The universe has been having fewer and fewer stellar "babies." Over the past 4.5 billion years, the rate of star birth has crashed to less than half of what it once was. But here's the twist: the most essential "fuel" for making stars has barely decreased.
This finding comes from an international team led by researchers from the Chinese Academy of Sciences (CAS), in collaboration with the Dark Energy Spectroscopic Instrument (DESI) project. They used China's Five-hundred-meter Aperture Spherical radio Telescope (FAST) to make high-precision measurements of cosmic neutral atomic hydrogen over the past 4.5 billion years.
The study reveals that while cosmic star formation has declined dramatically over this period, neutral atomic hydrogen (HI)—a vital gas reservoir for galaxies—has only decreased slightly.
This finding was published online in Nature Astronomy on Sept. 1.
Why has it become harder for the universe to form new stars? That question has long been central to the study of galaxy formation and evolution. A natural explanation is that, as the universe ages, the cold gas that fuels star birth is steadily consumed, inevitably driving a decline in star formation. If that were true, however, the drop in the star formation rate should be matched by a simultaneous sharp depletion of the cold gas reservoir. But such depletion has not been detected.
HI is a key cold-gas reservoir in galaxies that links the large-scale cosmic gas cycle to internal star formation and is mainly detected via its extremely faint 21-centimeter radio emission line. Unfortunately, individual signals for distant galaxies are often swamped by background noise.
For this reason, astronomers long faced a frustrating dilemma: deep observations could not cover large areas, while wide-field surveys lacked the sensitivity to detect faint signals. As a result, the evolution of total HI mass in the low-to-intermediate redshift universe has remained difficult to measure directly and reliably.
To overcome this bottleneck, the new study integrated FAST's ultra-high radio sensitivity with DESI's massive optical spectroscopic survey. The researchers analyzed a vast sample of about 2.5 million galaxies, covering nearly one-third of the sky. Using an innovative HI spectral stacking technique, they aligned and stacked the faint, otherwise undetectable radio signals based on precise galaxy redshifts. This allowed them to extract the average HI signal from the noise, tracing the evolution of cosmic neutral hydrogen with unprecedented statistical precision and sample size.
The highly precise measurements revealed a striking disparity: 4.5 billion years ago, the cosmic star formation rate was about 2.5 times higher than today, while the corresponding neutral atomic hydrogen density was only about 1.4 times its current level. This indicates that while star formation activity plummeted, the cosmic HI reservoir did not dry up synchronously. This striking contrast shows that rapid hydrogen depletion cannot explain the decline in star formation.
Shifting focus
According to the researchers, this breakthrough effectively shifts the scientific focus from "whether the gas is depleting" to "why it is increasingly difficult to form stars despite abundant neutral hydrogen reserves."
Stars are primarily born in denser molecular gas clouds, and neutral atomic hydrogen sits at a critical intermediate stage between the large-scale cosmic gas supply and the formation of molecular gas. The study suggests that the most important late-time changes may occur not in the total HI reservoir itself, but rather in the gas flow through the baryon cycle. As the gas supply from the cosmic web weakens and gas density declines, the efficiency of converting HI into molecular hydrogen likely drops. This mechanism allows the HI reservoir to stay relatively stable, even as the molecular gas that directly feeds stars gradually dwindles.
As a result, the significance of this work goes far beyond merely measuring how much hydrogen is in the universe. Instead, it provides crucial new clues for understanding why the massive stellar engines of the universe are gradually shutting down.
According to the researchers, the joint observation by FAST and DESI establishes a brand-new observational benchmark for unraveling the late-stage cosmic gas cycle, the decline of star formation, and the broader evolution of galaxies.
The research was led by scientists from the National Astronomical Observatories of China, the Shanghai Astronomical Observatory of CAS, and Shanghai Jiao Tong University, in collaboration with researchers participating in DESI. The study includes contributors from research institutions across Asia, North America, and Europe, highlighting the scientific power of combining sensitive radio observations with large-scale optical spectroscopy.
