Extragalactic Water Masers, Geometric Estimation of H_o and   Characterization of Dark Energy
  High precision estimation of the equation of state of dark energy depends on constraints external to analyses of Cosmic Microwave Background fluctuations. A geometric estimation of the local expansion rate, H_o, would provide the most direct and robust constraint. Traditional techniques to estimate H_o have depended on observations of standard candles for which systematic effects can be 10% or more. Observations of water maser sources in the accretion disks that feed the central engines of active galaxies enable simplified, robust, and largely geometric analyses. Many thousand maser sources will be discovered in studies with the SKA, owing to its great sensitivity. Spectroscopic monitoring and interferometric mapping - with intercontinental baselines - will allow estimation of H_o to 1% and possibly better. 
L.J. Greenhill