FAST and DESI reveal slow star formation despite stable cosmic hydrogen

Cosmic star formation has plummeted by more than half over the past 4.5 billion years, yet the universe’s neutral atomic hydrogen reservoir has remained nearly stable. This surprising mismatch was uncovered by an international research team led by scientists from the Chinese Academy of Sciences, working in partnership with the Dark Energy Spectroscopic Instrument project and utilizing China's Five-hundred-meter Aperture Spherical radio Telescope.

For years, researchers operated under the assumption that aging galaxies were simply running out of the cold gas required to sustain stellar births. However, the new measurements reveal that the rapid exhaustion of atomic hydrogen cannot be the primary driver behind the fading rate of star formation, forcing astronomers to rethink how matter flows and converts within galaxies.

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Neutral atomic hydrogen, designated as HI, serves as a vital cold gas reservoir linking broader cosmic supplies to the environments where stars actually ignite. Detecting this gas from distant regions has historically challenged observers because the 21-centimeter radio emission line is exceedingly faint and easily swallowed by background noise. Deep surveys could achieve adequate sensitivity but covered restricted patches of the sky, while wide-area surveys lacked the necessary detection power.

To overcome this observational hurdle, the research team combined the high radio sensitivity of FAST with the extensive optical spectroscopy data set provided by DESI. By cross-matching data across roughly 12,000 square degrees, the scientists analyzed galaxies spanning the past 4.5 billion years of cosmic history, according to reporting by SSB Crack News.

Instead of attempting to resolve individual faint signals, the researchers applied an HI spectral stacking method. By aligning the individual radio observations according to each galaxy's precise redshift, random background noise cancelled out, allowing the shared hydrogen signal to emerge clearly.

The resulting data laid bare a sharp contradiction. According to findings published in Nature Astronomy on Sept. 1, 2026, and highlighted by Yahoo News, the cosmic star formation rate dropped by a factor of 2.46 over the observed 4.5-billion-year timeline. By contrast, the raw density of cosmic neutral hydrogen declined by a factor of only 1.35 ± 0.10. After applying conservative corrections for possible systematic effects, that modest change shrank further to just 1.12 ± 0.10.

Stars do not form directly from most neutral atomic hydrogen. Instead, they ignite within much denser clouds of molecular gas, placing HI in an intermediate evolutionary position. While atomic hydrogen has stayed relatively abundant, observations indicate that molecular gas densities decline much more steeply, tracking the drop in star formation far more closely.

The findings shift the fundamental scientific question from whether galaxies are depleting their gas to why conversion mechanisms are stalling. As gas inflows from the cosmic web weaken and overall densities drop, galaxies may simply become less efficient at channeling neutral atomic hydrogen into molecular clouds. Under this framework, the overall atomic reservoir remains stable while downstream star-forming fuel dwindles.

This dynamic presents galaxy-formation models with a demanding new benchmark. Successful simulations must now simultaneously account for nearly stable atomic hydrogen reservoirs alongside steep reductions in molecular gas and stellar output. Existing models handle this challenge differently: Yahoo News notes that IllustrisTNG predicts an approximately constant cosmic HI density over the measured redshift range, whereas SIMBA projects a modest decline of about 1.5 times, bringing it closer to observational records.

Whether future modeling can fully reconcile these contrasting trends depends on how well simulations incorporate gas recycling, disk pressure, turbulence, and metallicity shifts as cosmic structures continue to evolve.

Observational Synergy and Modeling Challenges

The collaborative breakthrough relied directly on the union of two vast astronomical surveys, as detailed in reports from the Chinese Academy of Sciences and mirrored in coverage by Yahoo News. By merging the All-Sky H I survey capabilities of FAST with the precise optical redshifts compiled by the Dark Energy Spectroscopic Instrument's Bright Galaxy Survey, researchers gained access to a combined sample size encompassing millions of individual galaxies. This cross-matching technique spanned a colossal sky area of roughly 12,000 square degrees, charting cosmic evolution over a timeline stretching back 4.5 billion years.

• FAST contribution: Exceptional radio sensitivity detecting faint 21-centimeter emission lines.
• DESI contribution: Enormous optical spectroscopy datasets providing precise galaxy redshifts.
• Methodology: HI spectral stacking to cancel random background noise and isolate average hydrogen signals.
• Scope: Millions of galaxies analyzed across roughly 12,000 square degrees of the sky.

Refining cosmological simulations to account for turbulence, disk pressure, and gas recycling remains the next step for researchers striving to explain why stellar birth rates have stalled while raw atomic fuel remains abundant.