The MeerKLASS Collaboration unveils its first independent detection of cosmic hydrogen clustering on large scales, advancing a technique that could transform how cosmologists map the Universe. After a decade of preparation and work, an international group of researchers using the South African MeerKAT radio telescope for the MeerKAT Large Area Synoptic Survey (MeerKLASS) has directly measured how neutral hydrogen ‘clumps’ in the distant Universe. This has been possible via a technique known as hydrogen intensity mapping, which combines hydrogen 21-cm emission from many galaxies within large patches of sky, capturing the collective pattern even when individual galaxies cannot be distinguished. “Mapping how hydrogen is distributed tells us how all matter is distributed, and from there how the Universe has evolved and what it is made of. For the first time, we have detected this signal on cosmological scales using our own data alone, without relying on other surveys. It shows the technique is now mature: hydrogen intensity mapping has become a new observable of the large-scale structure of the Universe”, says Isabella Paola Carucci, researcher at INAF-OATS and member of the MeerKLASS Collaboration. To achieve this detection, the team analysed MeerKAT observations spanning approximately 236 square degrees of sky, an area of more than a thousand full moons. The faint signal they uncovered came from 4.5 billion years ago, when the Universe was just over 9 billion years old. Extracting it required overcoming challenging problems such as removing astrophysical ‘foreground’ emission about 10,000 times stronger than the hydrogen signal itself. Finding it is like trying to make out a whispered conversation from the front row of a roaring rock concert. Signal-processing methods developed at the INAF Astronomical Observatory of Trieste were crucial to this task. Born from the RadioGaGa project led by Carucci and funded by the European Union’s NextGenerationEU programme through Italy’s National Recovery and Resilience Plan (PNRR), these statistical learning techniques are designed to separate the faint hydrogen signal from instrumental effects and the bright emission of our own Galaxy. First tested on MeerKAT data in 2025, they became a key ingredient in the collaboration’s analysis behind this detection. The measurement relied on a new observing mode for MeerKAT. Rather than combining its 64 dishes into a single interferometer, the team operated each dish as an independent telescope, trading sharp angular resolution for high survey speed. This produced many independent maps from different dishes and observing times that could be cross-compared. Real cosmic structure recurs across them, while differing instrumental effects are suppressed. Combining these maps was crucial to reducing systematics and confirming a signal more than six times above the statistical noise. “This is a milestone in establishing 21-cm intensity mapping as a novel tool for cosmology”, says Prof. Mário Santos, Principal Investigator of the MeerKLASS survey at the University of the Western Cape. “This measurement took a decade of patient work by researchers spread across seven countries and three continents […]. Such an ambitious result requires sustained and genuine teamwork from many people. And none of it would exist without the South African Radio Astronomy Observatory (SARAO), which built and operates MeerKAT and helped sort out the many challenges with this survey along the way. It is a reminder that science at this level is a large collaborative effort.” Since 2023, the MeerKLASS Collaboration has continued collecting data at lower radio frequencies, corresponding to hydrogen emission from even more distant past times. Eventually, MeerKLASS expects to map the Universe across a volume over a thousand times larger than that used for this detection. This will allow the team to probe the Universe’s large-scale structure over larger swathes of its history and investigate the growth of cosmic structure and the Universe's expansion. As a precursor to the SKA Observatory, MeerKAT shows how these techniques can be scaled up to survey much larger areas of the sky.
References:
MeerKLASS Collaboration et al. (2026). HI intensity mapping with MeerKAT: A measurement of the 21-cm auto-spectrum on cosmological scales. Monthly Notices of the Royal Astronomical Society, stag1911. doi:10.1093/mnras/stag1911 · arXiv:2610.12158.
INAF press release (in Italian)
SARAO press release
Image credit: E. Sacchetti/INAF
