A research team has precisely measured the temperature of the cosmic microwave background radiation (CMB) at redshift z =0.89—corresponding to about 7 billion years ago—by analyzing archival data from the Atacama Large Millimeter/submillimeter Array (ALMA) toward the quasar PKS1830−211. The team consisted of Keio University researchers, including doctoral student Tatsuya Kotani from the Graduate School of Science and Technology, and Professor Tomoharu Oka from the Faculty of Science and Technology Department of Physics, in collaboration with the National Astronomical Observatory of Japan. The team determined the CMB temperature at that epoch to be 5.13 ± 0.06 K, the most precise measurement ever obtained at intermediate redshift. This temperature is roughly twice the current CMB temperature (≈ 2.7 K) and is in agreement with the underlying prediction of big bang cosmology that the CMB temperature increases in proportion to (1 + z ) when looking back in time. This result provides a stringent observational test of the standard cosmological model and places important constraints on our understanding of the universe’s thermal history.
This research was published on October 29, 2025, in The Astrophysical Journal.
