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“Megamasers, the Hubble Constant, and Dark Energy” Dr. Jim 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. I will 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. I will highlight recent results, including observations of UGC 3789, NGC 6323, and Mrk 1419, that demonstrate the technique is viable out to at least 100 Mpc. |