A Comprehensive Measurement of the Local Value of the Hubble Constant with 1 km/s/Mpc Uncertainty from the Hubble Space Telescope and the SH0ES Team

TL;DR

The paper presents a high-precision local determination of the Hubble constant H0 with an uncertainty of 1 km s−1 Mpc−1, using HST observations of Cepheids in the hosts of 42 SNe Ia to calibrate the SN Ia distance scale. Cepheids are geometrically anchored to Gaia EDR3 parallaxes, megamasers in NGC 4258, and detached eclipsing binaries in the LMC, and measured with uniform WFC3 photometry to minimize zeropoint errors. The SN Ia calibration leverages the Pantheon+ compilation with full covariance, and ~70 analysis variants test sensitivity to anchors, SN surveys, dust and metallicity treatments, and sample selections. The baseline result is H0 = 73.04 ± 1.04 km s−1 Mpc−1, with cross-checks yielding H0 = 72.53 ± 0.99 when TRGB consistency is included, and H0 = 73.30 ± 1.04 when high-z SN Ia data are added; the analysis finds a 5-sigma tension with Planck+LCDM, with the discrepancy not explained by measurement errors or the tested systematics. This reinforces the persistence of the H0 tension and highlights the need for new physics or previously unknown systematics.

Abstract

We report observations from HST of Cepheids in the hosts of 42 SNe Ia used to calibrate the Hubble constant (H0). These include all suitable SNe Ia in the last 40 years at z<0.01, measured with >1000 orbits, more than doubling the sample whose size limits the precision of H0. The Cepheids are calibrated geometrically from Gaia EDR3 parallaxes, masers in N4258 (here tripling that Cepheid sample), and DEBs in the LMC. The Cepheids were measured with the same WFC3 instrument and filters (F555W, F814W, F160W) to negate zeropoint errors. We present multiple verifications of Cepheid photometry and tests of background determinations that show measurements are accurate in the presence of crowding. The SNe calibrate the mag-z relation from the new Pantheon+ compilation, accounting here for covariance between all SN data, with host properties and SN surveys matched to negate differences. We decrease the uncertainty in H0 to 1 km/s/Mpc with systematics. We present a comprehensive set of ~70 analysis variants to explore the sensitivity of H0 to selections of anchors, SN surveys, z range, variations in the analysis of dust, metallicity, form of the P-L relation, SN color, flows, sample bifurcations, and simultaneous measurement of H(z). Our baseline result from the Cepheid-SN sample is H0=73.04+-1.04 km/s/Mpc, which includes systematics and lies near the median of all analysis variants. We demonstrate consistency with measures from HST of the TRGB between SN hosts and NGC 4258 with Cepheids and together these yield 72.53+-0.99. Including high-z SN Ia we find H0=73.30+-1.04 with q0=-0.51+-0.024. We find a 5-sigma difference with H0 predicted by Planck+LCDM, with no indication this arises from measurement errors or analysis variations considered to date. The source of this now long-standing discrepancy between direct and cosmological routes to determining the Hubble constant remains unknown.

Figures (2)

• Figure 1: Sources of data for distance ladder. Red block shows data from this work.
• Figure 2: HST observations of Cepheids in 37 hosts of 42 "ideal" SNe Ia and NGC$\,$4258, collected over 20 yr with 4 cameras and $>1000$ orbits of HST time. In most cases, 60 to 90-day campaigns in F555W and F814W or in F350LP were used to identify Cepheids from their light curves with occasional observations years later to identify longer-period Cepheids. NIR follow-up observations in F160W are used to reduce the effects of host-galaxy extinction, sensitivity to metallicity, and breaks in the $P$--$L$ relation. Data sources: (1) HST Key Project, freedman01; (2) HST SN Ia Luminosity Calibration Program, sandage06; (3) riess05; (4) macri06; (5) Mager:2013; (6) Yuan:2021_N4258.