Two Routes to the Expansion Rate: The Hubble Tension Near 5σ
A local distance-ladder measurement and the value inferred from the cosmic microwave background under ΛCDM disagree by about 4.9σ for one widely quoted comparison. JWST has tested an important Cepheid-crowding systematic, while DESI finds a dataset-dependent preference for evolving dark energy that does not by itself resolve the Hubble tension. The remaining possibilities include unrecognised systematics and physics beyond the standard cosmological model.