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Strong Limits on a Variable Proton-to-Electron Mass Ratio from Ammonia Inversion Lines

The Standard Model of particle physics assumes that the so-called fundamental constants are universal and unchanging. Absorption lines arising in molecular clouds along quasar sightlines offer a precise test for variations in the proton-to-electron mass ratio over cosmological time and distance scales. The inversion lines of ammonia (NH3) are particularly sensitive to this ratio with respect to rotational transitions. Comparing available inversion and rotational spectra toward the quasars B0218+357 and PKS1830-211, first detailed measurements with this technique are presented, limiting relative deviations from the laboratory value to less than one part per million at more than half the Universe's current age. Constraints to the temperature of the cosmic background radiation at redshift 0.9 will also be given.

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