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

S. Muller, A. Beelen, M. Guelin, J. H. Black, F. Combes, H. L. Bethlem, M. Gerin, C. Henkel, K. M. Menten, M. T. Murphy, W. Ubachs, N. Wozny

First listed 2025-12-16 | Last updated 2025-12-16

Abstract

Absorption spectroscopy toward high-redshift quasars provides strong constraints on the putative variation of fundamental constants of physics on cosmological time scales. 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 of alpha, the fine structure constant, and mu, 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 PKS1830-211 (z_abs=0.88582) and B0218+357 (z_abs=0.68466). 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 delta_v = -0.048 pm 0.028 km/s and -0.13 pm 0.14 km/s (1 sigma confidence level) between CH and H2O in the direction of PKS1830-211(NE) and B0218+357(SW), respectively. These values convert into the 3sigma upper limits |Delta_alpha/alpha| < 0.55 ppm and 1.5 ppm, respectively, taking into account the independent upper limits on |Delta_mu/mu| previously obtained for these systems. These constraints on |Delta_alpha/alpha|, 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.

Short digest

Simultaneous ALMA spectroscopy of CH ground-state doublets and H2O toward the lens absorbers at z_abs=0.88582 (PKS 1830-211) and 0.68466 (B 0218+357) compares velocity centroids to test for cosmological variation of the fine-structure constant. Bulk shifts are Δv = −0.048 ± 0.028 km s−1 toward PKS 1830-211 (NE) and −0.13 ± 0.14 km s−1 toward B 0218+357 (SW), with tightly correlated CH–H2O opacities across many narrow components. Using independent limits on Δμ/μ for these systems, the shifts imply 3σ bounds |Δα/α| < 0.55 ppm and 1.5 ppm at look-back times ≈ half the age of the Universe. These single-system limits are 2–4× tighter than previous high-z constraints, enabled by the simultaneous setup that minimizes calibration, variability, and chemical-segregation biases.

Key figures to inspect

• Fig. 1 (CH level diagram): Identify which Λ-doublet and hyperfine components fall in the ALMA setup and their relative strengths—crucial for understanding the later hyperfine deconvolution and line-weighting in velocity comparisons.
• Fig. 2 (Multi-epoch spectra): Compare CH and H2O profiles for PKS 1830-211 NE/SW and B 0218+357 SW across observing dates; verify the stability of component centroids despite intrinsic variability, the alignment of CH–H2O features, and the role of the H2^18O spectrum in checking opacity scaling.
• Fig. 3 (Opacity overlays and common fit): Inspect the hyperfine-deconvolved CH profile overlaid with H2O scaled by the opacity ratio; the common fit visualizes the small bulk offset, the two-dex span in optical depths, and the isolated PKS 1830-211(SW) component used as a clean test case.