Hubble tension

Hubble tension is the persistent mismatch between the present expansion rate inferred from the early universe and rates measured using nearby cosmic distance indicators.

Hubble constant: The current rate at which the universe’s expansion increases distance between sufficiently far-apart galaxies. It is the shared quantity whose estimates disagree.
Planck (spacecraft): A European Space Agency observatory that mapped the cosmic microwave background from 2009 to 2013. Its observations produced a precise early-universe estimate under Lambda-CDM.
Systematic error: A consistent measurement bias that shifts results away from the true value. Unrecognized systematics could make either measurement route appear discrepant.
Cosmological parameter: A numerical quantity specifying properties of a cosmological model, such as matter density or expansion rate. The Hubble constant is a key parameter whose uncertainty propagates through cosmological inference.
Cosmic microwave background: The relic radiation released when the early universe became transparent, about 380,000 years after the Big Bang. Its measured pattern anchors early-universe estimates of the expansion rate.
SH0ES: An observational program that measures the Hubble constant using Cepheids and Type Ia supernovae. Its local distance-ladder result is substantially higher than Planck’s model-based estimate.
Early dark energy: A proposed component that briefly contributes significant energy density before recombination. It could alter the sound horizon and raise the inferred Hubble constant.
Age of the universe: The time elapsed since the universe began expanding from its hot, dense early state. Different expansion histories imply different estimates of the universe’s age.
Lambda-CDM model: The standard cosmological model, combining a cosmological constant with cold dark matter and ordinary matter. Extrapolating this model from the cosmic microwave background predicts a lower Hubble constant.
Tip of the red-giant branch: A stellar distance indicator based on the nearly uniform maximum brightness of red-giant stars. It provides an alternative calibration that has tested Cepheid-based distance estimates.

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