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 (CH${}_3$OH) lines in the $z=0.88582$ 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 GHz and 60.531 GHz lines, $[\Delta mu/\mu ]\le 1.1\times 10^{-7}$ ($2\sigma$) over $0<z\le 0.88582$, a factor of $\approx 2.5$ more sensitive than the best earlier results. However, the higher signal-to-noise ratio (by a factor of $\approx 2$) 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 CH${}_3$OH lines (at $>4\sigma$ significance). The sensitivity of the above result, and that of all earlier CH${}_3$OH 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 $[\Delta \mu /\mu ]\lesssim 4\times 10^{-7}$, 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 $\approx 7.5$ Gyrs.

6 pages, 1 figure; accepted for publication in MNRAS Letters

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• Further consistency tests of the stability of fundamental couplings
• [CI], [CII] and CO emission lines as a probe for alpha variations at low and high redshifts
• Methanol absorption in PKS B1830-211 at milliarcsecond scales
• Constraining spacetime variations of nuclear decay rates from light curves of type Ia supernovae
• Testing bound dark energy with cosmological parameter and fundamental constant evolution
• The Relation Between Fundamental Constants and Particle Physics Parameters