Practical Limitations on Astrophysical Observations of Methanol to   Investigate Variations in the Proton-to-Electron Mass Ratio
  The possibility of using astrophysical observations of rotational transitions in the methanol molecule to measure, or constrain temporal and spatial variations in the proton-to-electron mass ratio (mu) has recently been investigated by several groups. Here we outline some of the practical considerations of making such observations, including both the instrumental and astrophysical limitations which exist at present. This leads us to conclude that such observations are unlikely to be able to improve evidence either for, or against the presence of variations in the proton-to-electron mass ratio by more than an order of magnitude beyond current limits. 
Simon Ellingsen, Maxim Voronkov, Shari Breen
astro-ph.CO astro-ph.GA