[Paper Review] 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

Adam G. Riess, Wenlong Yuan+16 more|arXiv (Cornell University)|Dec 8, 2021

Gamma-ray bursts and supernovae194 citations

TL;DR

This paper presents a local determination of H0 with 1 km/s/Mpc uncertainty by analyzing Cepheids in 42 SNe Ia hosts using HST, Gaia, and maser/LMC/MW anchors, and explores extensive analysis variants.

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.

Motivation & Objective

• Quantify the local expansion rate H0 with minimized systematics via a Cepheid–SN Ia distance ladder.
• Calibrate Cepheid luminosities with geometric anchors (Gaia parallaxes, NGC 4258 masers, LMC DEBs).
• Cross-calibrate SN Ia absolute magnitudes and the Hubble-flow SN sample to derive H0.
• Assess robustness by evaluating ~70 analysis variants across anchors, SN surveys, redshift ranges, and dust/metallicity treatments.

Proposed method

• Formulate a three-rung distance ladder linking Cepheids in SN Ia hosts to calibrated SN Ia magnitudes.
• Use Wesenheit magnitudes to deredden Cepheids and model Cepheid luminosity as mH^W = μ0 + MH,1^W + bW(log P − 1) + ZW[O/H].
• Simultaneously fit for H0, SN Ia luminosity, and Cepheid standardization parameters via a linear-algebra framework with a covariance-aware χ2 objective.
• Incorporate geometric distance priors (NGC 4258, MW, LMC) and include covariance among SN data and host properties.
• Compare local H0 with high-redshift SN Ia constraints and, optionally, include TRGB anchors to test consistency.

Experimental results

Research questions

• RQ1 What is the local value of the Hubble constant H0 when using a expanded Cepheid–SN Ia distance ladder with extensive anchors?
• RQ2 How do geometric anchors (Gaia, NGC 4258, LMC) and SN systematics affect the H0 uncertainty and central value?
• RQ3 How consistent is the resulting H0 with TRGB-based estimates and Planck ΛCDM predictions?
• RQ4 How robust is H0 to variations in anchor choices, dust/metallicity treatment, P-L relation form, and SN fitting variants?
• RQ5 What is the impact of including high-redshift SNe Ia on the inferred expansion history (q0) and H0?

Key findings

• Baseline H0 from the Cepheid–SN Ia sample is 73.04 ± 1.04 km s−1 Mpc−1 (including systematics).
• Including TRGB anchors yields H0 = 72.53 ± 0.99 km s−1 Mpc−1.
• Including high-redshift SNe Ia yields H0 = 73.30 ± 1.04 km s−1 Mpc−1 and q0 = −0.51 ± 0.024.
• The results show a ~5σ difference with Planck CMB ΛCDM predictions, with no evidence that the discrepancy arises from measurement uncertainties or the analyzed variants.