Large Magellanic Cloud Cepheid Standards Provide a 1% Foundation for the
  Determination of the Hubble Constant and Stronger Evidence for Physics Beyond
  LambdaCDM
  We present an improved determination of the Hubble constant (H0) from Hubble
Space Telescope (HST) observations of 70 long-period Cepheids in the Large
Magellanic Cloud. These were obtained with the same WFC3 photometric system
used to measure Cepheids in the hosts of Type Ia supernovae. Gyroscopic control
of HST was employed to reduce overheads while collecting a large sample of
widely-separated Cepheids. The Cepheid Period-Luminosity relation provides a
zeropoint-free link with 0.4% precision between the new 1.2% geometric distance
to the LMC from Detached Eclipsing Binaries (DEBs) measured by Pietrzynski et
al (2019) and the luminosity of SNe Ia. Measurements and analysis of the LMC
Cepheids were completed prior to knowledge of the new LMC distance. Combined
with a refined calibration of the count-rate linearity of WFC3-IR with 0.1%
precision (Riess et al 2019), these three improved elements together reduce the
full uncertainty in the LMC geometric calibration of the Cepheid distance
ladder from 2.5% to 1.3%. Using only the LMC DEBs to calibrate the ladder we
find H0=74.22 +/- 1.82 km/s/Mpc including systematic uncertainties, 3% higher
than before for this particular anchor. Combining the LMC DEBs, masers in NGC
4258 and Milky Way parallaxes yields our best estimate: H0 = 74.03 +/- 1.42
km/s/Mpc, including systematics, an uncertainty of 1.91%---15% lower than our
best previous result. Removing any one of these anchors changes H0 by < 0.7%.
The difference between H0 measured locally and the value inferred from Planck
CMB+LCDM is 6.6+/-1.5 km/s/Mpc or 4.4 sigma (P=99.999% for Gaussian errors) in
significance, raising the discrepancy beyond a plausible level of chance. We
summarize independent tests which show this discrepancy is not readily
attributable to an error in any one source or measurement, increasing the odds
that it results from a cosmological feature beyond LambdaCDM.
