A sub-ppm upper limit on the cosmological variations of the fine structure constant α

Journal article, 2026

Absorption spectroscopy toward high-redshift quasars provides strong constraints on the putative variation of fundamental constants of physics on cosmological timescales. The submillimeter ground-state transitions of methylidyne (CH) and water (H2O), both molecules widespread and coeval in the interstellar medium, provide a sensitive test for variations in α, the fine structure constant, and μ, the proton-to-electron mass ratio, taking advantage of the unmatched spectral resolution and frequency reliability of radio techniques. We used ALMA simultaneous observations of the two species to constrain any velocity offset between their absorption profiles toward the radio-bright lensed quasars PKS 1830−211 (z abs = 0.89) and B 0218+357 (z abs = 0.68). Our observational setup minimizes instrumental errors and known sources of systematics, such as time variability of the absorption profile and frequency-dependent morphology of the background quasar. The excellent correlation between CH and H2O opacities, the large number of individual narrow velocity components, and the number of independent spectra obtained due to the intrinsic time variability of the absorption profiles ensure that even the chemical segregation bias is minimized. We obtained bulk velocity shifts of δv = −0.048 ± 0.028 km s⁻¹ and −0.13 ± 0.14 km s⁻¹ (1σ confidence level) between CH and H2O in the direction of PKS 1830−211(NE) and B 0218+357(SW), respectively. These values convert into the 3σ upper limits |Δα/α|< 0.55 ppm and 1.5 ppm, respectively, taking into account the independent upper limits on |Δμ/μ| previously obtained for these systems. These constraints on |Δα/α|, at look-back times of about half the present age of the Universe, are two to four times deeper than previous constraints on any other single high-z system.