Star formation across the universe has fallen by a factor of 2.46 over the past 4.5 billion years, yet the supply of neutral atomic hydrogen gas has declined far less, according to a large-scale analysis of approximately 2.5 million galaxies published in Nature Astronomy on September 1, 2026. Researchers combined radio observations from the Five-hundred-meter Aperture Spherical radio Telescope (FAST) with optical redshifts from the Dark Energy Spectroscopic Instrument (DESI) to reveal a striking cosmic mismatch between available raw fuel and stellar output.
The Great Mismatch in Cosmic Gas Inventories
For decades, standard astrophysical models pointed to a straightforward culprit for the fading glow of the late universe: exhaustion. As galaxies age, they burn through their cold gas reserves, leaving behind insufficient fuel to sustain massive rates of star formation. However, recent data upends that baseline assumption.
While optical spectroscopy from DESI supplied precise galaxy distances, FAST captured the notoriously faint 21-centimetre radio emission line emitted by atomic hydrogen.
The raw measurements showed that cosmic atomic-hydrogen density fell by a factor of 1.35 ± 0.10 over the span of 4.5 billion years. After applying a conservative forward-model correction for survey and measurement effects, the inferred decline dropped to a mere 1.12 ± 0.10. Meanwhile, the cosmic star-formation-rate density plunged by a factor of 2.46. The universe is running out of stellar nurseries, but it is certainly not running out of hydrogen.
Separating Raw Material from Usability
Atomic hydrogen is not the final ignition fuel inside a stellar nursery. Before gravity can successfully orchestrate a collapse into protostars, diffuse gas must cool, densify, and convert into molecular hydrogen. Several complex mechanisms regulate this critical phase transition, including radiative heating, internal turbulence, magnetic fields, stellar feedback from supernovae, and the ongoing supply of fresh gas streaming in from the cosmic web.
As reported by Soz6, the new data forces astronomers to shift the locus of the mystery. Galaxies retain vast, stable reservoirs of neutral atomic gas in their broad envelopes and disks, yet they have become significantly worse at channeling that matter into the dense molecular clouds required to build new stars. Molecular-gas measurements consistently evolve more closely with the actual star-formation rate, reinforcing the widening gap between raw atomic abundance and productive usability.
Unanswered Questions in Late-Universe Evolution
While the FAST and DESI survey successfully captures the average aggregate signal of a massive galaxy population, stacking spectra means individual hydrogen maps for every single target remain obscured. Furthermore, correcting the raw 1.35-fold decline down to 1.12 relies heavily on forward modeling, leaving room for systematic uncertainties.
The study does not pinpoint a single regulatory switch that put the brakes on cosmic star formation. Reduced gas accretion rates, inefficient molecular conversion chemistries, energetic feedback loops driven by active galactic nuclei and black holes, and large-scale shifts in galactic morphology all likely contribute to the slowdown. Ultimately, rapid gas exhaustion is ruled out as the primary driver, leaving modern astrophysics to decode why the abundant fuel of the cosmos sits largely idle.
