Measurements of the Hubble Constant: Tensions in Perspective*

Measurement of the distances to nearby galaxies has improved rapidly in recent decades. The ever-present challenge is to reduce systematic effects, especially as greater distances are probed and the uncertainties become larger. In this paper, we combine several recent calibrations of the tip of the red giant branch (TRGB) method. These calibrations are internally self-consistent at the 1% level. New Gaia Early Data Release 3 data provide an additional consistency check at a (lower) 5% level of accuracy, a result of the well-documented Gaia angular covariance bias. The updated TRGB calibration applied to a sample of Type Ia supernovae from the Carnegie Supernova Project results in a value of the Hubble constant of H (0) = 69.8 +/- 0.6 (stat) +/- 1.6 (sys) km s(-1) Mpc(-1). No statistically significant difference is found between the value of H (0) based on the TRGB and that determined from the cosmic microwave background. The TRGB results are also consistent to within 2 sigma with the SHoES and Spitzer plus Hubble Space Telescope (HST) Key Project Cepheid calibrations. The TRGB results alone do not demand additional new physics beyond the standard (?CDM) cosmological model. They have the advantage of simplicity of the underlying physics (the core He flash) and small systematic uncertainties (from extinction, metallicity, and crowding). Finally, the strengths and weaknesses of both the TRGB and Cepheids are reviewed, and prospects for addressing the current discrepancy with future Gaia, HST, and James Webb Space Telescope observations are discussed. Resolving this discrepancy is essential for ascertaining if the claimed tension in H (0) between the locally measured and CMB-inferred values is physically motivated.

Automated summary

Recent advancements in measuring the distances to nearby galaxies have led to improved calibrations of the tip of the red giant branch (TRGB) method, resulting in consistent values for the Hubble constant. The TRGB results show no statistically significant difference with values determined from the cosmic microwave background, and are also in agreement with other calibration methods such as SHoES and Spitzer plus Hubble Space Telescope (HST) Key Project Cepheid calibrations. The simplicity and small systematic uncertainties of the TRGB method make it a promising tool for exploring cosmological models without the need for additional physics beyond the standard (?CDM) cosmological model.