Star formation across the universe has significantly decreased over the past 4.5 billion years. However, new research has revealed an unexpected finding: the amount of neutral hydrogen, a crucial component in forming stars, has not dropped nearly as much. Using data from China's FAST radio telescope and observations of about 2.5 million galaxies from the Dark Energy Spectroscopic Instrument (DESI), scientists found that star formation was approximately 2.5 times more active 4.5 billion years ago, while neutral hydrogen levels were only about 1.4 times higher than they are today. This discovery suggests that the decline in star formation cannot be fully explained by the depletion of neutral hydrogen alone. The study highlights a key distinction between the evolution of star formation and the availability of neutral hydrogen. While the rate of star formation has dropped significantly, the density of neutral atomic hydrogen has not declined at the same pace, indicating that other factors are at play. The researchers propose that recent changes in the universe may be more closely tied to how gas moves within the baryon cycle—essentially the process by which gas is recycled between galaxies and the intergalactic medium—rather than the total amount of neutral hydrogen available. As the flow of gas from the cosmic web weakens and gas densities decrease, galaxies may become less efficient at converting neutral hydrogen into molecular hydrogen, which is the form of gas that directly leads to star formation. This means that even if the overall supply of neutral hydrogen remains stable, the amount of molecular gas available to form stars could be decreasing. 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, in collaboration with researchers from the DESI project. Scientists from institutions across Asia, North America, and Europe contributed to the study, showcasing the power of combining advanced radio observations with large-scale optical surveys to uncover new insights into the universe's evolution.