Constraints on changes in the proton–electron mass ratio using methanol lines
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 K-mu to changes in the proton-electron mass ratio mu; a comparison between the line redshifts thus allows us to test for temporal evolution in mu. We obtain a stringent statistical constraint on changes in mu by comparing the redshifted 12.179 and 60.531 GHz lines, [Delta mu/mu] 4 sigma 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 2 sigma constraint [Delta mu/mu] less than or similar to 4 x 10(-7), the most stringent current constraint on changes in mu. We thus find no evidence for changes in the proton-electron mass ratio over a lookback time of approximate to 7.5 Gyr.