Hubble's law

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Hubble's law describes the observation in physical cosmology that the velocity at which various galaxies are receding from the Earth is proportional to their distance from us. The law was first derived from the General Relativity equations by Georges Lemaître in 1927. Edwin Hubble derived it empirically in 1929[3] after nearly a decade of observations. The recession velocity of the objects was inferred from their redshifts, many measured earlier by Vesto Slipher (1917) and related to velocity by him. It is considered the first observational basis for the expanding space paradigm and today serves as one of the pieces of evidence most often cited in support of the Big Bang model.

The law is often expressed by the equation v = H0D, with H0 the constant of proportionality (the Hubble constant) between the distance D to a galaxy and its velocity v. The SI unit of H0 is s-1 but it is most frequently quoted in (km/s)/Mpc, thus giving the speed in km/s of a galaxy one Megaparsec away. The reciprocal of H0 is the Hubble time.

The most recent observational determination of the proportionality constant obtained in 2010 based upon measurements of gravitational lensing by using the Hubble Space Telescope (HST) yielded a value of H0 = 70.6 ± 3.1 (km/sec)/Mpc. In 2009 also using the Hubble Space Telescope the measure was 74.2 ± 3.6 (km/s)/Mpc. The results agree closely with an earlier measurement of H0 = 72 ± 8 km/s/Mpc obtained in 2001 also by the HST. In August 2006, a less-precise figure was obtained independently using data from NASA's Chandra X-ray Observatory: H0 = 77 (km/s)/Mpc or about 2.5×10−18 s−1 with an uncertainty of ± 15%. NASA summarizes existing data to indicate a constant of 70.8 ± 1.6 (km/s)/Mpc if space is assumed to be flat, or 70.8 ± 4.0 (km/s)/Mpc otherwise.