Colloquia

Dark energy dominates the expansion of the universe and will determine its ultimate fate. The best complement to cosmic microwave background data for constraining the nature of dark energy is an accurate measurement of the current expansion rate (Hubble constant). The goal of the Megamaser Cosmology Project is to measure the Hubble constant by using the Green Bank Telescope and the Very Long Baseline Array to discover and image 22 GHz water masers orbiting the nuclei of Seyfert galaxies. We can show that these compact nuclei contain supermassive black holes, not just dense clusters of stars, and determine their masses. In the past year we improved our measurement of the angular-size distance to the galaxy UGC 3789, imaged four more masing galaxies, and derived a preliminary estimate for the Hubble constant.