The Universe Still Has Plenty of Fuel, So Why Fewer Stars?

The Universe is forming far fewer new stars than it did billions of years ago, even though one of the key ingredients for star formation has barely declined, according to a new study combining observations from two major astronomical surveys. An international team led by researchers from the Chinese Academy of Sciences (CAS), working with the Dark Energy Spectroscopic Instrument (DESI) project, published the findings in the journal Nature Astronomy.

Over the past 4.5 billion years, the Universe's cosmic star formation rate has fallen to less than half its former level. The researchers used China's Five-hundred-meter Aperture Spherical radio Telescope (FAST) to make precise measurements of neutral atomic hydrogen, a major gas reservoir within galaxies, across that same period. Neutral atomic hydrogen, called HI, is detected through a faint 21-centimeter radio emission line that is difficult to measure in distant galaxies.

Combining FAST's radio sensitivity with the optical spectroscopic data gathered by DESI, the team examined about 2.5 million galaxies across nearly one-third of the sky, using a technique that combines many otherwise undetectable radio signals. The results showed that about 4.5 billion years ago, the Universe was forming stars at roughly 2.5 times today's rate, while the density of neutral atomic hydrogen was only about 1.4 times greater than its current level. Star formation, in other words, declined much faster than the neutral hydrogen supply.

The researchers said this shifts the central question from whether the gas is depleting to why it has become harder to form stars despite abundant neutral hydrogen reserves. Stars do not form directly from neutral atomic hydrogen; they form inside much denser clouds of molecular gas, and HI sits between the large-scale gas supply and that molecular fuel. The team said galaxies may be becoming less efficient at converting neutral hydrogen into molecular hydrogen as the flow of gas from the wider cosmic web weakens.