Hubble Constant

Edwin Hubble's 1929 paper plotted the distances of 24 galaxies against their measured redshifts and found a straight line: the further away, the faster the recession. Lemaître had derived the same relation from general relativity two years earlier in a French-language journal that few read. Hubble's own slope was about 500 km/(s·Mpc), seven times too steep, because the Cepheid calibration of the day conflated two different classes of variable star and his "nebulae" were far more distant than he thought.

The modern problem is not accuracy but disagreement. Planck's measurement of the microwave background, extrapolated forward with the standard ΛCDM model, gives 67.4 ± 0.5 km/(s·Mpc), while the SH0ES team's direct distance ladder of Cepheids and Type Ia supernovae gives 73.0 ± 1.0 — a gap of five sigma or more that has survived a decade of cross-checks, independent methods such as tip-of-the-red-giant-branch distances and gravitational-wave sirens, and JWST re-observation. This "Hubble tension" is either a subtle systematic nobody has found or a sign that ΛCDM is incomplete. Note also that H₀ is not constant in time; the subscript zero means "now".