Cosmology and the Hubble Constant: On the Megamaser Cosmology Project   (MCP)
  The Hubble constant Ho describes not only the expansion of local space at redshift z ~ 0, but is also a fundamental parameter determining the evolution of the universe. Recent measurements of Ho anchored on Cepheid observations have reached a precision of several percent. However, this problem is so important that confirmation from several methods is needed to better constrain Ho and, with it, dark energy and the curvature of space. A particularly direct method involves the determination of distances to local galaxies far enough to be part of the Hubble flow through water vapor (H2O) masers orbiting nuclear supermassive black holes. The goal of this article is to describe the relevance of Ho with respect to fundamental cosmological questions and to summarize recent progress of the the `Megamaser Cosmology Project' (MCP) related to the Hubble constant. 
C. Henkel, J. A. Braatz, M. J. Reid, J. J. Condon, K. Y. Lo, C. M. V.   Impellizzeri, C. Y. Kuo
astro-ph.CO