Science with the ngVLA: H$_2$O Megamaser Cosmology
  In combination with observations of the Cosmic Microwave Background, a
measurement of the Hubble Constant provides a direct test of the standard
$\Lambda$CDM cosmological model and a powerful constraint on the equation of
state of dark energy. Observations of circumnuclear water vapor megamasers at
22 GHz in nearby active galaxies can be used to measure their distances,
geometrically, and thereby provide a direct, one step measurement of the Hubble
Constant. The measurement is independent of distance ladders and standard
candles. With present-day instrumentation, the Megamaser Cosmology Project is
expected to reach a $\sim$4% measurement using the megamaser technique, based
on distances to fewer than 10 megamaser systems. A goal of the observational
cosmology community is to reach a percent-level measurement of H$_0$ from
several independent astrophysical measurements to minimize the systematics. The
ngVLA will provide the sensitivity at 22 GHz required to discover and measure
the additional megamaser disks that will enable an H$_0$ measurement at the
$\sim$1% level.
