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 (2112.04510v3)

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.

• The paper refines the local Hubble Constant to 73.04 km/s/Mpc with a 1% uncertainty using extensive HST observations and Cepheid calibrations.
• The paper employs a three-step distance ladder, integrating Gaia EDR3 parallaxes, masers, and eclipsing binary data for robust geometric calibration.
• The paper identifies a significant 5σ tension with Planck CMB predictions, suggesting the need to explore new physics or revisit systematic errors.

A Comprehensive Measurement of the Local Value of the Hubble Constant

In the paper titled "A Comprehensive Measurement of the Local Value of the Hubble Constant with 1\% Uncertainty from the Hubble Space Telescope and the SH0ES Team," Riess et al. present an extensive analysis aimed at refining the local determination of the Hubble Constant (H$_0$). This research leverages observations from the Hubble Space Telescope (HST) of Cepheid variables in 42 host galaxies of Type Ia supernovae (SNe Ia), collected over four decades. The paper focuses on providing a high-precision measurement of H$_0$, contributing significantly to the ongoing discourse on its potential discrepancy with values inferred from the Cosmic Microwave Background (CMB) under the ΛCDM cosmological model.

Key Contributions and Methodology

The paper extensively utilizes data obtained from the HST, with careful calibration using the Wide Field Camera 3 (WFC3) photometric system. Cepheid variables serve as primary distance indicators due to their luminosity, periodicity, and well-defined period-luminosity ($P$--$L$) relationship. The analysis employs geometric calibrations of Cepheids via Gaia EDR3 parallaxes, masers in NGC 4258, and detached eclipsing binaries in the Large Magellanic Cloud (LMC).

The authors employ a three-step distance ladder approach to determine H$_0$:

1. Geometric Calibration: Establishing the luminosity of Cepheids through geometric distance measurements, particularly using Cepheids in anchors like NGC 4258 and the LMC.
2. Cepheid and SN Ia Calibration: Utilizing Cepheids in SN Ia host galaxies to standardize their luminosity and calibrate the $P$--$L$ relation.
3. Hubble Flow Measurement: Applying the calibrated luminosities to SNe Ia in the Hubble flow, using their independently measured redshifts to derive H$_0$.

The research carefully addresses systematic errors through robust statistical techniques, employing an extensive covariance matrix account to model uncertainties and correlations adequately. The final results demonstrate a remarkable internal consistency within the analyzed data, with a global fit using a $\chi^2$ statistical approach.

Findings and Implications

The paper presents a baseline value of the Hubble Constant H$_0$ = 73.04 km s$^{-1}$ Mpc$^{-1}$, boasting an uncertainty inclusive of systematics at the 1\% level. Importantly, this value highlights a significant discrepancy -- approximately a 5σ difference -- with the value predicted by the Planck CMB observations when considered under the ΛCDM framework. This persistent "Hubble tension" alludes to potential new physics or unaccounted-for systematics either in the local measurements or in the underlying assumptions of the cosmological models.

Future Directions

The research underscores the critical need for further investigations to resolve the Hubble tension. Future work could involve increasingly precise methods, such as observations from the James Webb Space Telescope (JWST), along with cross-validation through different astronomical distance indicators beyond Cepheids and SNe Ia. Moreover, explorations into alternative cosmological models that could reconcile differences between early and late Universe measurements offer fertile ground for theoretical advancements.

In summary, Riess et al.'s paper is a noteworthy contribution to precision cosmology, offering an incrementally refined measurement of the local Hubble Constant. The commitment to methodological rigor and the broad implications of the findings ensure that this paper will remain a pivotal point of reference in the narrative of the Hubble tension.