| Dark Energy (DE) accounts for 74% of the mass-energy of the universe, dominates its present expansion, and
determines its ultimate fate. Understanding DE may be the most important problem in fundamental physics today.
The best complement to the CMB to constrain the DE equation of state parameter w would be a measurement of
H 0 to better than 3% (Hu, 2005). The best prospect for reaching such accuracy involves measuring direct geometric
distances to circumnuclear masers in galaxies well into the Hubble flow, 50--200 Mpc distant. The Megamaser
Cosmology Project (MCP) aims to find suitable masers and measure their distances using the maser technique
pioneered on NGC 4258. Our successful pilot programs have demonstrated the feasibility of measuring distances to
such faint, distant masers using the GBT and the HSA. The MCP is an ambitious ``Large Project'' that aims to
measure H 0 directly with 3% accuracy within about 5 years. |
