A Comprehensive Measurement of the Local Value of the Hubble Constant
  with 1 km/s/Mpc Uncertainty from the Hubble Space Telescope and the SH0ES
  Team
  We report observations from HST of Cepheids in the hosts of 42 SNe Ia used to
calibrate the Hubble constant (H0). These include all suitable SNe Ia in the
last 40 years at z<0.01, measured with >1000 orbits, more than doubling the
sample whose size limits the precision of H0. The Cepheids are calibrated
geometrically from Gaia EDR3 parallaxes, masers in N4258 (here tripling that
Cepheid sample), and DEBs in the LMC. The Cepheids were measured with the same
WFC3 instrument and filters (F555W, F814W, F160W) to negate zeropoint errors.
  We present multiple verifications of Cepheid photometry and tests of
background determinations that show measurements are accurate in the presence
of crowding. The SNe calibrate the mag-z relation from the new Pantheon+
compilation, accounting here for covariance between all SN data, with host
properties and SN surveys matched to negate differences. We decrease the
uncertainty in H0 to 1 km/s/Mpc with systematics. We present a comprehensive
set of ~70 analysis variants to explore the sensitivity of H0 to selections of
anchors, SN surveys, z range, variations in the analysis of dust, metallicity,
form of the P-L relation, SN color, flows, sample bifurcations, and
simultaneous measurement of H(z).
  Our baseline result from the Cepheid-SN sample is H0=73.04+-1.04 km/s/Mpc,
which includes systematics and lies near the median of all analysis variants.
We demonstrate consistency with measures from HST of the TRGB between SN hosts
and NGC 4258 with Cepheids and together these yield 72.53+-0.99. Including
high-z SN Ia we find H0=73.30+-1.04 with q0=-0.51+-0.024. We find a 5-sigma
difference with H0 predicted by Planck+LCDM, with no indication this arises
from measurement errors or analysis variations considered to date. The source
of this now long-standing discrepancy between direct and cosmological routes to
determining the Hubble constant remains unknown.
