  We report the outcome of a 3-day workshop on the Hubble constant (H_0) that
took place during February 6-8 2012 at the Kavli Institute for Particle
Astrophysics and Cosmology, on the campus of Stanford University. The
participants met to address the following questions. Are there compelling
scientific reasons to obtain more precise and more accurate measurements of H_0
than currently available? If there are, how can we achieve this goal? The
answers that emerged from the workshop are (1) better measurements of H_0
provide critical independent constraints on dark energy, spatial curvature of
the Universe, neutrino physics, and validity of general relativity, (2) a
measurement of H_0 to 1% in both precision and accuracy, supported by rigorous
error budgets, is within reach for several methods, and (3) multiple paths to
independent determinations of H_0 are needed in order to access and control
systematics.

Cosmology and Nongalactic Astrophysics