### The Megamaser Cosmology Project: Overview

Dark Energy (DE) accounts for 74% of the mass of the universe, dominates its present expansion and ultimate fate, and retards the formation of large-scale structure. Understanding DE may be the most important problem in all of physics today.

Given the exquisite cosmic microwave background radiation (CMBR) data from redshift z ~ 1100, when DE was a negligible component of the universe, a measurement of the Hubble constant H0 at z ~ 0 to better than 3% would be the best single constraint on the nature of DE (Hu 2005). An accurate H0 would also constrain other critical cosmological parameters, including the geometry (open, closed, or flat) of the universe and the fraction of the critical density contributed by matter.

The best current value of the Hubble Constant, from the HST Key Project (Freedman et al. 2001), is based on Cepheid variables used as standard candles. The measurement is limited by systematic errors at the 10% level. The best prospect for measuring H0 with 3% or better precision involves direct geometric distance measurements to circumnuclear H

_{2}O megamasers in galaxies well into the Hubble flow, at distances 50 -- 200 Mpc. The Megamaser Cosmology Project (MCP) aims to find suitable megamasers and measure their distances using the technique pioneered on the nearby galaxy NGC 4258.

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JimBraatz - 05 Jun 2009