Scientists studying the evolution of the universe have found new evidence challenging a long-standing explanation for why star formation has slowed dramatically in recent cosmic history.

An international team led by researchers from China has used observations from the Five-hundred-meter Aperture Spherical radio Telescope (FAST) and the Dark Energy Spectroscopic Instrument (DESI) to examine how neutral hydrogen has changed over the past 4.5 billion years.

The study involved researchers from the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC), the Shanghai Astronomical Observatory (SHAO), Shanghai Jiao Tong University and other institutions. By combining highly sensitive radio observations with DESI’s large spectroscopic survey, scientists analyzed about 2.5 million galaxies covering roughly one-third of the sky.

Their results provide an important clue to one of astronomy’s central questions: why has the universe become increasingly inefficient at producing new stars?

Hydrogen Reserves Did Not Fall as Quickly as Star Formation

For decades, scientists have considered the gradual depletion of cold gas a possible explanation for the decline in star formation. Galaxies rely on cold gas as the raw material from which stars eventually form, suggesting that fewer available reserves should lead to fewer new stars.

Neutral atomic hydrogen is among the most important components of these gas reservoirs. However, detecting it in distant galaxies is extremely difficult because its 21-centimeter radio signal is exceptionally faint and can easily be hidden by background noise.

The new observations provide a much clearer picture.

Researchers found that about 4.5 billion years ago, the cosmic star formation rate was approximately 2.5 times higher than it is today. Yet the density of neutral atomic hydrogen was only about 1.4 times its present level.

This difference suggests that the decline in star formation cannot simply be explained by the rapid disappearance of neutral hydrogen.

Why Are Fewer Stars Being Born?

The researchers believe the answer may lie in what happens after neutral hydrogen accumulates in galaxies.

Stars are primarily formed inside much denser molecular clouds. For neutral hydrogen to become molecular hydrogen, the conditions within galaxies must allow that conversion to take place efficiently.

As the universe has evolved and the overall density of gas has decreased, the researchers suggest that the conversion from neutral atomic hydrogen to molecular hydrogen has also become less efficient.

As a result, the supply of molecular gas available to directly fuel star formation has gradually declined, even though large quantities of neutral hydrogen remain.

Jiang Peng, a researcher at NAOC, said understanding why the universe has become less capable of forming stars is a central problem in research into galaxy formation and evolution.

The findings therefore point toward a more complex explanation for the cosmic slowdown in star formation—one involving the changing efficiency of gas conversion rather than simply the exhaustion of hydrogen.

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FAST and DESI Provide a Larger Cosmic Picture

The study demonstrates the value of combining different types of astronomical observations.

FAST, the world’s largest single-dish radio telescope, can detect extremely faint signals associated with neutral hydrogen. DESI, meanwhile, provides extensive information about the positions and properties of galaxies across the sky.

Together, the two datasets allowed researchers to study the evolution of cosmic neutral hydrogen on an unprecedented statistical scale.

The results offer a new framework for understanding how galaxies obtain, transform and eventually use the gas needed to produce stars.

The study was published Tuesday in the international journal Nature Astronomy.