Key points
On September 1, 2026, *Nature Astronomy* published a paper by an international team from the Chinese Academy of Sciences' National Astronomical Observatories, Shanghai Astronomical Observatory, Shanghai Jiao Tong University, and partners. Using joint data from China's FAST (Five-hundred-meter Aperture Spherical Telescope) and the Dark Energy Spectroscopic Instrument (DESI), the team performed a high-precision measurement of cosmic neutral hydrogen (H I) evolution over the past 4.5 billion years — and overturned a long-standing assumption: the star formation rate has dropped to ~40% of its value 4.5 billion years ago, yet neutral hydrogen reserves still stand at over 71% of that level. The fuel hasn't run out; the conversion chain is the bottleneck.
The open question
Why does the universe increasingly struggle to form new stars? The intuitive explanation has been that stars form from cold gas (mainly neutral hydrogen); as star formation consumes cold gas, star formation should decline in step with gas reserves. Until now, equipment lacked the sensitivity to test this directly — the 21 cm line signal of neutral hydrogen is so weak that single distant galaxies are buried in noise. As Dr. Zhang Chuanpeng of NAOC put it, "it's like trying to hear someone whispering in a noisy plaza."
Extracting the signal from noise
The observational strategy had three steps:
1. Large sample: align radio signals from ~2.5 million galaxies across ~1/3 of the sky by redshift; 2. Line stacking: individually undetectable weak signals, when stacked, reveal a statistically meaningful average signal; 3. Cross-matching with DESI: DESI provides precise redshifts and galaxy properties; FAST H I data are stacked bin by bin at those redshifts.
This stacking approach says nothing about individual galaxies but is powerful for average properties and evolutionary trends.
Key numbers: asymmetric decline
| Dimension | 4.5 Gyr ago | Today | |---|---|---| | Cosmic star formation rate | ≈ 2.5× today | 1.0 (baseline) | | Neutral atomic hydrogen density | ≈ 1.4× today | 1.0 (baseline) | | Ratio (H I / SFR) | ≈ 0.56 | 1.00 |
Star formation dropped ~60% while neutral hydrogen dropped only ~29%. The two curves did not decline in step — directly refuting "fuel exhaustion" as the main cause of declining star formation.
The real story: the "factory" slowed, not the fuel
Stars do not form directly from neutral hydrogen; H I must first convert into denser molecular hydrogen clouds — the true "nurseries" of stars. Zhang Chuanpeng's causal chain:
> "What's really changing 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 actually available for star formation decreases, while neutral hydrogen is preserved."
Researcher Guo Hong summarized: "What we're seeing is not a universe running out of gas, but gas becoming harder to 'process' into stars."
FAST at ten years: from catching up to defining benchmarks
FAST, completed September 25, 2016, in Pingtang, Guizhou, turns ten in 2026. Deputy Director Jiang Peng reflected: "From chasing pulsars and capturing fast radio bursts to mapping the evolution of cosmic neutral hydrogen, FAST has continuously produced world-class scientific results over its first decade, driving a historic leap for Chinese radio astronomy."
FAST's single-dish sensitivity plus DESI's large-sample precision redshifts together broke the long-standing trade-off between "deep but narrow" and "wide but shallow" observations for the first time in a systematic way.
Context in the 2026 H2 astronomy narrative
This result joins a series of recent developments shifting astronomy from "resolving single objects" to "tracking physical quantities across cosmic scales":
- Purple Mountain Observatory's Milky Way H I survey with intermediate-mass black hole candidates;
- Stacking analysis of 217 "little red dots" revealing extended host-galaxy emission;
- JWST's 8.7-hour NIRSpec mapping of NGC 4696's black hole cold-gas self-feeding cycle;
- RHIC STAR results challenging the 53-year-old quark-sum rule for baryon number;
- IRIS detection of 3D magnetic reconnection flare ribbon bright kernels.
References
1. "The evolution of neutral hydrogen over the past 4.5 Gyr," *Nature Astronomy*, published online 2026-09-01 2. Guangming Daily, "中国天眼重绘宇宙演化图景," 2026-09-02, p. 8 3. CCTV Finance / The Paper coverage, 2026-09-01 4. Xinhua, "Ten Years Watching the Stars: FAST Redraws Cosmic Evolution," 2026-09-01 5. FAST: 500 m aperture spherical radio telescope, Pingtang County, Guizhou, completed 2016-09-25 6. DESI: Dark Energy Spectroscopic Instrument, 4 m prime-focus survey instrument, 5-year survey 7. 21 cm line: hyperfine transition of neutral atomic hydrogen, 1420 MHz