The universe is forming stars at less than half the rate it did roughly 4.5 billion years ago. A new survey now shows that this decline cannot be explained simply by galaxies running out of atomic hydrogen, the raw gas reservoir from which star-forming clouds are built.
An international team combined observations from China’s Five-hundred-meter Aperture Spherical radio Telescope, or FAST, with spectra from the Dark Energy Spectroscopic Instrument, DESI. Together, the two surveys allowed the researchers to measure faint hydrogen signals across about 2.5 million galaxies covering nearly one-third of the sky.
A mismatch between gas and newborn stars
Stars form when gas becomes dense enough to collapse under its own gravity. Astronomers have therefore expected a long-term fall in star formation to be accompanied by a comparable loss of the cold gas that feeds it.
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The new measurements do not show that. The cosmic star-formation rate was about 2.5 times higher 4.5 billion years ago than it is today, while the density of neutral atomic hydrogen was only about 1.4 times higher. After the team accounted for systematic effects in its modelling, the inferred change in the hydrogen reservoir was smaller still.
That difference matters because it rules out rapid depletion of atomic hydrogen as the main reason the universe has become quieter. Galaxies still contain much of the material, yet they are turning less of it into new stars.
Finding a faint signal in millions of galaxies
Neutral hydrogen is usually detected through a weak radio emission line at a wavelength of 21 centimetres. For a distant individual galaxy, the signal can disappear into noise. FAST provides the sensitivity, while DESI supplies accurate distances and redshifts for a huge optical sample.
The researchers used those redshifts to line up the radio spectra of millions of galaxies and combine them statistically. The average signal that emerged was too faint to see in most single objects, but strong enough in the stacked data to trace how the hydrogen reservoir changed over time.
This approach does not produce a photograph of each galaxy’s gas. It measures the population as a whole, giving astronomers a much firmer estimate of the average atomic-hydrogen density across the recent history of the universe.
Atomic gas is only one step
The result also clarifies where the missing explanation may be hiding. Atomic hydrogen is an intermediate stage. Before it can collapse into stars, it must usually become denser molecular gas, and that conversion depends on pressure, temperature, dust and the way gas moves through a galaxy.
The supply from the cosmic web may also be weakening. If fresh gas arrives more slowly and the average density inside galaxies falls, the conversion from atomic to molecular hydrogen can become less efficient even while the larger atomic reservoir remains comparatively stable.
The likely bottleneck is therefore the transition from atomic to molecular gas, or the flow of gas into the denser clouds where stars form. That would allow the larger atomic reservoir to remain relatively stable while the material immediately available for star formation becomes scarcer.
A new test for galaxy-evolution models
The FAST–DESI result gives theorists a quantitative target. Models of galaxy evolution must now explain how star formation can drop by roughly a factor of 2.5 while the atomic-hydrogen supply declines far more slowly.
That question connects to the broader work of mapping how matter moves through the universe. As our account of the Great Attractor explains, astronomers often reconstruct invisible structures from the motion and faint signals of enormous numbers of galaxies. Here, the same statistical logic is being used to follow the fuel cycle inside those galaxies.
The new measurements do not identify one final mechanism. They show that the simple fuel-shortage explanation is insufficient and set a sharper benchmark for studying gas accretion, phase conversion and the gradual slowing of cosmic star birth.
