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Where do the stars in the universe come from? During the most active era of star formation, what conditions allowed galaxies to continuously produce vast numbers of stars? Dr. Yi-Kuan Chiang, an Assistant Research Fellow at the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), has made the first cosmic-scale measurement of the background glow produced by carbon emission lines. His study finds that, during the peak epoch of star formation, the amount of molecular gas in the universe might have been nearly twice that of previous estimates. This result not only fills an important gap in our understanding of the cosmic matter cycle but also reveals the key driving force behind the birth of stars. The findings were published in Nature Astronomy in early March 2026.

An Innovative Observational Method Reveals a Cosmic Fuel Supply Nearly Twice that of Previous Estimates

In the early universe, star formation was as vibrant as the lights of a city at night. Galaxies functioned like star-forming factories, constantly producing new stars. Yet over time the universe gradually became less “lively.” Astronomers have long sought to understand why cosmic star formation slowed down. Traditional galaxy surveys resemble an incomplete population census: they can only identify bright, massive galaxies, while numerous faint and small galaxies remain undetected. As a result, the true inventory of gas in the universe has been difficult to determine.

To fill in this missing piece, Chiang adopted an innovative observational technique known as line-intensity mapping, which captures the faint background glow of the entire universe and overcomes the limitations of traditional galaxy observations. Using this method, he achieved the first observational measurement of the cosmic mean background emission from carbon monoxide (CO) and ionized carbon ([CII]). Gas clouds within galaxies emit extremely faint radiation in these spectral lines. When the signals from all galaxies are combined, they form a diffuse background glow permeating the universe. Chiang explains that “it’s like looking down at a city at night from an airplane. Even if you cannot distinguish individual buildings, the overall brightness distribution still allows you to estimate the size of the city.”

By combining data from the Planck satellite, the Herschel Space Observatory, and the Infrared Astronomical Satellite, Chiang effectively performed an unprecedented cosmic “gas tomography,” revealing signals that had previously been hidden in the darkness. Carbon monoxide traces the amount of molecular gas, the raw fuel for star formation, while ionized carbon reflects the cooling associated with star-forming activity. Statistical analysis of the CO signal enabled scientists to estimate the total amount of molecular gas in the universe for the first time, reconstructing its evolution over roughly 12 billion years of cosmic history.

The study finds that, during the universe’s peak star-forming epoch, the cosmic density of molecular gas (ΩH2) in galaxies was nearly twice that of previous estimates. This suggests that the universe once possessed a far larger “reservoir of stellar fuel” than scientists had imagined. The result explains how galaxies in that era were able to produce stars at extraordinarily high rates—because the universe itself contained an abundance of raw material.

Why Did Cosmic Star Formation Decline? Gas Supply Drives the History of Star Formation

Beyond revising the cosmic gas inventory, the study also provides evidence that the rise and fall of star formation in the universe is primarily controlled by the supply of gas. According to Chaing’s results, cosmic star formation peaked about 10 billion years ago and has gradually declined since then, largely because of changes in the availability of gas. When galaxies have abundant fuel, stars form rapidly; when gas becomes scarce, stars form slowly. Without replenishment, the molecular gas in galaxies would be converted into stars and depleted within roughly one billion years. Thus the history of star formation depends on a vast cosmic supply chain: gas flows from the large-scale cosmic web into galaxies, cools to form molecular clouds, and eventually collapses to ignite new stars.

By measuring the cosmic mean background intensities of CO and ionized carbon for the first time, Chiang's study uses the faint “carbon glow” of the universe to reveal a previously hidden gaseous cosmos. The results demonstrate that the molecular gas reservoir in the early universe was nearly twice as large as previously thought, offering a clearer picture of how galaxies grow, how stars form, and how matter is distributed across the universe. The work also opens a new window for exploring the large-scale structure and evolutionary history of the cosmos. This research was supported by Academia Sinica and the National Science and Technology Council of Taiwan.