Star News | Why Is the Universe Finding It Harder and Harder to Make New Stars?

(FAST redraws a new picture of cosmic evolution)

Why is star formation slowing down as the Universe ages? This question sits at the heart of galaxy formation and evolution. Stars are born inside dense molecular gas clouds, and for a long time astronomers have generally believed a simple “fuel story”: as cosmic time goes on, the cold gas that can form stars is steadily used up, so the star formation rate declines—and the reservoir of cold gas should drop in step.

One of the most important pieces of that cold-gas puzzle is neutral atomic hydrogen (often written as H I, “neutral hydrogen”). It is a major gas reservoir in galaxies, a key intermediate stage between large-scale gas supply and the formation of molecular gas, and a crucial link connecting a galaxy’s large-scale gas cycle to its internal star formation.

Why has this been so hard to measure?

H I in distant galaxies is mainly detected through an extremely faint radio signal: the 21-centimeter line (the H I “21 cm spectral line”). For faraway galaxies, the signal from any single object is easily drowned out by background noise. Because of these observational limits, past studies often faced a trade-off—either “deep but narrow” or “wide but not deep enough.” As a result, astronomers have long lacked reliable direct observational evidence for how the total amount of H I in the low- to intermediate-redshift Universe changes over time.

A new FAST + DESI view of the last 4.5 billion years

A new international collaboration led by teams from the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC), the Shanghai Astronomical Observatory, and Shanghai Jiao Tong University, among others, combined observations from China’s Five-hundred-meter Aperture Spherical Radio Telescope (FAST)—often called the “China Sky Eye”—with the Dark Energy Spectroscopic Instrument (DESI).

Using these two facilities together, the team carried out a high-precision measurement of how cosmic H I has evolved over the past 4.5 billion years.

The key result: stars fade fast, but H I fades slowly

The study finds that, over the last ~4.5 billion years, cosmic star formation has weakened dramatically, yet the amount of neutral atomic hydrogen—a major gas reservoir—has changed only slowly. This challenges the traditional picture that star formation is declining mainly because neutral hydrogen is being rapidly depleted.

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To achieve this, the researchers merged FAST’s high-sensitivity radio observations with DESI’s large spectroscopic survey, analyzing data for about 2.5 million galaxies spread across roughly one-third of the sky.

How did they detect such faint signals?

They used a technique called H I spectral-line stacking. In simple terms: many individual H I signals are too weak to detect on their own, but if you precisely align them by each galaxy’s redshift and then stack (add) them together, an average H I signal can be extracted from the noise. With this statistical method and a very large sample, the team reconstructed the evolutionary track of cosmic neutral hydrogen with high precision.

Numbers that reshape the story

The measurements indicate that:

• About 4.5 billion years ago, the cosmic star formation rate was roughly 2.5 times the present-day value.
• Over the same period, the cosmic H I density was only about 1.4 times today’s value.

In other words, star formation fell sharply, but the neutral-hydrogen reservoir did not dry up at the same pace. This mismatch in evolution rates undercuts the older idea that “rapid H I depletion drives star-formation decline.” Instead, it shifts the focus to a deeper question:

If neutral hydrogen remains relatively abundant, why is forming stars becoming increasingly difficult?

A new clue: the bottleneck may be the H I → H₂ conversion

The study suggests that the key driver of late-time star-formation decline may lie in the process that turns neutral atomic hydrogen (H I) into molecular hydrogen (H₂)—the form of gas most directly involved in making stars.

As the cosmic web’s gas supply weakens and gas densities drop, the efficiency of converting H I into H₂ may decrease. That would allow the H I reservoir to remain comparatively stable, while the molecular gas needed to actually form stars gradually diminishes.

Why it matters

By precisely quantifying long-term changes in cosmic neutral hydrogen, this work provides new evidence and a fresh direction for tackling the mystery of star-formation decline. The combined FAST–DESI approach also sets a new observational benchmark for understanding gas cycling in the late Universe, the fading of star formation, and the broader path of galaxy evolution.

The results have been published in Nature Astronomy.

Source: Chinese Academy of Sciences (CAS) — “FAST redraws the picture of cosmic evolution” (published September 2, 2026).