The universe is producing fewer stellar "babies," and scientists are scratching their heads over why. Over the past 4.5 billion years, the rate at which new stars form has plummeted to less than half its previous level. Yet the supply of one of the most critical "fuels" for star formation - neutral atomic hydrogen - has barely budged. It's like a bakery running out of customers while the flour shelves remain stocked.

An international research team, led by scientists from the Chinese Academy of Sciences (CAS) and working with the Dark Energy Spectroscopic Instrument (DESI) project, used China's Five hundred meter Aperture Spherical radio Telescope (FAST) to make precise measurements of cosmic neutral atomic hydrogen across the last 4.5 billion years. Their findings, published online in Nature Astronomy on Sept. 1, reveal a striking mismatch: star formation has declined sharply, while the amount of neutral atomic hydrogen (HI) has only modestly decreased.

Understanding why star formation has become less active as the universe ages is a major question in galaxy formation and evolution. A seemingly obvious explanation is that galaxies have simply run out of cold gas needed to produce stars. But if dwindling gas supplies were the culprit, astronomers would expect the dramatic fall in star formation to be accompanied by a similarly large decline in available gas. Observations, however, haven't shown such a sharp depletion.

HI is a key player in this cosmic puzzle. It's an important cold gas reservoir inside galaxies, linking the broader cosmic gas supply to the processes that forge new stars. Astronomers usually detect HI via its extremely faint 21-centimeter radio emission line - a signal so weak that it's often swamped by background noise, especially from distant galaxies.

For years, astronomers faced a Catch-22: deep surveys could achieve the needed sensitivity but couldn't scan large sky regions, while wide-area surveys lacked the sensitivity to detect such faint signals. This made it tough to directly and reliably measure how the universe's total HI mass has changed over time.

The new research cleverly combined FAST's exceptional radio sensitivity with DESI's enormous optical spectroscopy dataset. The team studied about 2.5 million galaxies spanning nearly one-third of the sky. Using an HI spectral stacking method, they aligned weak radio signals - each too faint to detect individually - based on precise galaxy redshifts, allowing the average HI signal to emerge from the noise. This approach gave them an unprecedented sample size and exceptionally high statistical precision.

The results exposed a glaring discrepancy: About 4.5 billion years ago, the cosmic star formation rate was roughly 2.5 times higher than today, but neutral atomic hydrogen density was only about 1.4 times higher. In other words, star formation has crashed without a comparable disappearance of the universe's HI reservoir. The findings suggest that rapidly exhausting neutral hydrogen can't by itself explain the decline.

Instead, the researchers propose, the real issue may be how gas moves through the baryon cycle, not how much HI exists overall. As gas flows from the cosmic web weaken and gas densities drop, galaxies may become less efficient at converting HI into molecular hydrogen - the dense clouds where stars actually form. So the HI reservoir stays stable while the molecular gas that directly fuels star formation dwindles.

The implications extend beyond just measuring hydrogen. The findings offer a new clue to why the universe's star-forming engines are gradually sputtering. Combining FAST and DESI data provides a fresh observational benchmark for studying the cosmic gas cycle, the long-term decline in star formation, and broader galaxy-shaping processes.

The research was led by scientists from the National Astronomical Observatories of China, the Shanghai Astronomical Observatory of CAS, and Shanghai Jiao Tong University, alongside DESI collaborators from institutions across Asia, North America, and Europe. It's a stellar example of what happens when you pair hyper-sensitive radio ears with a massive optical survey - science gets a little closer to understanding why the universe's star factory is slowing down, even with plenty of raw material on hand.