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.

• hubble-tension-viewpoint-systematic-error - Holds that the tension most likely reflects an unidentified systematic error in one or more measurement chains, particularly in the local distance-ladder calibration (e.g., Cepheid period-luminosity relations, supernova standardization, or stellar parallax zero-point corrections), rather than a fundamental problem with the standard cosmological model.
• hubble-tension-viewpoint-new-physics - Holds that the tension is statistically robust enough across independent methods that it likely points to physics beyond Lambda-CDM, such as early dark energy, a non-standard dark sector, or modified expansion history.
• hubble-tension-viewpoint-data-driven-resolution - Holds that ongoing improvements in instrumentation and datasets-including Gaia astrometric releases, the James Webb Space Telescope, and expanded gravitational-wave standard siren measurements-will likely resolve the discrepancy without requiring new physics, by progressively narrowing systematic uncertainties on both sides.
• hubble-tension-viewpoint-model-independent-measurement - Holds that resolving the tension requires prioritizing measurement techniques less dependent on cosmological model assumptions, such as gravitational lensing time delays or gravitational-wave standard sirens, over either traditional method.

• Disagreement over whether Cepheid photometry used in SH0ES distance-ladder calibrations is affected by uncorrected systematic effects, including disputes over JWST-based reanalyses of Cepheid crowding corrections. See hubble-tension-controversy-cepheid-calibration-controversy.

• History: Hubble Tension - History
• Debate: hubble-tension-debate-cause
• Viewpoint Debate: hubble-tension-viewpoint-new-physics-viewpoint-debate
• Controversy: hubble-tension-controversy-cepheid-calibration-controversy
• Related Main Topics: Standard Cosmological Model, Gaia Mission, Stellar Parallax, Dark Energy

1. Riess, Adam G., et al. “A Comprehensive Measurement of the Local Value of the Hubble Constant with 1 km/s/Mpc Uncertainty from the Hubble Space Telescope and the SH0ES Team.” *Astrophysical Journal Letters* 934, no. 1 (2022): L7.
2. Planck Collaboration. “Planck 2018 Results. VI. Cosmological Parameters.” *Astronomy & Astrophysics* 641 (2020): A6.
3. Verde, Licia, Tommaso Treu, and Adam G. Riess. “Tensions between the Early and Late Universe.” *Nature Astronomy* 3 (2019): 891-895.
4. Freedman, Wendy L., et al. “Calibration of the Tip of the Red Giant Branch.” *Astrophysical Journal* 891, no. 1 (2020): 57.
5. Di Valentino, Eleonora, et al. “In the Realm of the Hubble Tension-A Review of Solutions.” *Classical and Quantum Gravity* 38, no. 15 (2021): 153001.