The MeerKLASS Collaboration has reported its first measurement of the large-scale clustering of cosmic hydrogen, advancing a technique that could transform how cosmologists chart the Universe.

Decade of preparation

After a decade of preparation, an international team of researchers working with data from the South African MeerKAT radio telescope on the MeerKAT Large Area Synoptic Survey, known as MeerKLASS, has directly measured how neutral hydrogen “clumps” in the distant Universe, demonstrating that its distribution follows the structure of the cosmic web. Mapping this structure on the largest scales of the Universe has been a goal of cosmology for decades.

The detection was made possible through a technique known as 21-cm intensity mapping, which combines hydrogen emission from many galaxies within large patches of sky, capturing the collective pattern even when individual galaxies cannot be distinguished. This enables the creation of a large 3D map of the Universe.

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For this detection, the team analysed MeerKAT observations across approximately 250 square degrees of sky — roughly the area of 1 250 full moons. The observed signal originated about 4.5 billion years ago, when the Universe was a little older than 9 billion years. Extracting it required overcoming extremely challenging problems, such as removing astrophysical “foreground” emission around 10 000 times stronger than the hydrogen signal itself. Finding it is a little like trying to make out a whispered conversation from the front row of a roaring rock concert.

The measurement relied on a completely 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 very 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 systematic errors and confirming a signal standing 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 — and many of them are early-career scientists who have spent their whole career on this project. 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, 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.”

Sensitivity

Foundation for the futureThis observation is possible only because of the excellent sensitivity and, crucially, stability of the MeerKAT telescope — the culmination of world-class design and engineering from SARAO, the South African Radio Astronomy Observatory. MeerKAT’s achievement provides a foundation for the next stage. As a precursor to the SKA Observatory, it is demonstrating how repeated radio observations can become reliable measurements of the Universe’s large-scale structure. This will help cosmologists understand how the Universe evolved, with the ultimate aim of understanding the physical laws that govern it.

Since 2023, the MeerKLASS Collaboration has continued collecting data at lower radio frequencies, corresponding to hydrogen emission originating from even more distant times. Eventually, MeerKLASS expects to map the Universe across a volume over a thousand times larger than the one used for this detection. This will allow the team to probe the Universe’s large-scale structure over ever larger swathes of its history and eventually investigate the growth of cosmic structure and the expansion of the Universe, helping to test models of gravity and dark energy.

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