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

@article{Murphy2008StrongLO,
  title={Strong Limit on a Variable Proton-to-Electron Mass Ratio from Molecules in the Distant Universe},
  author={Michael T. Murphy and Victor V. Flambaum and S{\'e}bastien Muller and Christian Henkel},
  journal={Science},
  year={2008},
  volume={320},
  pages={1611 - 1613}
}
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… 
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