The Hubble tension refers to a persistent discrepancy between measurements of the present-day rate of cosmic expansion, known as the Hubble constant (H₀), obtained through different observational methods. One class of measurements, based on the cosmic microwave background and the standard cosmological model (Lambda-CDM), yields values around 67-68 km/s/Mpc. Another class, based on direct local-universe measurements using a “distance ladder” anchored by Cepheid variables, Type Ia supernovae, and related calibrators, yields values around 73-74 km/s/Mpc. Whether this discrepancy reflects unresolved systematic errors in one or both measurement techniques, or signals new physics beyond the standard cosmological model, is itself a matter of active dispute; see hubble-tension-viewpoint-systematic-error and hubble-tension-viewpoint-new-physics.
Measurements of H₀ generally fall into two broad categories. “Early-universe” methods infer H₀ indirectly from the cosmic microwave background (CMB), most notably data from the Planck satellite, combined with the assumptions of the Lambda-CDM model; these consistently yield values near 67.4 km/s/Mpc. “Late-universe” or “local” methods measure distances and recession velocities of nearby galaxies directly, typically using the cosmic distance ladder-parallax and Cepheid variable stars calibrated in part through Gaia mission astrometry, followed by Type Ia supernovae as standardized candles reaching further into the universe. The SH0ES collaboration, led by Adam Riess, has consistently measured values near 73 km/s/Mpc using this approach. Other independent local methods, including gravitational lensing time delays and the tip of the red giant branch (TRGB) technique, have produced results that sometimes track closer to the early-universe value and sometimes closer to the late-universe value, depending on the analysis and calibration choices used.
The discrepancy between the two principal values is currently reported at a statistical significance of roughly 4 to 6 sigma, depending on which datasets and methods are compared. Proposed explanations span a wide range, including undetected systematic errors in distance-ladder calibration, unrecognized systematics in CMB analysis or its underlying model assumptions, and genuine new physics requiring modification of the standard cosmological model, such as early dark energy, varying dark energy density, or non-standard neutrino properties. No explanation has yet achieved broad acceptance among researchers; see hubble-tension-debate-cause.