The Hubble Tension

The programme that measures the Hubble constant from the local distance ladder — anchoring Cepheids with parallax, using them to calibrate Type Ia supernovae, and reading the expansion rate from supernovae in the smooth Hubble flow. Its value comes out consistently higher than the one inferred from the early universe, and that gap is the Hubble tension.

A proposed but unconfirmed resolution of the Hubble tension in which a small, transient dark-energy-like component acts in the early universe around the time of recombination, shrinking the sound horizon and so raising the Hubble constant inferred from the CMB toward the local value. It is one of several competing ideas — alongside extra relativistic species and modified gravity — and none is yet established.