Hubble constant

Article By:

Freedman, Wendy L. Observatories of the Carnegie Institution for Science, Pasadena, California.

Last reviewed:August 2022

DOI:https://doi.org/10.1036/1097-8542.324300

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• Hubble constant, published December 2019:Download PDF Get Adobe Acrobat Reader

A number that characterizes the expansion rate of the universe and is required to determine the universe's age and its observable size. In the standard big bang model of cosmology, the local universe expands uniformly according to the Hubble law (also known as the Hubble–Lemaitre law), v = H₀, where v is the velocity of a galaxy at a distance d, and H₀ is the Hubble constant. The wavelengths of radiation (light) are stretched due to the expansion of space so that the spectra of objects become progressively longer, or "redder," toward or past the red end of the visible electromagnetic spectrum at greater distances. (For nearby objects, where the expansion velocities are small, the observed redshift can be described as a Doppler effect.) The constant is named after the U.S. astronomer, Edwin Hubble, who discovered that the velocity of recession of a galaxy is proportional to its distance. A reliable and accurate measurement of the Hubble constant, an independent estimate of the ages of the oldest objects in the universe, and an independent measurement of the average mass-energy density in the universe are all separately required in order to test and ultimately provide strong constraints on cosmological models. Measuring an accurate value of H0 was one of the motivating reasons for building the Hubble Space Telescope, which launched in 1990, and other major observatories since. Observations of certain kinds of stars in nearby galaxies, the explosions of stars as supernovae (see illustration), emissions from the vicinity of supermassive black holes, the relic radiation of the big bang, and gravitational waves are methods that have been used to measure the Hubble constant and will be discussed in this article. The differing methods yield different Hubble constant values for the early, distant universe and the modern, local universe, with values generally ranging from 67 km/s/Mpc (kilometers per second per megaparsec) to 74 km/s/Mpc. Resolving this discrepancy, called the Hubble tension, is an active area of research. See also: Big bang theory; Cosmology; Doppler effect; Hubble Space Telescope; Light; Redshift; Star; Universe

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