FAST telescope's new discovery reshapes understanding of early universe

BEIJING -- China's Five-hundred-meter Aperture Spherical Radio Telescope (FAST) has detected 156,411 neutral hydrogen sources across nearly half the sky, marking a major breakthrough in radio astronomy.

The findings, published as a cover story in the journal Science China Physics, Mechanics & Astronomy, position FAST as the new global reference in hydrogen survey research.

The 156,411 galactic "identity cards" are expected to advance multiple frontier fields, including galaxy formation and evolution, dark matter distribution and large-scale cosmic structures, said Zhang Chuanpeng, lead author of the study and associate researcher at the National Astronomical Observatories of the Chinese Academy of Sciences.

Neutral hydrogen, the most abundant element in the universe, formed about 13.8 billion years ago when the post-Big Bang universe cooled enough for protons and electrons to combine. It serves as the raw material for stars and galaxies and carries critical information about the universe's transition from darkness to light. The element emits a distinct 21-centimeter spectral line that penetrates interstellar dust and travels billions of light-years to Earth. FAST, with its unmatched sensitivity, is uniquely equipped to capture these faint cosmic signals.

The latest data release, dubbed FASHI DR2, comes two years after the team published its first results. In January 2024, the initial release set a world record with 41,741 detections. That first dataset has since been cited more than 100 times in scientific literature, and over 10 international research teams have expressed interest in collaborating on topics ranging from galaxy evolution and cosmology to large-scale structure.

FASHI DR2 now covers 19,500 square degrees, nearly half the celestial sphere, and includes 156,411 sources -- four times the number in DR1. The survey's typical sensitivity features about 30 percent sharper than its predecessor, enabling the detection of much fainter signals. International peer reviewers have hailed the dataset as a long-awaited milestone and a future standard reference in the field.

Using the data, the research team has reliably extended the neutral hydrogen mass function down to galaxies of roughly one million solar masses, and determined the local neutral hydrogen density with a statistical precision of 0.6 percent -- the most accurate measurement to date.

Located in a naturally deep and round karst depression in Southwest China's Guizhou province, FAST has a reception area equal to 30 standard football fields. As the world's largest single-dish radio telescope, FAST started formal operations in January 2020 and was officially opened to the world in March 2021.

All data from FASHI DR2 have been made publicly available, enabling the global scientific community to participate in further exploration, Zhang added.