Star Formation Has Halved in 4.5 Billion Years, But the Universe's Hydrogen Barely Has

The universe is producing far fewer new stars than it once did: the rate of star formation has fallen to less than half its previous level over the past 4.5 billion years. Yet the supply of neutral atomic hydrogen, an important gas reservoir inside galaxies and one of the key raw materials for star formation, has changed surprisingly little in that time, according to an international team led by scientists from the Chinese Academy of Sciences (CAS) working with the Dark Energy Spectroscopic Instrument (DESI) project. The findings were published online in Nature Astronomy on September 1.

Using China's Five-hundred-meter Aperture Spherical Telescope (FAST), the researchers measured cosmic neutral atomic hydrogen, known as HI, across roughly 2.5 million galaxies spanning nearly one-third of the sky. Because the radio signal from HI is very faint, the team used a technique called spectral stacking, combining many individually undetectable signals using each galaxy's precise redshift from DESI to make the average signal emerge from background noise.

The results showed a striking mismatch: about 4.5 billion years ago, the cosmic star formation rate was roughly 2.5 times higher than it is today, but the density of neutral atomic hydrogen back then was only about 1.4 times higher than its present level. In other words, star formation collapsed far faster than the universe's hydrogen supply did.

Because stars are born mainly inside denser clouds of molecular gas rather than directly from neutral atomic hydrogen, the researchers say the results shift the central question from whether the gas is depleting to why galaxies are finding it harder to turn abundant neutral hydrogen into the molecular gas that fuels new stars. They suggest the answer may lie in how gas moves through the broader cosmic gas cycle rather than in how much hydrogen exists overall. The work was led by researchers from the National Astronomical Observatories of China, the Shanghai Astronomical Observatory of CAS and Shanghai Jiao Tong University, with contributors from institutions across Asia, North America and Europe.