What makes FAST telescope essential for measuring hydrogen in distant galaxies?

FAST's 500-meter aperture gives it a collecting area roughly 40 times larger than any previous single-dish radio telescope. That sensitivity makes it possible to detect the faint 21-centimeter hydrogen signal from galaxies billions of light-years away, enabling the spectral stacking technique used in this study.

Star birth is down 60% — the universe’s hydrogen supply is mostly intact

Q: Does this study prove the fuel crisis theory of galaxy aging is wrong?

It refutes the simplest version: that declining atomic hydrogen supply directly explains declining star formation. The hydrogen supply has not fallen nearly fast enough to account for the observed drop in star birth. A more complex process involving how inefficiently hydrogen converts to the molecular phase needed for star formation appears to be at work.

Star birth has fallen by more than half across the cosmos in the last 4.5 billion years, and the leading explanation just collapsed. A survey of 2.5 million galaxies — combining the world’s most sensitive radio telescope with the largest optical sky survey ever assembled — found that the universe still holds most of the neutral hydrogen it had at its star-forming peak. Whatever is shutting down the cosmic nurseries, it is not a shortage of raw material.

The finding, in Nature Astronomy, comes from researchers at the Chinese Academy of Sciences and their international partners in the DESI collaboration. It is the largest direct measurement of cosmic hydrogen reserves ever made. It resolves a decades-old question about why the universe is aging — and the answer is not what most astronomers expected.

The scale of the decline is stark. At its peak roughly 10 billion years ago, the universe was producing stars around ten times more vigorously than it does today. Four and a half billion years ago — when Earth was just forming — the cosmic star formation rate was still 2.5 times higher than it is now. If that trajectory continues, the universe is on a slow and irreversible path toward a sky with ever fewer new suns. The question has always been: why?

How they measured the universe’s hydrogen supply

The dominant theory has been intuitive: as galaxies age, they exhaust their gas reserves. Less fuel means fewer stars. To test that hypothesis at the scale of the observable universe, researchers needed to measure neutral atomic hydrogen — the most abundant form of hydrogen in interstellar space — across billions of light-years simultaneously. That had never been possible before at this precision.

The team combined two instruments that together make the measurement tractable for the first time. FAST — the Five-hundred-meter Aperture Spherical radio Telescope in Guizhou province, China, currently the world’s largest single-dish radio telescope — detects the faint 21-centimeter radio emission that neutral hydrogen naturally broadcasts. DESI, the Dark Energy Spectroscopic Instrument in Arizona, provided precise optical redshifts for the 2.5 million galaxies in the study, establishing their distances and therefore the cosmic epoch at which each was observed.

The technique, called HI spectral stacking, addressed the central challenge: no single distant galaxy emits enough hydrogen signal to be individually detected at cosmological distances. By aligning and stacking millions of faint radio spectra according to the precise galaxy redshifts that DESI supplied, the team converted noise into statistical signal — equivalent to listening to millions of quiet conversations simultaneously instead of trying to isolate a single whisper across a room. The resulting survey covered roughly one third of the entire sky, extending back 4.5 billion years in cosmic time. Nothing at this scale and sensitivity has been attempted before.

What the numbers actually show

The results were unambiguous — and inconvenient for the standard model. Neutral hydrogen density across the surveyed volume has declined by a factor of only 1.4 over the same 4.5 billion years that saw the star formation rate fall by a factor of 2.5. In proportional terms: star birth fell by roughly 60 percent. Hydrogen supply fell by roughly 29 percent.

The gap between those two numbers is the problem for existing theory. If hydrogen depletion were the primary driver of the star formation collapse, the two curves would track each other closely. They do not. The mismatch means that billions of cubic light-years of neutral hydrogen are sitting unused in the spaces between and within galaxies — available in principle to form stars, but not doing so. The universe is not converting its hydrogen into stars fast enough to account for the observed decline, and the shortfall in conversion cannot be explained by simply having less hydrogen available.

The comparison across cosmic time reinforces the point. At the epoch 4.5 billion years ago that corresponds to the study’s deepest measurements, the ratio of star-forming activity to available hydrogen was measurably higher than it is today. Something has changed in how efficiently gas converts to stars — and that something is not the quantity of raw material on hand.

