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 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.

We report observations from the Hubble Space Telescope (HST) of Cepheid variables in the host galaxies of 42 Type Ia supernovae (SNe Ia) used to calibrate the Hubble constant (H 0). These include the complete sample of all suitable SNe Ia discovered in the last four decades at redshift z ≤ 0.01, collected and calibrated from ≥1000 HST orbits, more than doubling the sample whose size limits the precision of the direct determination of H 0. The Cepheids are calibrated geometrically from Gaia EDR3 parallaxes, masers in NGC 4258 (here tripling that sample of Cepheids), and detached eclipsing binaries in the Large Magellanic Cloud. All Cepheids in these anchors and SN Ia hosts were measured with the same instrument (WFC3) and filters (F555W, F814W, F160W) to negate zero-point errors. We present multiple verifications of Cepheid photometry and six tests of background determinations that show Cepheid measurements are accurate in the presence of crowded backgrounds. The SNe Ia in these hosts calibrate the magnitude–redshift relation from the revised Pantheon+ compilation, accounting here for covariance between all SN data and with host properties and SN surveys matched throughout to negate systematics. We decrease the uncertainty in the local determination of H 0 to 1 km s−1 Mpc−1 including systematics. We present results for a comprehensive set of nearly 70 analysis variants to explore the sensitivity of H 0 to selections of anchors, SN surveys, redshift ranges, the treatment of Cepheid dust, metallicity, form of the period–luminosity relation, SN color, peculiar-velocity corrections, sample bifurcations, and simultaneous measurement of the expansion history. Our baseline result from the Cepheid–SN Ia sample is H 0 = 73.04 ± 1.04 km s−1 Mpc−1, which includes systematic uncertainties and lies near the median of all analysis variants. We demonstrate consistency with measures from HST of the TRGB between SN Ia hosts and NGC 4258, and include them simultaneously to yield 72.53 ± 0.99 km s−1 Mpc−1. The inclusion of high-redshift SNe Ia yields H 0 = 73.30 ± 1.04 km s−1 Mpc−1 and q 0 = −0.51 ± 0.024. We find a 5σ difference with the prediction of H 0 from Planck cosmic microwave background observations under ΛCDM, with no indication that the discrepancy arises from measurement uncertainties or analysis variations considered to date. The source of this now long-standing discrepancy between direct and cosmological routes to determining H 0 remains unknown.