The physics behind the detection is elegantly simple in principle. Neutral hydrogen atoms, the most abundant form of matter in the cosmos, naturally emit radio waves at a wavelength of 21 centimetres, produced by a subtle flip in the relative orientation of the proton and electron in each atom. As the Universe expands, this characteristic emission is stretched to longer wavelengths on its journey to our telescopes, a redshifting effect that encodes the distance and therefore the look-back time of the emitting gas. By tuning receivers to different radio frequencies, astronomers can in effect slice the Universe at different epochs of cosmic history, turning the sky into a three-dimensional map of hydrogen across billions of light years.

What makes intensity mapping revolutionary is that it abandons the traditional approach of cataloguing galaxies one at a time. Conventional galaxy surveys require painstakingly resolving and measuring individual objects, a process that becomes prohibitively slow and expensive at the vast distances where the bulk of cosmic history unfolded. Intensity mapping instead measures the combined radio emission from countless unresolved galaxies within each patch of sky. The individual galaxies blur together into a statistical signal, but that signal still carries the imprint of the cosmic web, the vast network of filaments, sheets and voids along which matter is distributed. This allows astronomers to survey enormous cosmic volumes with comparatively modest observing time, making it possible to probe structure on scales of several million light years, comparable to the distance between the Milky Way and its neighbour Andromeda.

The difficulty, and the reason a direct detection at these distances has remained elusive until now, lies in the sheer faintness of the target. The 21-centimetre signal from distant hydrogen is buried beneath foreground emission from our own Galaxy and from distant radio galaxies, which can be many thousands of times brighter. Human-made radio-frequency interference from satellites, transmitters and other technology adds further contamination, as do the subtle instrumental effects of the telescope itself, from calibration errors to the complex frequency-dependent response of the receivers. Separating the cosmological signal from this tangle of astrophysical and terrestrial noise demands an exceptionally detailed understanding of every contaminating process, and any residual error in that modelling can easily masquerade as, or mask, the true signal.

Previous reliable detections of the hydrogen signal at cosmological distances have typically relied on cross-correlating radio observations with optical galaxy surveys, using the independently mapped distribution of galaxies to confirm that the radio data really do contain the hydrogen signal. The new study, led by Dr Sourabh Paul, breaks with that dependency. By analysing approximately 96 hours of MeerKAT observations, the team extracted the hydrogen intensity mapping signal using radio data alone, detecting it at two distinct redshifts, corresponding to epochs when the Universe was considerably younger. The measurements trace hydrogen across scales of several million light years, providing a direct statistical picture of how neutral gas is distributed in the cosmic web at those epochs.

Dr Paul described the result as a very exciting milestone, noting that hydrogen intensity mapping has long been seen as a promising way to map the Universe efficiently, but that the signal is extremely faint and difficult to isolate from foreground emission, human-made interference and instrumental effects. Detecting it directly with MeerKAT, he said, shows that the technique is becoming a practical tool for cosmology. Professor Mário Santos, a co-author from the University of the Western Cape, emphasised how challenging the data analysis process was, requiring a detailed understanding of the many sources of contamination that can affect such a faint measurement. He highlighted something particularly remarkable about the achievement: the data used in the study were taken in 2018, when MeerKAT had only just begun science operations, and there is now a rich trove of MeerKAT data waiting to be explored with this method.

The significance of the result extends well beyond the technical feat itself. Neutral hydrogen is one of the key ingredients for understanding how galaxies form and evolve, as Dr Zhaoting Chen, a co-author of the study, explained. With intensity mapping, astronomers do not need to detect every individual galaxy; instead they can measure the collective signal from hydrogen across large cosmic volumes, offering a new way to study both galaxy evolution and the underlying matter distribution of the Universe. Because hydrogen traces the dark matter that dominates the cosmic mass budget, these maps also provide a route to testing how dark matter shapes the cosmic web, and potentially to constraining fundamental physics such as the properties of dark energy through precise measurements of the Universe’s expansion history.

The timing of the detection is especially propitious for the future of radio cosmology. MeerKAT, located in the Karoo region of South Africa, is a precursor telescope for the Square Kilometre Array Observatory, or SKAO, the multinational project that will become the most sensitive radio observatory ever built. Hydrogen intensity mapping is expected to be a major science driver for the SKAO, and the new result provides an early proof that the technique works with real data from precursor instruments. Professor Laura Wolz, a co-author from The University of Manchester, noted that MeerKAT continues to open new windows for cosmology, and that the fact this signal could be extracted from observations that were not originally designed for hydrogen intensity mapping is very encouraging. It demonstrates, she said, the enormous scientific value of MeerKAT data and points the way to future observations with the SKAO.

Looking ahead, the researchers say that future observations covering larger areas of the sky and using longer observing times will enable astronomers to map hydrogen in even greater detail. Such surveys promise to reveal how galaxies formed, how dark matter sculpts the cosmic web, and how the Universe has evolved over billions of years. For a technique that has spent years hovering at the boundary between promise and proof, the direct MeerKAT detection represents the moment the promise began to be fulfilled: a faint 21-centimetre whisper from the deep past, finally heard clearly enough to become one of cosmology’s most powerful new instruments.

Subject of Research: Direct detection of neutral hydrogen intensity mapping signal at cosmological distances using the MeerKAT radio telescope

Article Title: Astronomers use MeerKAT to directly detect faint hydrogen signal from the distant Universe

Article References: Astronomers use MeerKAT to directly detect faint hydrogen signal from the distant Universe. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: MeerKAT, hydrogen intensity mapping, 21-centimetre line, neutral hydrogen, cosmology, large-scale structure, radio astronomy, Square Kilometre Array, cosmic web, galaxy evolution, dark matter, University of Manchester