Why are stars being born 2.5 times slower in the universe?

An international team of astronomers led by the Chinese Academy of Sciences has presented unexpected results after measuring neutral hydrogen reserves on a large scale over the past 4.5 billion years. According to Ixbt.com, the rate of star formation in the universe has decreased by nearly 2.5 times during this period, while the density of neutral atomic hydrogen—the primary "fuel" that eventually forms molecular hydrogen and stars—has dropped by only 1.4 times. This is reported by Ixbt.com reports .

Two powerful scientific instruments were used to conduct this massive study. The first is the world's largest FAST radio telescope, located in China's Guizhou province with a diameter of 500 meters, which recorded radiation at a wavelength of 21 cm. The second is the DESI spectroscopic project operating on the Nicholas Mayall telescope in Arizona, USA. Together, these instruments collected data on nearly 2.5 million galaxies, covering one-third of the sky.

Fuel is sufficient, but 'production' is stalling

According to the logical assumption in the scientific community, stars emerge from cold molecular hydrogen clouds, which in turn are formed from neutral hydrogen. Therefore, the process should have ended once the reserves were depleted. However, data collected by the FAST and DESI telescopes refuted this pattern. Despite neutral hydrogen reserves remaining largely intact, the operation of the star factory has slowed down.

One of the study's leaders, Xun Gong, noted that the process observed by scientists covers the decline phase in the final stage of the universe. Indicators have been falling since the "cosmic noon"—the peak of star formation approximately 10 billion years ago. Nevertheless, the discrepancy between existing neutral hydrogen and rapidly declining star production requires a scientific explanation.

The importance of the baryon cycle

Scientists believe the root of this situation lies in the baryon cycle. The baryon cycle is the process of circulation of ordinary matter between galaxies and the intergalactic medium. Matter is pulled from the cosmic web into a galaxy, part of which turns into stars, while another part is expelled through supernova explosions and stellar winds.

The authors hypothesize that as the universe ages, the flow of gas from the large-scale web weakens. While the total reserve of neutral hydrogen remains stable, its efficiency in converting into molecular gas decreases. As a result, even though reserves are not yet exhausted, the gas already spent on stars is not replenished, and the process gradually stabilizes.

For now, the results reflect a specific moment in the late universe rather than the full history of the past. The FAST telescope can look back approximately 7.2 billion years, but deeper analyses in various ranges are required to study even earlier periods. This research serves as a solid foundation for future discoveries in the history of the universe.