The universe is making fewer stars, but it isn’t running out of fuel

A study published in Nature Astronomy finds that star formation across the universe has dropped sharply over the past 4.5 billion years, even though the supply of neutral atomic hydrogen — a key raw material for stars — has declined only modestly.

Researchers combined data from China's FAST telescope and Arizona's DESI instrument to study roughly 2.5 million galaxies, finding star formation ran about 2.5 times higher billions of years ago while hydrogen density was only 1.4 times higher than today.

The team suggests the slowdown isn't about galaxies running out of gas but about that gas becoming less efficient at converting into the denser molecular hydrogen that actually feeds new stars, though the exact mechanism remains unresolved.

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