On September 1, 2026, *Nature Astronomy* published online a paper by an international collaboration led by the National Astronomical Observatories of the Chinese Academy of Sciences, the Shanghai Astronomical Observatory, and Shanghai Jiao Tong University. Using joint data from China's FAST telescope (Five-hundred-meter Aperture Spherical Telescope) and the Dark Energy Spectroscopic Instrument (DESI), the team performed a high-precision measurement of cosmic neutral hydrogen evolution over the past 4.5 billion years. The result overturns a long-standing assumption: the star formation rate has dropped to 40% of its earlier value, yet neutral hydrogen reserves still stand at more than 71%. The fuel hasn't run out — the conversion chain is the bottleneck.
The Opening Question
Why is the universe becoming less capable of forming new stars?
The most intuitive explanation in astronomy: stars form from cold gas (mainly neutral hydrogen), which is continuously consumed by star formation, eventually leading to decline. If this picture is correct, then as the star formation rate falls, cold gas reserves should decline in step.
For decades, instruments weren't sensitive enough, and the "step-by-step decline" hypothesis lacked direct observational evidence — the 21 cm line of neutral hydrogen is extremely faint, and individual distant galaxies are nearly drowned in noise. Dr. Zhang Chuanpeng of the Chinese Academy of Sciences offered an analogy: "It's like trying to hear someone whisper in a noisy plaza."
This time, FAST's high sensitivity combined with DESI's massive spectroscopic survey allowed a joint observation to meet both depth and breadth requirements simultaneously.
How to Extract the "Breathing" of Neutral Hydrogen from Noise
The observational strategy is essentially three steps:
1. Large sample: Align radio signals from ~2.5 million galaxies covering about 1/3 of the sky by redshift; 2. Spectral stacking: Individual galaxy signals are buried in noise, but stacking 2.5 million weak signals is like millions of people whispering at once in a plaza — a statistically meaningful "average signal" emerges; 3. Cross-matching with DESI spectroscopy: DESI provides precise redshifts and galaxy properties, against which FAST's neutral hydrogen data are stacked bin by bin.
This approach cannot speak to individual galaxies, but it speaks authoritatively about average values and evolutionary trends. It is a classic stacking analysis — trading single-point breakthroughs for population statistics.
Key Numbers: An "Asymmetric Decline" Over 4.5 Billion Years
| Dimension | 4.5 Gyr ago | Now | |---|---|---| | Cosmic star formation rate | ≈ 2.5× today | 1.0 (baseline) | | Neutral atomic hydrogen density | ≈ 1.4× today | 1.0 (baseline) | | Ratio (H I / SFR) | 1.4 / 2.5 ≈ 0.56 | 1.00 |
The star formation rate has fallen to ~40% of its former value, but neutral hydrogen reserves have only dropped to ~71%.
The two curves did not decline in step. This directly refutes the hypothesis that fuel depletion is the main driver of declining star formation.
The Real Story: Fuel Remains, but the "Factory" Has Slowed
Stars do not form directly from neutral hydrogen. Neutral hydrogen must first be converted into denser molecular hydrogen clouds (molecular clouds) — the true "delivery rooms" of star formation.
Dr. Zhang Chuanpeng's causal chain:
> "What has really changed in the late universe may be the gas conversion efficiency — as cosmic web gas supply weakens and gas density drops, the efficiency of converting neutral hydrogen into molecular hydrogen declines. The molecular gas available for star formation gradually decreases, while neutral hydrogen is instead preserved."
Researcher Guo Hong's summary is more colloquial: "What we're seeing is not the universe running out of gas, but gas becoming harder to 'process' into stars."
Placing This in the Broader 2026 H2 Astronomy Narrative
Over the past month, "the physical basis of galaxy evolution" has been one of the repeatedly rewritten themes:
- Aug 26 – Purple Mountain Observatory's galactic portrait of the Milky Way + IMBH candidate: mapping Galactic structure with neutral hydrogen;
- Aug 26 – Stacking of 217 little red dots: extended host emission, early galaxy–black hole co-evolution;
- Aug 29 – JWST NGC 4696 8.7-hour NIRSpec mapping: a supermassive black hole self-feeding via cold gas filaments;
- Aug 31 – STAR collaboration: baryon number in the gluon junction: the quark-sharing assumption challenged after 53 years;
- Sep 1 – IRIS 3D magnetic reconnection flare bright kernels: 12 years of 1–2 s observations, 100× resolution gain;
- Sep 1 – FAST × DESI: neutral hydrogen declines out of step with star formation; the gas conversion chain is the bottleneck.
FAST at Ten: From Catching Up to Defining the Observational Baseline
Completed on September 25, 2016, FAST turns ten in 2026. This paper pushes its capability into a new dimension — large-scale evolution.
From Deputy Director Jiang Peng:
> "From chasing pulsars and capturing fast radio bursts to mapping the evolution of cosmic neutral hydrogen, in the decade since completion, FAST, rooted deep in the mountains, has continuously produced world-class scientific results, driving a historic leap for Chinese radio astronomy."
FAST's advantage is single-dish sensitivity (detecting extremely weak signals); DESI's is sample size (precise redshifts). Combining the two systematically breaks, for the first time, the old paradox of "seeing deep but not wide, or wide but not deep enough."
References
1. Collaboration paper: *The evolution of neutral hydrogen over the past 4.5 Gyr*, Nature Astronomy, published online 2026-09-01 (NAOC / Shanghai Astronomical Observatory / Shanghai Jiao Tong University, et al.) 2. Guangming Net: "China's FAST Redraws the Cosmic Evolution Picture," 2026-09-02 (*Guangming Daily*, 2026-09-02, p. 8) 3. CCTV Finance / The Paper: "Challenging conventional wisdom — major discovery from FAST," 2026-09-01 4. Xinhua: "A Decade of Watching the Stars: FAST Redraws the Cosmic Evolution Picture," 2026-09-01 5. FAST: 500 m Aperture Spherical Telescope, Pingtang County, Guizhou, completed 2016-09-25 6. DESI: Dark Energy Spectroscopic Instrument, 4 m prime-focus survey instrument, 5-year DESI Collaboration survey 7. 21 cm line: hyperfine transition of neutral atomic hydrogen, 1420 MHz (λ ≈ 21 cm) 8. Wikipedia: Star formation / Molecular cloud / Cold gas / Interstellar medium