Ammonia as a Tracer of Fundamental Constants
  Observing inversion lines of ammonia (NH3), complemented by rotational lines
of NH3 and other molecular species, provides stringent constraints on potential
variations of the proton-to-electron mass ratio. While a limit of one part per
million is derived for a lookback time of 7 billion years, nearby dark clouds
might show a statistically significant variation of order 20-30 parts per
billion, possibly being related to chameleon fields. The detection of
radio-loud quasars with strong molecular absorption lines at redshifts z > 1 as
well as the identification of a larger sample of nearby dark clouds with
exceptionally narrow lines (<0.2 km/s) would be essential to improve present
limits and to put the acquired results onto a firmer statistical basis.
