This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: The universe has been having fewer and fewer stellar "babies." Over the past 4.5 billion years, the rate of star birth has crashed to less than half of what it once was. But here's the twist: the most essential "fuel" for making stars has barely decreased.
This finding comes from an international team led by researchers from the Chinese Academy of Sciences (CAS), in collaboration with the Dark Energy Spectroscopic Instrument (DESI) project. They used China's Five-hundred-meter Aperture Spherical radio Telescope (FAST) to make high-precision measurements of cosmic neutral atomic hydrogen over the past 4.5 billion years. The study reveals that while cosmic star formation has declined dramatically over this period, neutral atomic hydrogen (HI)—a vital gas reservoir for galaxies—has decreased only slightly.
This finding was published online in Nature Astronomy on Sept. 1. Why has it become harder for the universe to form new stars? That question has long been central to the study of galaxy formation and evolution.
A natural explanation is that, as the universe ages, the cold gas that fuels star birth is steadily consumed, inevitably driving a decline in star formation. If that were true, however, the drop in the star formation rate should be matched by a simultaneous sharp depletion of the cold gas reservoir. But such depletion has not been detected.
HI is a key cold-gas reservoir in galaxies that links the large-scale cosmic gas cycle to internal star formation and is mainly detected via its extremely faint 21-centimeter radio emission line. Unfortunately, individual signals for distant galaxies are often swamped by background noise. For this reason, astronomers long faced a frustrating dilemma: Deep observations could not cover large areas, while wide-field surveys lacked the sensitivity to detect faint signals.
As a result, the evolution of total HI mass in the low-to-intermediate-redshift universe has remained difficult to measure directly and reliably. To overcome this bottleneck, the new study integrated FAST's ultra-high radio sensitivity with DESI's massive optical spectroscopic survey. The researchers analyzed a vast sample of about 2.5 million galaxies, covering nearly one-third of the sky.
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