Braatz

The Megamaser Cosmology Project
James Braatz (NRAO)

As a complement to observations of the Cosmic Microwave Background, a precise measurement of the Hubble constant would provide a valuable constraint on the equation of state of dark energy. Currently, the best measurements of the Hubble constant are based on standard candle observations and are limited by systematic uncertainties at about the 10% level. Here we describe an ambitious project that aims to determine the Hubble constant with a total uncertainty of 3% by measuring direct, geometric distances to galaxies in the Hubble flow. The technique, pioneered on NGC 4258, is based on observations of circumnuclear water vapor masers at 22 GHz. Maser disks analogous to the one in NGC 4258, but more distant, have been discovered and are now being studied in detail. We will highlight recent results, including observations of UGC 3789 and NGC 6323, that demonstrate the technique is viable for galaxies well into the Hubble flow. We will also discuss prospects for finding additional maser disks and measuring their distances.