Measurements of the Hubble Constant: Tensions in Perspective
  Measurement of the distances to nearby galaxies have 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 (EDR3) 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 distant sample of Type Ia supernovae from the Carnegie
Supernova Project results in a value of the Hubble constant of Ho = 69.8 $\pm$
0.6 (stat) $\pm$ 1.6 (sys) km/s/Mpc. No statistically significant difference is
found between the value of Ho based on the TRGB and that determined from
measurements of the cosmic microwave background. The TRGB results are also
consistent to within 2$\sigma$ with the SHoES and Spitzer plus HST Key Project
Cepheid calibrations. The TRGB results alone do not demand additional new
physics beyond the standard Lambda-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 JWST
observations are discussed. Resolving this discrepancy is essential for
ascertaining if the claimed tension in Ho between the locally-measured and the
CMB-inferred value is physically motivated.
