Hubble flow

Hubble's law is a statement in physical cosmology which states that the redshift in light coming from distant galaxies is proportional to their distance. The law was first formulated by Edwin Hubble and Milton Humason in 1929 after nearly a decade of observations. It is considered the first observational basis for the expanding space paradigm and today serves as one of the most often cited pieces of evidence in support of the Big Bang. The most recent calculation of the proportionality constant, using the satellite WMAP began in 2003, yielding a value of 71 ± 4 (km/s)/Mpc. In August, 2006, a less accurate figure was obtained independently using data from NASA's orbital Chandra X-ray Observatory: 77 (km/s)/Mpc with an uncertainty of ± 15%.
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The expansion of the universe. It was brought about by the big bang which caused space itself to expand. Since everything in the universe exists upon the spacetime continuum, although the celestial objects, such as stars and galaxies, do not expand, the space in between them does. Gravity> can overcome this expansion on scales smaller than that of clusters of galaxies. The clusters, however, are being forever driven apart by the Hubble flow. The further apart two clusters are, the greater the rate at which the distance in between them increases. The distance and recessional velocity are linked by a quantity known as the Hubble constant, which states that for every million parsecs between objects, the space between them grows by 50-100 kilometers every second. This makes it appear as if the objects are receding from one another and causes the electromagnetic radiation they release to be red shifted.