The international team, led by researchers at the University of Manchester and the University of the Western Cape, used South Africa's MeerKAT radio telescope to measure emissions from neutral hydrogen dating to a period when the Universe was several billion years younger than it is today.

Published in The Astrophysical Journal Letters, the results highlight the growing potential of hydrogen intensity mapping, a technique designed to survey enormous regions of space far more efficiently than methods that rely on identifying galaxies individually.

Mapping the Universe With Hydrogen

Neutral hydrogen naturally gives off a very weak radio emission known as the 21-centimeter line. Because the Universe is expanding, this signal becomes stretched to longer wavelengths as it travels through space. By measuring that shift, astronomers can study hydrogen from different eras in cosmic history.

Hydrogen intensity mapping takes advantage of this signal in a different way from conventional galaxy surveys. Instead of attempting to identify and measure one galaxy at a time, astronomers detect the combined radio glow produced by hydrogen in large numbers of galaxies that cannot be individually resolved.

That approach makes it possible to examine huge volumes of space and reconstruct a three-dimensional view of how matter is distributed across the Universe.

Previous reliable measurements of hydrogen at these distances generally required astronomers to combine radio telescope observations with data from optical galaxy surveys. In this case, the researchers were able to directly identify the hydrogen intensity mapping signal using MeerKAT radio observations by themselves.

A Signal From Billions of Years Ago

The researchers examined about 96 hours of MeerKAT observations and found the signal from two separate periods in cosmic history. The emissions had traveled for approximately four to five billion years before arriving at Earth.

The measurements allowed the team to trace hydrogen across distances of several million light-years, roughly comparable to the separation between the Milky Way and the neighboring Andromeda galaxy.

"This is a very exciting milestone," said Dr. Sourabh Paul, lead author of the study. "Hydrogen intensity mapping has long been seen as a promising way to map the Universe efficiently, but the signal is extremely faint and difficult to isolate from foreground emission, human-made radio-frequency interference, and instrumental effects. Detecting it directly with MeerKAT shows that this technique is becoming a practical tool for cosmology."

Extracting such a weak signal required the researchers to carefully account for many different sources of interference that could distort the measurements.

"This was a challenging data analysis process, requiring a detailed understanding of the many sources of contamination that can affect such a faint measurement," Professor Santos added. "It is particularly remarkable that the data used in this study were taken in 2018, when MeerKAT had only just started science operations. There is now a rich trove of MeerKAT data waiting to be explored with this method."

A New Tool for Studying Galaxy Evolution

According to the researchers, the results create new opportunities to measure neutral hydrogen across cosmological distances and investigate how galaxies have formed and changed throughout the history of the Universe.

Dr. Zhaoting Chen, co-author of the study, said: "Neutral hydrogen is one of the key ingredients for understanding how galaxies form and evolve.

"With intensity mapping, we do not need to detect every individual galaxy. Instead, we can measure the collective signal from hydrogen across large cosmic volumes, giving us a new way to study both galaxy evolution and the underlying matter distribution of the Universe."

The successful detection could also have major implications for upcoming cosmological surveys. Hydrogen intensity mapping is expected to become an important scientific focus for the Square Kilometre Array Observatory. MeerKAT serves as a precursor telescope for that facility.

Preparing for the Square Kilometre Array

Professor Laura Wolz, co-author of the study from the University of Manchester, added: "MeerKAT continues to open new windows for cosmology. The fact that this signal can be extracted from observations that were not originally designed for hydrogen intensity mapping is very encouraging. It shows the enormous scientific value of MeerKAT data and points the way to future observations with SKAO."

Future studies that observe larger portions of the sky for longer periods should allow astronomers to map neutral hydrogen with even greater precision.

Those observations could help researchers understand how galaxies developed, determine how dark matter influences the cosmic web, and reconstruct how the Universe has changed across billions of years.