We present an improved determination of the Hubble constant 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 extragalactic 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-independent link with 0.4 % precision between the new 1.2 % geometric distance to the LMC from Detached Eclipsing Binaries ( DEBs ) measured by and the luminosity of SNe Ia .
Measurements and analysis of the LMC Cepheids were completed prior to knowledge of the new DEB LMC distance .
Combined with a refined calibration of the count-rate linearity of WFC3-IR with 0.1 % precision ( ) , these three improved elements together reduce the overall uncertainty in the geometric calibration of the Cepheid distance ladder based on the LMC from 2.5 % to 1.3 % .
Using only the LMC DEBs to calibrate the ladder we find H _ { 0 } = 74.22 \pm 1.82 km s ^ { -1 } Mpc ^ { -1 } 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 : H _ { 0 } = 74.03 \pm 1.42 km s ^ { -1 } Mpc ^ { -1 } , including systematics , an uncertainty of 1.91 \% —15 % lower than our best previous result .
Removing any one of these anchors changes H _ { 0 } by less than 0.7 % .
The difference between H _ { 0 } measured locally and the value inferred from Planck CMB and \Lambda CDM is 6.6 \pm 1.5 km s ^ { -1 } Mpc ^ { -1 } 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 attributable to an error in any one source or measurement , increasing the odds that it results from a cosmological feature beyond \Lambda CDM .