very encouraging and points the way to future SKAO observations.”

Looking ahead, the team plans to point MeerKAT at larger portions of the sky for longer stretches of time, hoping to refine the hydrogen map and eventually piece together how the universe’s biggest filaments and voids formed over billions of years. “It was a challenging data‑analysis process, requiring a detailed understanding of many sources of contamination,” said Mario G. Santos of the University of the Western Cape. “It’s remarkable that the data we used were taken back in 2018, when MeerKAT had just started scientific operations. There’s a rich trove of observations waiting to be mined with this method.”

The findings were published in the July issue of The Astrophysical Journal Letters, and they may well become a cornerstone for future cosmological surveys that rely less on optical telescopes and more on the subtle glow of hydrogen itself.