The universe has been producing fewer stars over the past 4.5 billion years, and astronomers have found that a simple shortage of star-birth material does not explain the decline. An international research team led by Hong Guo of the Chinese Academy of Sciences used high-precision measurements to track cosmic neutral hydrogen and discovered that the universe’s star formation rate dropped substantially while its neutral atomic hydrogen reservoir changed very little.
High-Precision Measurements From FAST and DESI
The research team used the Five-hundred-meter Aperture Spherical radio Telescope (FAST) to measure cosmic neutral hydrogen (HI) through 21-centimeter radio emissions. Observations from the Dark Energy Spectroscopic Instrument (DESI) project complemented the radio data. The study examined a sample of roughly 2.5 million galaxies covering nearly a third of the sky.
The data revealed that 4.5 billion years ago, the star formation rate across the universe was about 2.5 times higher than it is today. Meanwhile, the neutral atomic hydrogen density was only about 1.4 times its current level. According to Hong Guo, this mismatch runs counter to standard expectations.
“What we find is that during the most recent 4.5 billion years, star formation continued to decline substantially, while the cosmic reservoir of neutral atomic hydrogen changed surprisingly little,” Hong said regarding the late-time phase of cosmic star formation. The cosmic star formation rate peaked much earlier around 10 billion years ago during an epoch known as “cosmic noon,” declining steadily ever since while neutral atomic hydrogen changed only weakly.
The Baryon Cycle and Missing Molecular Gas
Star formation relies on cold clouds of molecular hydrogen gas ($H_2$). Astronomers previously assumed that ongoing star formation consumed molecular hydrogen, leading to a sharp drop in star production in modern cosmic epochs. However, maintaining a supply of molecular hydrogen requires neutral atomic hydrogen ($HI$) as a link in the larger-scale cosmic gas cycle.
The study indicates that the changes driving modern star-form decline do not originate in the neutral hydrogen reservoir itself. Instead, researchers point to the baryon cycle—the exchange of normal baryonic matter between galaxies and the intergalactic environment. As the gas supply from the wider cosmic web weakens, the efficiency of converting neutral hydrogen into molecular hydrogen drops. The overall supply of neutral hydrogen remains stable, but molecular gas gradually disappears without replenishment, reducing the rate of star formation.
Future Observations and Extended Timelines
While the FAST-DESI study establishes a baseline for understanding recent star-form decline, the telescope’s limits restrict observations to the past 7.2 billion years. To trace conditions further back in time, researchers plan to conduct deeper surveys across a wide variety of wavelengths and frequencies to capture signals from earlier epochs of cosmic evolution.