A number that characterizes the expansion rate of the universe and is required to determine its age. In the standard big bang model, the local universe expands uniformly according to the Hubble law, v = H₀d, where v is the velocity of a galaxy at a distance d, and H₀ is the Hubble constant. The wavelength of radiation is stretched due to the expansion of space so that the spectra of objects become progressively redder at greater distances. (For nearby objects, the observed redshift can be described as a Doppler effect.) The constant is named after Edwin P. 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 a further 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.