Embedded Files
Released: March 13, 2026, Academia Sinica Institute of Astronomy & Astrophysics (ASIAA), Taiwan
Where do the stars in our universe come from? During the universe’s most active era of star formation, what allowed galaxies to produce vast numbers of new stars? Dr. Yi-Kuan Chiang, an Assistant Research Fellow at the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), has made the first measurement of the cosmic background glow from carbon spectral lines. His study finds that during the peak era of cosmic star formation, the amount of molecular gas in the universe may have been nearly twice as high as previous estimates. This result not only fills a missing piece in our understanding of the cosmic baryon cycle but also reveals the key driving force behind the birth of stars. The findings were published in Nature Astronomy in early March 2026.
A New Technique Reveals a Much Larger Cosmic Gas Reservoir
In the early universe, star formation was as active as a brightly lit city at night. Many galaxies functioned like star-forming factories, constantly producing new stars. Yet over time, the universe grew quieter and less active. Astronomers have long sought to understand why cosmic star formation slowed down. Traditional galaxy surveys resemble an incomplete population census: they detect bright, massive galaxies but miss the many faint, smaller ones. As a result, the total inventory of gas in the universe has been difficult to determine.
To address this gap, Chiang adopted an innovative observational technique known as line-intensity mapping, which captures the faint background glow of the universe and bypasses the limitations of traditional galaxy surveys. Using this method, he made the first observational measurement of the cosmic background from carbon monoxide (CO) and ionized carbon ([C II]). Gas clouds within galaxies emit faint radiation in these lines. When the signals from all galaxies are combined, they form a diffuse background glow permeating the universe. Chiang explains, “It’s like looking down at a city from an airplane at night. Even if you cannot distinguish individual buildings, the overall brightness distribution still allows you to estimate the size of the city.”
By combining all-sky diffuse maps from the Planck satellite, the Herschel Space Observatory, and the Infrared Astronomical Satellite (IRAS), Chiang carried out an unprecedented tomographic study of cosmic gas, revealing signals that had previously been hidden in the darkness. Carbon monoxide traces molecular gas—the raw fuel for star formation—while ionized carbon traces the cooling of gas heated by the newly formed stars. Statistical analysis of the CO signal allowed Chiang to estimate the total amount of molecular gas in the universe for the first time, tracing 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 as high as previously estimated. 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: the universe itself contained an abundance of raw material.
Why Did Cosmic Star Formation Decline? The Role of Gas Supply
Beyond revising the cosmic gas inventory, the study also shows that the rise and fall of star formation in the universe is largely governed by the supply of gas. Cosmic star formation peaked about 10 billion years ago and has declined since then as the amount of available gas decreased. When galaxies have abundant fuel, stars form rapidly; when the supply runs low, the process slows. Without replenishment, the molecular gas in galaxies would be converted into stars and exhausted within roughly a 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, where it cools into molecular clouds that serve as the raw material for the universe’s star-forming factories.
By measuring the cosmic background intensities of carbon monoxide and ionized carbon for the first time, this study uses the faint “carbon glow” of the universe to reveal a previously hidden reservoir of gas. The results show that the molecular gas content of 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 on the large-scale structure and evolution of the universe. This research was supported by Academia Sinica and the National Science and Technology Council (NSTC) of Taiwan.
Eleven sky maps used in this study, observed at different wavelengths by the Planck satellite, the Herschel Space Observatory, and the Infrared Astronomical Satellite (IRAS). Image Credit: Yi-Kuan Chiang/ASIAA.
History of cosmic molecular gas density inferred from the diffuse CO background (black curve and symbols), compared with lower limits from traditional galaxy surveys (light blue symbols) that detect only bright CO-emitting galaxies individually. The horizontal axis shows redshift, corresponding to cosmic time from the early universe (right) to the present (left). The diffuse intensity-mapping measurement reveals that a significant fraction of the universe’s molecular gas resides in faint galaxies that were previously missed. Image Credit: Yi-Kuan Chiang/ASIAA.
More Information:
This research was presented in a paper “Cosmic CO and [C II] backgrounds and the fuelling of star formation over 12 Gyr" by Chiang in Nature Astronomy published in March 2026.
Media Contact:
Media Contact:
Dr. Yi-Kuan Chiang, +886-2-2366-5470, ykchiang@asiaa.sinica.edu.tw
Dr. Mei-Yin Chou, +886-2-2366-5415, cmy@asiaa.sinica.edu.tw
Page updated
Report abuse