Cosmic hydrogen measurement marks breakthrough in mapping the universe

Cosmic hydrogen measurement marks breakthrough in mapping the universe Lisa Lock Scientific Editor Robert Egan Senior Editor An international research project is bringing scientists closer to mapping the universe. 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. After a decade of work, 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 study is published in the journal Monthly Notices of the Royal Astronomical Society. Dr. Steven Cunnington, senior research fellow at the University of Portsmouth's Institute of Cosmology and Gravitation, led the development of the analysis pipeline that enabled the detection and has been involved in the project for several years. Since rejoining the university as a Stephen Hawking fellow in April 2025, he has also co-led one of the largest working groups within the international MeerKLASS Collaboration. He said, "This result feels particularly special because it's the culmination of a journey that began when I started my Ph.D. at Portsmouth over a decade ago. I am now fortunate to be part of a much wider international collaboration sharing this common goal. "Many of the challenges addressed in this work have shaped my own research career, so seeing our team deliver this measurement is a remarkable milestone to witness." Mapping hydrogen beyond individual galaxies The international group of researchers worked with data from the South African MeerKAT radio telescope on MeerKAT's Large Area Synoptic Survey (MeerKLASS). This has been possible via 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 technique provides the ability to build a huge 3D map of the universe. For this detection, the team analyzed 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. Separating a faint signal from overwhelming noise Extracting the signal required overcoming extremely challenging problems such as removing astrophysical "foreground" emission about 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 instrumental effects that differ are suppressed. Combining these maps was crucial to beating down systematics and confirming a signal standing more than six times above the statistical noise. A decade of international teamwork "This is a milestone in establishing 21-cm intensity mapping as a novel tool for cosmology," said professor 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." Publication details Matilde Barberi-Squarotti et al., Hi intensity mapping with MeerKAT: A measurement of the 21-cm auto-spectrum on cosmological scales, Monthly Notices of the Royal Astronomical Society (2026). DOI: 10.1093/mnras/stag1911. On arXiv: DOI: 10.48550/arxiv.2610.12158 Journal information: arXiv , Monthly Notices of the Royal Astronomical Society Provided by University of Portsmouth