The Universe still has plenty of hydrogen, but its ability to turn that fuel into new stars appears to be fading.
The Universe is producing far fewer new stars than it did billions of years ago. Over the past 4.5 billion years, the cosmic star formation rate has fallen to less than half its former level. Yet one of the most important ingredients needed to make stars has declined only modestly.
An international team led by researchers from the Chinese Academy of Sciences (CAS), working with the Dark Energy Spectroscopic Instrument (DESI) project, reached this conclusion after studying the evolution of neutral atomic hydrogen across the Universe. The researchers used China’s Five-hundred-meter Aperture Spherical radio Telescope (FAST) to make exceptionally precise measurements spanning the past 4.5 billion years.
Their results show that cosmic star formation has dropped sharply while neutral atomic hydrogen (HI), a major gas reservoir within galaxies, has changed far less dramatically.
The findings were published online in Nature Astronomy on September 1.
A Longstanding Mystery of Star Formation
Why the Universe has become less productive at forming stars is one of the major questions in the study of how galaxies form and evolve.
One seemingly straightforward explanation is that galaxies gradually use up the cold gas needed to create new stars. If that were the main cause, then the dramatic decline in star formation should have been accompanied by a similarly steep reduction in the Universe’s cold gas supply. Astronomers, however, have not observed such a decline.
HI plays an important role in this process. It serves as a major cold gas reservoir in galaxies and helps connect the large-scale movement of gas through the Universe with the star formation taking place inside individual galaxies. Astronomers primarily detect HI through its extremely faint 21-centimeter radio emission line.
Detecting this signal from distant galaxies is difficult because it is often overwhelmed by background noise.
For years, that created a major observational challenge. Deep surveys were sensitive enough to detect faint signals but could examine only relatively small regions of the sky. Wider surveys could cover much larger areas but generally lacked the sensitivity needed to measure such weak radio emissions. Consequently, astronomers have struggled to directly and reliably track how the total amount of HI changed across the low-to-intermediate redshift Universe.
Combining FAST and DESI
The new research addressed this limitation by combining the extraordinary radio sensitivity of FAST with the enormous optical spectroscopic dataset gathered by DESI.
The team examined about 2.5 million galaxies spread across nearly one-third of the sky. Researchers then used an HI spectral stacking technique to combine many radio signals that would have been too faint to detect individually.
Using precise measurements of each galaxy’s redshift, they aligned the weak signals before stacking them together. This approach allowed the average HI signal to emerge from the surrounding noise, giving the team an unusually detailed picture of how cosmic neutral hydrogen has evolved.
The results revealed a striking mismatch.
About 4.5 billion years ago, the Universe was forming stars at roughly 2.5 times the rate seen today. During the same period, however, the density of neutral atomic hydrogen was only about 1.4 times greater than its current level.
In other words, star formation declined much faster than the Universe’s reservoir of HI. The neutral hydrogen supply did not disappear at anything close to the same rate.
That makes it difficult to explain the decline in star formation simply as the result of galaxies rapidly exhausting their hydrogen.
Why Is the Universe Struggling to Make Stars?
According to the researchers, the findings shift the central question from “whether the gas is depleting” to “why it is increasingly difficult to form stars despite abundant neutral hydrogen reserves.”
Stars do not usually form directly from neutral atomic hydrogen. Instead, they are born inside much denser clouds of molecular gas. HI occupies an important intermediate position between the large-scale supply of cosmic gas and the molecular hydrogen that ultimately feeds star formation.
The results suggest that the major changes occurring later in the Universe’s history may involve how gas moves through the baryon cycle rather than a simple decline in the overall HI supply.
As the flow of gas from the cosmic web weakens and gas densities decrease, galaxies may become less efficient at transforming HI into molecular hydrogen. Under this scenario, the Universe could retain a relatively stable reservoir of neutral atomic hydrogen even while the molecular gas needed to make stars gradually becomes less abundant.
That would help explain how the Universe can still contain large amounts of potential star-forming material while producing fewer and fewer new stars.
A New View of the Cosmic Gas Cycle
The importance of the findings extends beyond measuring the total amount of hydrogen in the Universe. They provide new evidence about why the enormous stellar engines inside galaxies have been gradually slowing down.
According to the researchers, the combined FAST and DESI observations establish a new benchmark for studying the late-stage cosmic gas cycle, the long-term decline in star formation, and the broader evolution of galaxies.
Reference: “Weak evolution of cosmic atomic hydrogen over the past 4.5 billion years” by Chuan-Peng Zhang, Hong Guo, Yizhou Gu, Amélie Saintonge, Xiaohu Yang, Dirk Scholte, Ming Zhu, Peng Jiang, Hu Zou, Manasvee Saraf, Wenlin Ma, Yirong Wang, Y. P. Jing, Zheng Zheng, Zhejie Ding, J. Aguilar, S. Ahlen, D. Bianchi, D. Brooks, T. Claybaugh, A. de la Macorra, P. Doel, E. Gaztañaga, G. Gutierrez, M. Ishak, R. Joyce, S. Juneau, R. Kehoe, D. Kirkby, A. Kremin, O. Lahav, C. Lamman, M. Landriau, L. Le Guillou, M. Manera, A. Meisner, R. Miquel, J. Moustakas, S. Nadathur, W. J. Percival, F. Prada, I. Pérez-Ràfols, G. Rossi, E. Sanchez, D. Schlegel, M. Schubnell, H. Seo, J. Silber, D. Sprayberry, G. Tarlé and B. A. Weaver, 32 August 2026, Nature Astronomy.
DOI: 10.1038/s41550-026-02965-9
The research was led by scientists from the National Astronomical Observatories of China, the Shanghai Astronomical Observatory of CAS, and Shanghai Jiao Tong University, together with researchers involved in DESI. Contributors came from institutions across Asia, North America, and Europe, demonstrating the value of combining highly sensitive radio measurements with large-scale optical spectroscopy.
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