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We present an improved determination of the Hubble constant (H0) from HubbleSpace Telescope (HST) observations of 70 long-period Cepheids in the LargeMagellanic Cloud. These were obtained with the same WFC3 photometric systemused to measure Cepheids in the hosts of Type Ia supernovae. Gyroscopic controlof HST was employed to reduce overheads while collecting a large sample ofwidely-separated Cepheids. The Cepheid Period-Luminosity relation provides azeropoint-free link with 0.4% precision between the new 1.2% geometric distanceto the LMC from Detached Eclipsing Binaries (DEBs) measured by Pietrzynski etal (2019) and the luminosity of SNe Ia. Measurements and analysis of the LMCCepheids were completed prior to knowledge of the new LMC distance. Combinedwith 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 thefull uncertainty in the LMC geometric calibration of the Cepheid distanceladder from 2.5% to 1.3%. Using only the LMC DEBs to calibrate the ladder wefind H0=74.22 +/- 1.82 km/s/Mpc including systematic uncertainties, 3% higherthan before for this particular anchor. Combining the LMC DEBs, masers in NGC4258 and Milky Way parallaxes yields our best estimate: H0 = 74.03 +/- 1.42km/s/Mpc, including systematics, an uncertainty of 1.91%---15% lower than ourbest previous result. Removing any one of these anchors changes H0 by < 0.7%.The difference between H0 measured locally and the value inferred from PlanckCMB+LCDM is 6.6+/-1.5 km/s/Mpc or 4.4 sigma (P=99.999% for Gaussian errors) insignificance, raising the discrepancy beyond a plausible level of chance. Wesummarize independent tests which show this discrepancy is not readilyattributable to an error in any one source or measurement, increasing the oddsthat it results from a cosmological feature beyond LambdaCDM.

astrophysical journal/the astrophysical journal

Large Magellanic Cloud Cepheid Standards Provide a 1% Foundation for the  Determination of the Hubble Constant and Stronger Evidence for Physics Beyond  LambdaCDM