What else could be stopping the stars

The paper identifies where the bottleneck is most likely to be found. Neutral atomic hydrogen does not collapse directly into stars. It must first convert to molecular hydrogen — the denser, colder phase that can actually condense under gravity into stellar nurseries. The efficiency of that conversion, rather than the supply of atomic hydrogen upstream of it, is now the prime suspect for what is slowing the universe’s star factories.

What controls that conversion efficiency? Several mechanisms are candidates. Feedback from supermassive black holes at galactic centers — energetic jets and winds that heat surrounding gas — can prevent atomic hydrogen from cooling into the molecular phase needed for star formation. Radiation from earlier generations of stars can ionize gas faster than gravity can pull it together. The properties of the circumgalactic medium, the diffuse gas halos that surround galaxies, may have shifted over cosmic time in ways that reduce the rate at which atomic hydrogen settles into star-forming clouds. No single mechanism has been confirmed as the dominant one. The study’s contribution is to establish, with unprecedented certainty and scale, that none of them work by depleting the hydrogen supply itself.

What this study does not settle

The measurement is a population-level result averaged across millions of galaxies, not a diagnosis of any individual galaxy’s evolution. The stacking method gives statistically robust cosmic averages; it cannot isolate what is happening inside specific galaxy types, mass ranges, or environments. A galaxy-by-galaxy picture of why star formation is suppressed remains out of reach of this technique.

More fundamentally, the study measured atomic hydrogen — the HI phase. What ultimately fuels star formation is molecular hydrogen, the H₂ phase, which is far harder to detect at cosmological distances. Mapping molecular hydrogen across the same volume as this survey would be a far more demanding observational task and is not yet technically feasible at this scale. The bottleneck between atomic and molecular gas, which this paper identifies as the likely culprit, remains unmeasured at the scale required to resolve the question.

The authors note that multiple physical processes likely interact to produce the observed star formation decline across different environments and cosmic epochs. Any complete theory will need to account not only for these hydrogen measurements but also for observations of individual galaxy formation histories, star formation environments, and the role of galactic-scale gas flows.

Common questions about the cosmic star-birth decline

Why does the star formation rate matter for the future of the universe?

Stars forge the heavy elements that make up planets, chemistry, and life. As the rate of new star formation falls, the universe’s capacity to build new planetary systems and replenish the elements they require declines with it. The trend measured by this study represents the empirical signature of a cosmos in slow, irreversible decline — the beginning of the end of the stellar epoch.

Is our Sun affected by this cosmic slowdown?

No. The Sun formed 4.6 billion years ago and will continue burning for roughly another 5 billion years. The slowdown describes the rate at which new star systems ignite across the universe, not the behavior of stars already formed. Our Solar System is insulated from the trend by its age.

What makes FAST essential for this type of measurement?

FAST’s 500-meter aperture gives it a collecting area roughly 40 times larger than any previous single-dish radio telescope. That sensitivity is what makes it possible to detect the faint 21-centimeter hydrogen signal from galaxies billions of light-years away. Without FAST’s collecting power, the stacking technique would not produce a statistically significant result at these distances.

Does this study mean the ‘fuel crisis’ theory of galaxy aging is wrong?

It challenges the simplest version: that declining atomic hydrogen supply directly drives declining star formation. The hydrogen supply has not fallen nearly fast enough to account for the observed drop in star birth. A more complex process — most likely involving how inefficiently that hydrogen converts to the molecular phase needed for star formation — appears to be at work.

The team’s immediate next step is measuring molecular hydrogen at comparable cosmic scales, a task that next-generation facilities including the Square Kilometre Array may eventually make tractable. Until then, the universe’s most consequential open question about its own future has grown by one confirmed paradox: the cosmos still has most of its hydrogen, and it is still running out of stars.

Reference: et al., “Cosmic star formation rate decline is not driven by neutral hydrogen depletion,” Nature Astronomy, 2026. DOI: 10.1038/s41550-026-02965-9

Tags: astronomy, cosmology, DESI, FAST telescope, neutral hydrogen, star formation