Science

Astronomers find universe's star formation drop outpaces neutral hydrogen decline

Data from FAST and DESI show neutral atomic hydrogen dropped only modestly over 4.5 billion years while star production fell by more than half.

Updated Sep 3, 2026 · 08:18 UTC1 reports

The short version

A study published in Nature Astronomy reveals that cosmic star formation fell by more than half over the past 4.5 billion years despite neutral atomic hydrogen remaining largely stable.
Researchers utilized radio data from China's FAST telescope alongside spectroscopic data from 2.5 million galaxies surveyed by DESI.
The mismatch suggests declining star formation stems from lower efficiency in converting atomic hydrogen into star-forming molecular gas rather than an outright depletion of cosmic gas reserves.

Over the past 4.5 billion years, cosmic star formation rates declined by roughly 60% (from 2.5 times present levels), while cosmic neutral atomic hydrogen density decreased by only about 30% (from 1.4 times present levels).[ScienceDaily]
The international study, led by Chinese Academy of Sciences researchers and DESI collaborators, was published online in Nature Astronomy on September 1, 2026.[ScienceDaily]
The researchers applied an HI spectral stacking technique to align and aggregate faint 21-centimeter radio signals across approximately 2.5 million galaxies covering nearly one-third of the sky.[ScienceDaily]
The findings contradict the traditional hypothesis that the slowdown in star formation was primarily driven by the exhaustion of cold atomic gas reservoirs.[ScienceDaily]

The exact physical mechanisms hindering galaxies from converting neutral atomic hydrogen into denser molecular gas required for star formation remain unproven hypotheses.[ScienceDaily]