Star formation decline outpaces hydrogen supply decrease, posing a cosmic mystery

Astronomers have uncovered a surprising mismatch between the dramatic decline in star formation across the universe over the past 4.5 billion years and the relatively modest decrease in the supply of neutral hydrogen, a key ingredient for making stars. Using China's FAST radio telescope and data from 2.5 million galaxies, researchers found that star formation was 2.5 times higher 4.5 billion years ago, while neutral hydrogen levels were only 1.4 times higher. This suggests that changes in how gas moves through the baryon cycle, rather than overall hydrogen abundance, may be driving the collapse in star formation. The findings were published in Nature Astronomy.