Cosmic Star Formation Declining Despite Adequate Hydrogen Supplies, Study Reveals

An international team using radio telescopes and spectroscopic surveys has found that the universe's star formation rate has dropped to about 40% of what it was 4.5 billion years ago, yet the available neutral hydrogen—a key ingredient for star birth—has remained relatively constant. This unexpected finding suggests that factors beyond hydrogen depletion are responsible for the slowdown in stellar birth rates in recent cosmic history. The research analyzed data from approximately 2.5 million galaxies across nearly one-third of the observable sky.

Researchers examined approximately 2.5 million galaxies spanning roughly one-third of the observable universe to track changes in star formation and hydrogen availability over the past 4.5 billion years. The investigation combined data from two major observatories: FAST, which detects neutral hydrogen through radio signals at a specific wavelength, and DESI, which conducted spectroscopic analysis. The findings revealed a striking discrepancy—while the star formation rate has plummeted to roughly 40% of its earlier level, the total amount of neutral hydrogen in the cosmos has remained relatively stable, shifting only slightly.

The research suggests that hydrogen scarcity is not the culprit behind declining star birth. Instead, the team proposes that changes in the baryon cycle—the process controlling how matter flows between galaxies and intergalactic space—may explain the slowdown. As cosmic gas supplies weaken over time, the conversion efficiency of neutral hydrogen into the molecular hydrogen necessary for star formation may decrease, creating an indirect throttle on stellar genesis independent of raw fuel availability.