Astronomers using China’s FAST telescope and the DESI survey measured cosmic neutral hydrogen over the past 4.5 billion years, discovering that gas reserves did not dry up despite a sharp drop in star formation. The study, published Tuesday in Nature Astronomy, challenges long-standing assumptions about galaxy evolution.
Scientists have long grappled with a fundamental astronomical puzzle: why the universe produced new stars with dazzling frequency in its early epochs, yet sees a markedly slower rate of star formation today. To investigate this decline, an international collaboration turned to two powerful instruments. Researchers combined ultra-sensitive radio observations from China’s Five-hundred-meter Aperture Spherical radio Telescope (FAST) with the extensive spectroscopic sky mapping of the Dark Energy Spectroscopic Instrument (DESI), a global project involving more than 70 institutions.
The project brought together multiple research bodies, including the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC), the Shanghai Astronomical Observatory (SHAO) of the Chinese Academy of Sciences, and Shanghai Jiao Tong University. By pooling their observational power, the research team analyzed roughly 2.5 million galaxies spread across approximately one-third of the sky, allowing them to trace cosmic neutral hydrogen with high statistical precision.
Measuring Neutral Hydrogen Across 4.5 Billion Years
Neutral atomic hydrogen functions as one of the primary cold gas reservoirs inside galaxies, serving as the raw material that eventually fuels star creation and acting as a critical link between large-scale gas cycling and internal star formation. However, tracking this gas in distant galaxies has historically proved difficult because neutral hydrogen is primarily detected through its extremely faint 21-centimeter spectral line. For distant galaxies, individual signals are often buried in background noises, resulting in a persistent lack of reliable, direct observational evidence, as noted by Guo Hong, a researcher at SHAO.
To overcome this observational barrier, the study utilized a stacked spectral approach across an unprecedented sample size. The resulting measurements captured how cosmic hydrogen evolved over a span of 4.5 billion years, yielding precise figures that complicate traditional theories of stellar decline.
“Why is it becoming increasingly difficult for the universe to form new stars? This is a core question in the field of galaxy formation and evolution,” said Jiang Peng, a researcher at NAOC.
Challenging the Exhaustion Hypothesis
A widely held and intuitive explanation among astronomers has been that as the universe evolves, the cold gas that nurtures stars is gradually consumed, eventually leading to the decline in star formation. Under this model, the drop in the star formation rate should inevitably be accompanied by a commensurate depletion of cold gas reservoirs.
The joint FAST and DESI data directly challenge that premise. The precise historical timeline reveals that 4.5 billion years ago, the cosmic star formation rate was about 2.5 times that of today, while the density of neutral atomic hydrogen at that time was only about 1.4 times its current value.
In short, even as star-forming activity plummeted over the billions of years, the universe’s neutral hydrogen reserves did not dry up in tandem. That mismatch directly rules out the simplistic scenario that a rapid exhaustion of neutral hydrogen caused the decline in star formation.
“If neutral hydrogen reserves are still abundant, what is making star formation increasingly difficult?” Jiang said.
Shifting Focus From Atomic Gas to Molecular Clouds
With atomic hydrogen reserves still relatively abundant, researchers suggest that stars are mainly born within denser molecular gas clouds rather than directly from diffuse atomic hydrogen.
As the overall cosmic gas density drops, the efficiency of converting neutral hydrogen into molecular hydrogen decreases accordingly, causing the molecular gas that can directly incubate stars to gradually diminish.
The findings provide a new baseline for understanding how galaxies process their fuel over cosmological timescales. The complete study was published Tuesday in the international journal Nature Astronomy, establishing a robust observational framework for future work by participating global institutions.
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