We use the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST) to reduce the uncertainty in the local value of the Hubble constant from 3.3% to 2.4%. The bulk of this improvement comes from new near-infrared (NIR) observations of Cepheid variables in 11 host galaxies of recent type Ia supernovae (SNeIa), more than doubling the sample of reliable SNeIa having a Cepheid-calibrated distance to a total of 19; these in turn leverage the magnituderedshift relation based on ∼300 SNeIa at z<0.15. All 19 hosts as well as the megamaser system NGC 4258 have been observed with WFC3 in the optical and NIR, thus nullifying cross-instrument zeropoint errors in the relative distance estimates from Cepheids. Other noteworthy improvements include a 33% reduction in the systematic uncertainty in the maser distance to NGC 4258, a larger sample of Cepheids in the Large Magellanic Cloud (LMC), a more robust distance to the LMC based on late-type detached eclipsing binaries (DEBs), HST observations of Cepheids in M31, and new HST-based trigonometric parallaxes for Milky Way (MW) Cepheids. We consider four geometric distance calibrations of Cepheids: (i) megamasers in NGC 4258, (ii) 8 DEBs in the LMC, (iii) 15 MW Cepheids with parallaxes measured with HST/FGS, HST/WFC3 spatial scanning and/or Hipparcos, and (iv) 2 DEBs in M31. The Hubble constant from each is 72.25±2.51, 72.04±2.67, 76.18±2.37, and 74.50±3.27 km s −1 Mpc −1 , respectively. Our best estimate of H 0 =73.24±1.74 km s −1 Mpc −1 combines the anchors NGC 4258, MW, and LMC, yielding a 2.4% determination (all quoted uncertainties include fully propagated statistical and systematic components). This value is 3.4σ higher than 66.93±0.62 km s −1 Mpc −1 predicted by ΛCDM with 3 neutrino flavors having a mass of 0.06eV and the new Planck data, but the discrepancy reduces to 2.1σ relative to the prediction of 69.3±0.7 km s −1 Mpc −1 based on the comparably precise combination of WMAP+ACT+SPT+BAO observations, suggesting that systematic uncertainties in CMB radiation measurements may play a role in the tension. If we take the conflict between Planck high-redshift measurements and our local determination of H 0 at face value, one plausible explanation could involve an additional source of dark radiation in the early universe in the range of ΔN eff ≈0.4-1. We anticipate further significant improvements in H 0 from upcoming parallax measurements of long-period MW Cepheids.

We present here the Ðnal results of the Hubble Space T elescope (HST ) Key Project to measure the Hubble constant. We summarize our method, the results, and the uncertainties, tabulate our revised distances, and give the implications of these results for cosmology. Our results are based on a Cepheid calibration of several secondary distance methods applied over the range of about 60È400 Mpc. The analysis presented here beneÐts from a number of recent improvements and reÐnements, including (1) a larger LMC Cepheid sample to deÐne the Ðducial period-luminosity (PL) relations, (2) a more recent HST Wide Field and Planetary Camera 2 (WFPC2) photometric calibration, (3) a correction for Cepheid metallicity, and (4) a correction for incompleteness bias in the observed Cepheid PL samples. We adopt a distance modulus to the LMC (relative to which the more distant galaxies are measured) of mag, or 50 kpc. New, revised distances are given for the 18 spiral galaxies for k 0 (LMC) \ 18.50^0.10 which Cepheids have been discovered as part of the Key Project, as well as for 13 additional galaxies with published Cepheid data. The new calibration results in a Cepheid distance to NGC 4258 in better agreement with the maser distance to this galaxy. Based on these revised Cepheid distances, we Ðnd values (in km s~1 Mpc~1) of (random)^6 (systematic) (Type Ia supernovae),

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 Pietrzyński et al. (2019) 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 (Riess et al. 2019), 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 ± 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 ± 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 ΛCDM is 6.6±1.5 km s −1 Mpc −1 or 4.4 σ (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 ΛCDM.

We present an expanded sample of 75 Milky Way Cepheids with Hubble Space Telescope (HST) photometry and Gaia EDR3 parallaxes, which we use to recalibrate the extragalactic distance ladder and refine the determination of the Hubble constant. All HST observations were obtained with the same instrument (WFC3) and filters (F555W, F814W, F160W) used for imaging of extragalactic Cepheids in Type Ia supernova (SN Ia) hosts. The HST observations used the WFC3 spatial scanning mode to mitigate saturation and reduce pixel-to-pixel calibration errors, reaching a mean photometric error of 5 millimags per observation. We use new Gaia EDR3 parallaxes, greatly improved since DR2, and the period–luminosity (P–L) relation of these Cepheids to simultaneously calibrate the extragalactic distance ladder and to refine the determination of the Gaia EDR3 parallax offset. The resulting geometric calibration of Cepheid luminosities has 1.0% precision, better than any alternative geometric anchor. Applied to the calibration of SNe Ia, it results in a measurement of the Hubble constant of 73.0 ± 1.4 km s−1 Mpc−1, in good agreement with conclusions based on earlier Gaia data releases. We also find the slope of the Cepheid P–L relation in the Milky Way, and the metallicity dependence of its zero-point, to be in good agreement with the mean values derived from other galaxies. In combination with the best complementary sources of Cepheid calibration, we reach 1.8% precision and find H 0 = 73.2 ± 1.3 km s−1 Mpc−1, a 4.2σ difference with the prediction from Planck CMB observations under ΛCDM. We expect to reach ∼1.3% precision in the near term from an expanded sample of ∼40 SNe Ia in Cepheid hosts.