University of Pennsylvania Physics & Astronomy David Rittenhouse Lecture

The Hubble constant remains one of the most important parameters in the cosmological model, setting the size and age scales of the Universe. Present uncertainties in the cosmological model including the nature of dark energy, the properties of neutrinos and the scale of departures from flat geometry can be constrained by measurements of the Hubble constant made to higher precision than was possible with the first generations of Hubble Telescope instruments. Streamlined distances ladders constructed from infrared observations of Cepheids and type Ia supernovae with ruthless attention paid to systematics now provide 3.5% precision and offer the means to do much better. We will discuss a new round of improvements to the measurement of the Hubble constant including additional observations of Cepheids in recent SN hosts and a new technique, Parallel Astrometric Spatial Scanning (PASS), to measure parallax distances beyond a kiloparsec.