The 1% Concordance Hubble Constant
  The determination of the Hubble constant has been a central goal in
observational astrophysics for nearly 100 years. Extraordinary progress has
occurred in recent years on two fronts: the cosmic distance ladder measurements
at low redshift and cosmic microwave background (CMB) measurements at high
redshift. The CMB is used to predict the current expansion rate through a
best-fit cosmological model. Complementary progress has been made with baryon
acoustic oscillation (BAO) measurements at relatively low redshifts. While BAO
data do not independently determine a Hubble constant, they are important for
constraints on possible solutions and checks on cosmic consistency. A precise
determination of the Hubble constant is of great value, but it is more
important to compare the high and low redshift measurements to test our
cosmological model. Significant tension would suggest either uncertainties not
accounted for in the experimental estimates, or the discovery of new physics
beyond the standard model of cosmology. In this paper we examine in detail the
tension between the CMB, BAO, and cosmic distance ladder data sets. We find
that these measurements are consistent within reasonable statistical
expectations, and we combine them to determine a best-fit Hubble constant of
69.6+/-0.7 km/s/Mpc. This value is based upon
WMAP9+SPT+ACT+6dFGS+BOSS/DR11+H_0/Riess; we explore alternate data combinations
in the text. The combined data constrain the Hubble constant to 1%, with no
compelling evidence for new physics.
