Star formation has therefore fallen roughly twice as fast as the hydrogen supply.

How did the researchers measure cosmic hydrogen?

For decades, astronomers struggled to directly track the universe's neutral hydrogen. The signal they need is a faint 21-centimeter radio emission line, easily drowned out by background noise.

Surveys deep enough to detect that signal could only cover small patches of sky. Surveys covering large areas lacked the sensitivity to find it. The team tackled this by combining FAST's exceptional radio sensitivity with DESI's enormous catalog of optical spectra. They analyzed about 2.5 million galaxies spread across nearly one-third of the sky.

To extract the average hydrogen signal, the researchers used an HI spectral stacking method. They aligned the faint radio signals from each galaxy using precise redshift measurements — the wavelength shifts that reveal distance — then combined them so the collective signal rose above the noise. The approach yielded what the authors describe as a sample of unprecedented size and statistical precision.

Where does star formation actually happen?

Stars do not form directly from most neutral atomic hydrogen. They mainly condense inside much denser clouds of molecular hydrogen. Neutral atomic hydrogen occupies an intermediate position in the gas cycle, sitting between the cosmic reservoir and the molecular gas that ignites stars.

The researchers suggest the recent cosmic slowdown involves how gas moves through the baryon cycle — the recycling of ordinary matter between galaxies and surrounding space — rather than the total amount of neutral hydrogen. As the flow of gas from the cosmic web weakens and densities fall, galaxies may become less efficient at converting neutral hydrogen into molecular hydrogen.

Under that scenario, the overall reservoir can stay relatively stable while the molecular gas that directly fuels stars gradually shrinks.

What's next?

The findings offer a new observational benchmark for studying the cosmic gas cycle during the universe's later evolution and the long-term decline in star formation, the team reports.

The study was led by scientists from the National Astronomical Observatories of China, the Shanghai Astronomical Observatory of the Chinese Academy of Sciences, and Shanghai Jiao Tong University, with collaborators across Asia, North America, and Europe participating through DESI.

By joining FAST's sensitive radio measurements with DESI's optical spectroscopy, the collaboration demonstrated the scientific power of pairing instruments that cover different parts of the electromagnetic spectrum.