Strong limit on a variable proton-to-electron mass ratio from molecules in the distant universe

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, m, over cosmological time and distance scales. The inversion transitions of ammonia are particularly sensitive to m as compared to molecular rotational transitions. Comparing the available ammonia spectra observed toward the quasar B0218+357 with new, high- quality rotational spectra, we present the first detailed measurement of m with this technique, limiting relative deviations from the laboratory value to \Delta mu/mu\ < 1.8 x 10(-6) ( 95% confidence level) at approximately half the universe's current age- the strongest astrophysical constraint to date. Higher- quality ammonia observations will reduce both the statistical and systematic uncertainties in these observations.