The universe is producing fewer stellar “babies.” Over the past 4.5 billion years, the rate at which new stars form has fallen to less than half its previous level. Yet the supply of one of the most important types of “fuel” for star formation has changed surprisingly little.
That finding comes from an international research team led by scientists from the Chinese Academy of Sciences (CAS), working with the Dark Energy Spectroscopic Instrument (DESI) project. Using China’s Five hundred meter Aperture Spherical radio Telescope (FAST), the researchers made precise measurements of cosmic neutral atomic hydrogen across the past 4.5 billion years.
Their results show a striking mismatch. Star formation has declined sharply, while the amount of neutral atomic hydrogen (HI), an important reservoir of gas within galaxies, has decreased only modestly.
The findings were published online in Nature Astronomy on Sept. 1.
Why Is the Universe Making Fewer Stars?
Understanding why star formation has become less active as the universe ages is a major question in research on galaxy formation and evolution. One seemingly straightforward explanation is that galaxies have gradually consumed the cold gas needed to produce stars.
If dwindling supplies of cold gas were primarily responsible, however, astronomers would expect the dramatic fall in star formation to be accompanied by a similarly large decline in the available gas. So far, observations have not shown such a sharp depletion.
HI plays a central role in this puzzle. It is an important cold gas reservoir inside galaxies, connecting the broader cosmic supply of gas with the processes that eventually produce new stars. Astronomers primarily detect HI through its extremely faint 21-centimeter radio emission line.
Detecting that signal from distant galaxies is difficult because it is often overwhelmed by background noise.
FAST and DESI Survey Millions of Galaxies
For years, astronomers faced a major observational challenge. Very deep surveys could achieve the necessary sensitivity but could not examine large regions of the sky. Surveys covering much larger areas, meanwhile, generally lacked the sensitivity needed to detect such faint radio signals.
As a result, scientists have struggled to directly and reliably determine how the universe’s total HI mass has changed across the low- to intermediate-redshift universe.
The new research tackled this problem by combining the exceptional radio sensitivity of FAST with the enormous optical spectroscopy data set provided by DESI. The team studied about 2.5 million galaxies spread across nearly one-third of the sky.
The researchers used an HI spectral stacking method to combine radio signals that would have been too faint to detect individually. Using precise measurements of each galaxy’s redshift, they aligned the weak signals and stacked them together. This process allowed the average HI signal to emerge from the background noise.
The approach enabled the scientists to track changes in cosmic neutral hydrogen using a sample of unprecedented size and with exceptionally high statistical precision.
Star Formation Fell Much Faster Than Hydrogen
The results exposed a major difference between the evolution of star formation and the supply of neutral hydrogen.
About 4.5 billion years ago, the cosmic star formation rate was approximately 2.5 times higher than it is today. Over the same period, however, neutral atomic hydrogen density was only about 1.4 times higher than its present level.
In other words, star formation dropped dramatically without a comparable disappearance of the universe’s HI reservoir. The findings indicate that rapidly exhausting neutral hydrogen cannot by itself explain why star formation has declined so strongly.
The Cosmic Mystery Shifts
According to the researchers, the results change the central question from “whether the gas is depleting” to “why it is increasingly difficult to form stars despite abundant neutral hydrogen reserves.”
Stars do not form directly from most neutral atomic hydrogen. They are mainly born inside much denser clouds of molecular gas. Neutral atomic hydrogen occupies an important intermediate position between the universe’s larger gas supply and the molecular hydrogen that can ultimately fuel star formation.
The researchers suggest that the most important changes in the more recent universe may involve how gas moves through the baryon cycle rather than how much HI exists overall.
As the flow of gas from the cosmic web becomes weaker and gas densities decrease, galaxies may become less efficient at converting HI into molecular hydrogen. Under this scenario, the overall HI reservoir can remain relatively stable while supplies of the molecular gas directly responsible for creating stars gradually decline.
Why the Universe’s Star Factories Are Fading
The implications therefore extend beyond simply determining how much hydrogen exists in the universe. The findings offer an important new clue to why the universe’s enormous star-forming engines have been gradually slowing down.
According to the researchers, combining observations from FAST and DESI provides a new observational benchmark for studying the cosmic gas cycle during the universe’s later evolution, the long-term decline in star formation, and the broader processes shaping galaxies.
The research was led by scientists from the National Astronomical Observatories of China, the Shanghai Astronomical Observatory of CAS, and Shanghai Jiao Tong University, together with researchers participating in DESI.
Contributors came from research institutions across Asia, North America, and Europe. The collaboration demonstrates the scientific potential of combining highly sensitive radio observations with enormous optical spectroscopy surveys.
