Why does the Hubble tension persist?

Different methods of measuring the present-day expansion rate of the universe (the Hubble constant) yield values that disagree at the 5σ level. Early-universe inferences from the CMB give ~67 km/s/Mpc; late-universe ladder measurements give ~73 km/s/Mpc.

Whether this is a systematic error in one of the methods or a sign of new physics is the leading question in cosmology today.