ABSTRACT

The expansion rate and its evolution must be empirically determined
for our Universe to reveal its composition, scale, age, and fate. The Hubble Space Telescope
is unique in its ability to measure the keystones of cosmic expansion, distant type Ia
supernovae and Cepheid variables in their hosts. In 1998, high-redshift SNe Ia provided
the first and only direct evidencefor an accelerating Universe and the existence of dark energy.
More recently, ACS and NICMOS on HST have become tools to calibrate the Hubble diagram of
SNe Ia with modern data.

We report results from a program to determine the Hubble constant to 5% precision from a
refurbished distance ladder based on extensive use of differential measurements. We show that
the factor of 2.2 improvement in the precision of H_0 is a significant aid to the
determination of the equation-of-state parameter of dark energy, w = P/(rho c^2).
We show that future improvements in the measurement of H_0 are likely with the new
WFC3 instrument on Hubble and should further contribute to multi-technique studies of dark energy.