Strong limit on a variable proton-toelectron mass ratio from molecules

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 |Dm/m | < 1.8 × 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. The Standard Model of particle physicsassumes that the fundamental constantsof nature (or, at least, their low-energy limits) are the same everywhere and at every epoch in the universe. However, it cannot itself justify this assumption, nor can it predict their values. Our confidence in their constancy stem