BEIJING, 3rd September, 2026 (WAM) -- Chinese scientists using the Five-hundred-meter Aperture Spherical Telescope (FAST) and the Dark Energy Spectroscopic Instrument (DESI) have found that while star formation has declined sharply over the past 4.5 billion years, the universe's neutral atomic hydrogen reservoir has remained relatively stable.
According to a Global Times report, the findings offer new clues to why galaxies are becoming less efficient at forming stars.
Why the universe is producing fewer new stars remains a major question in galaxy research. One explanation is that galaxies have gradually depleted the cold gas needed for star formation. If so, the amount of cold gas should have declined at a similar rate.
Neutral atomic hydrogen is one of the most important sources of this gas, but its 21-centimetre radio signal is extremely weak, particularly from distant galaxies, making it difficult to observe.
To address this, researchers combined sensitive radio observations from FAST with DESI's large-scale galaxy survey, studying around 2.5 million galaxies across roughly one-third of the sky.
By aligning and combining weak signals from large numbers of galaxies according to their redshifts, the team detected an average neutral hydrogen signal that could not be observed in individual galaxies.
The results showed that around 4.5 billion years ago, the universe's star formation rate was about 2.5 times higher than today, while the density of neutral hydrogen was only around 1.4 times higher.
This indicates that star formation has declined much faster than the supply of neutral hydrogen, suggesting that the universe is not simply running out of the gas needed to form stars.
Instead, the findings raise a broader question over why galaxies are becoming less efficient at converting their remaining neutral hydrogen into new stars.
The study provides a new clue to understanding the gradual decline of star formation and the evolution of galaxies across the universe.
The research was conducted by an international team including the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC), the Shanghai Astronomical Observatory (SHAO) of the Chinese Academy of Sciences, Shanghai Jiao Tong University and other institutions.