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Hubble's law is the name for the astronomical observation in physical cosmology first made by Edwin Hubble, that: (1) all objects observed in deep space (interstellar space) are found to have a doppler shift observable relative velocity to Earth, and to each other; and (2) that this dopplershift measured velocity, of various galaxies receding from the Earth is proportional to their distance from the Earth and all other interstellar bodies. In effect, the spacetime volume of the observable universe is expanding (from a smaller past to a larger future); and Hubble's Law is the direct physical observation of this process, as it unfolds.^{[1]} The law was first derived from the General Relativity equations by Georges Lemaître in 1927.^{[2]} 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.^{[4]} 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 = H_{0}D, with H_{0} the constant of proportionality (the Hubble constant) between the "proper distance" D to a galaxy (which can change over time, unlike the comoving distance) and its velocity v (i.e. the derivative of proper distance with respect to cosmological time coordinate; see Comoving distance#Uses of the proper distance for some discussion of the subtleties of this definition of 'velocity'). The SI unit of H_{0} 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 H_{0} is the Hubble time.
The most recent observational determination of the proportionality constant obtained in 2010^{[5]} based upon measurements of gravitational lensing by using the Hubble Space Telescope (HST) yielded a value of H_{0} = 70.6 ± 3.1 (km/s)/Mpc. WMAP sevenyear results, also from 2010, gave an estimate of H_{0} = 71.0 ± 2.5 (km/s)/Mpc based on WMAP data alone, and an estimate of H_{0} = 70.4 +1.3
−1.4 (km/s)/Mpc based on WMAP data with Gaussian priors based on earlier estimates from other studies.^{[6]} In 2009 also using the Hubble Space Telescope the measure was 74.2 ± 3.6 (km/s)/Mpc.^{[7]} The results agree closely with an earlier measurement, based on observations by the HST of Cepheid variable stars, of H_{0} = 72 ± 8 km/s/Mpc obtained in 2001.^{[8]} In August 2006, a lessprecise figure was obtained independently using data from NASA's Chandra Xray Observatory: H_{0} = 77 (km/s)/Mpc or about 2.5×10^{−18} s^{−1} with an uncertainty of ± 15%.^{[9]} NASA's WMAP site 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,^{[10]} although these estimates have been on the site since January 2007^{[11]} and may not take into account the more recent studies discussed above.
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