Hubble constant

The present-day rate of cosmic expansion, expressed as recession velocity per unit distance. Its measured value sets the current scale of the universe’s expansion.

Hubble–Lemaître law: The observed proportionality between a galaxy’s recession velocity and its distance at sufficiently large scales. The law uses the Hubble constant as its proportionality factor.
Hubble tension: The persistent discrepancy between early-universe and late-universe determinations of the Hubble constant. This disagreement is the central unresolved issue surrounding its measured value.
Georges Lemaître: Belgian priest, physicist, and astronomer who derived an expanding-universe solution and linked recession to distance. His 1927 work derived the expansion relation before its observational confirmation.
Hubble parameter: The time-dependent fractional expansion rate of the universe, usually written H(t). The Hubble constant is specifically the present-day value of this evolving parameter.
Scale factor (cosmology): A function describing how distances between comoving points in the universe change over time. Its present fractional growth rate defines the Hubble constant.
Type Ia supernova: A thermonuclear explosion of a white dwarf whose standardized brightness makes it useful for measuring cosmic distances. Supernova distances anchor prominent late-universe estimates of the constant.
Edwin Hubble: American astronomer whose observations established that galaxies lie beyond the Milky Way and helped reveal cosmic expansion. His 1929 distance–velocity analysis made the expansion relation widely known.
Deceleration parameter: A dimensionless measure of whether the cosmic expansion rate is slowing or accelerating. It describes how expansion changes, rather than its present rate.
Redshift: An increase in the wavelength of light, commonly caused by a source moving away or by cosmic expansion. Cosmological redshift provides a key observable for estimating recession and expansion.
Cosmic microwave background: Relic radiation released when the early universe became transparent, now observed across the sky at microwave wavelengths. Its measured patterns constrain early-universe models that predict the present expansion rate.

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