Constraints on changes in the proton–electron mass ratio using methanol lines

Abstract

We report Karl G. Jansky Very Large Array (VLA) absorption spectroscopy in four methanol (CH3OH) lines in the z = 0.885 82 gravitational lens towards PKS1830−211. Three of the four lines have very different sensitivity coefficients Kμ to changes in the proton–electron mass ratio μ; a comparison between the line redshifts thus allows us to test for temporal evolution in μ. We obtain a stringent statistical constraint on changes in μ by comparing the redshifted 12.179 and 60.531 GHz lines, [Δμ/μ] ≤ 1.1 × 10−7 (2σ) over 0 < z ≤ 0.885 82, a factor of ≈2.5 more sensitive than the best earlier results. However, the higher signal-to-noise ratio (by a factor of ≈2) of the VLA spectrum in the 12.179 GHz transition also indicates that this line has a different shape from that of the other three CH3OH lines (at >4σ significance). The sensitivity of the above result, and that of all earlier CH3OH studies, is thus likely to be limited by unknown systematic errors, probably arising due to the frequency-dependent structure of PKS1830−211. A robust result is obtained by combining the three lines at similar frequencies, 48.372, 48.377 and 60.531 GHz, whose line profiles are found to be in good agreement. This yields the 2σ constraint [Δμ/μ] ≲ 4 × 10−7, the most stringent current constraint on changes in μ. We thus find no evidence for changes in the proton–electron mass ratio over a lookback time of ≈7.5 Gyr.