Hubble tension

The Hubble constant, usually written H₀, describes the present-day proportionality between the distance to a sufficiently distant galaxy and its recession velocity as part of cosmic expansion. It is expressed in kilometres per second per megaparsec. Its value is important for estimating the expansion history and age of the universe, but measurements that use different data and assumptions need not be direct like-for-like observations.

A local or late-universe approach builds a cosmic distance ladder. Geometric distances calibrate nearby objects such as Cepheid variable stars; Cepheids then help calibrate Type Ia supernovae in more distant galaxies, whose brightnesses can be compared with their redshifts. Analyses of this kind have found values around 70–76 km/s/Mpc, with the precise result depending on the dataset and method. A second approach measures the cosmic microwave background (CMB), relic radiation from the early universe, and infers H₀ by fitting a cosmological model to those data. Under the base ΛCDM model, the Planck 2018 analysis found H₀ = 67.4 ± 0.5 km/s/Mpc. This inferred value is model-dependent, not a direct measurement of the nearby expansion rate.

The difference has persisted as the precision of several measurements improved. Webb observations of Cepheids, used alongside Hubble measurements, supported the reliability of the distance-ladder observations and reduced the likelihood that a particular source of Cepheid measurement error fully explains the discrepancy. This does not by itself establish that the explanation must be new physics: uncertainty in data, calibration, analysis choices and cosmological modelling continues to be scrutinized. If robust and not attributable to unrecognized systematics or assumptions, the mismatch could indicate that the standard cosmological model is incomplete.

Researchers have investigated possibilities that alter the inferred early-universe expansion or the calibration scale, including additional energy or particle components before recombination, changes in the physics of recombination, and modifications to late-time cosmic expansion. These are proposals under test, not established explanations. Other distance indicators, independent observatories and improved calibration can help distinguish among measurement effects and changes to cosmological theory. The tension is therefore a live research problem rather than a settled discovery of a specific new component of the universe.