Strong Limit on a Variable Proton-to-Electron Mass Ratio from Molecules in the Distant Universe

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Science  20 Jun 2008:
Vol. 320, Issue 5883, pp. 1611-1613
DOI: 10.1126/science.1156352

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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, μ, over cosmological time and distance scales. The inversion transitions of ammonia are particularly sensitive to μ 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 μ with this technique, limiting relative deviations from the laboratory value to |Δμ/μ| < 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.

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