Estimation of the Hubble Constant and Constraint on Descriptions of Dark
  Energy
  Joint analysis of Cosmic Microwave Background, Baryon Acoustic Oscillation,
and supernova data has enabled precision estimation of cosmological parameters.
New programs will push to 1% uncertainty in the dark energy equation of state
and tightened constraint on curvature, requiring close attention to
systematics. Direct 1% measurement of the Hubble constant (H0) would provide a
new constraint. It can be obtained without overlapping systematics directly
from recessional velocities and geometric distance estimates for galaxies via
the mapping of water maser emission that traces the accretion disks of nuclear
black holes. We identify redshifts 0.02<z<0.06 as best for small samples, e.g.,
10 widely distributed galaxies, each with 3% distance uncertainty. Knowledge of
peculiar radial motion is also required. Mapping requires very long baseline
interferometry (VLBI) with the finest angular resolution, sensitivity to
individual lines of a few mJy-km/s, and baselines that can detect a complex of
~10 mJy lines (peak) in < 1 min. For 2010-2020, large ground apertures (50-100m
diameter) augmenting the VLBA are critical, such as EVLA, GBT, Effelsberg, and
the Large Millimeter Telescope, for which we propose a 22 GHz receiver and VLBI
instrumentation. A space-VLBI aperture may be required, thus motivating US
participation in the Japanese VSOP-2 mission (launch c.2013). This will provide
3-4x longer baselines and ~5x improvement in distance uncertainty. There are
now 5 good targets at z>0.02, out of ~100 known masers. A single-dish discovery
survey of >10,000 nuclei (>2500 hours on the GBT) would build a sample of tens
of potential distance anchors. Beyond 2020, a high-frequency SKA could provide
larger maser samples, enabling estimation of H0 from individually less accurate
distances, and possibly without the need for peculiar motion corrections.
