<P> For galaxies more distant than the Local Group and the nearby Virgo Cluster, but within a thousand megaparsecs or so, the redshift is approximately proportional to the galaxy's distance . This correlation was first observed by Edwin Hubble and has come to be known as Hubble's law . Vesto Slipher was the first to discover galactic redshifts, in about the year 1912, while Hubble correlated Slipher's measurements with distances he measured by other means to formulate his Law . In the widely accepted cosmological model based on general relativity, redshift is mainly a result of the expansion of space: this means that the farther away a galaxy is from us, the more the space has expanded in the time since the light left that galaxy, so the more the light has been stretched, the more redshifted the light is, and so the faster it appears to be moving away from us . Hubble's law follows in part from the Copernican principle . Because it is usually not known how luminous objects are, measuring the redshift is easier than more direct distance measurements, so redshift is sometimes in practice converted to a crude distance measurement using Hubble's law . </P> <P> Gravitational interactions of galaxies with each other and clusters cause a significant scatter in the normal plot of the Hubble diagram . The peculiar velocities associated with galaxies superimpose a rough trace of the mass of virialized objects in the Universe . This effect leads to such phenomena as nearby galaxies (such as the Andromeda Galaxy) exhibiting blueshifts as we fall towards a common barycenter, and redshift maps of clusters showing a Fingers of God effect due to the scatter of peculiar velocities in a roughly spherical distribution . This added component gives cosmologists a chance to measure the masses of objects independent of the mass to light ratio (the ratio of a galaxy's mass in solar masses to its brightness in solar luminosities), an important tool for measuring dark matter . </P> <P> The Hubble law's linear relationship between distance and redshift assumes that the rate of expansion of the Universe is constant . However, when the Universe was much younger, the expansion rate, and thus the Hubble "constant", was larger than it is today . For more distant galaxies, then, whose light has been travelling to us for much longer times, the approximation of constant expansion rate fails, and the Hubble law becomes a non-linear integral relationship and dependent on the history of the expansion rate since the emission of the light from the galaxy in question . Observations of the redshift - distance relationship can be used, then, to determine the expansion history of the Universe and thus the matter and energy content . </P> <P> While it was long believed that the expansion rate has been continuously decreasing since the Big Bang, recent observations of the redshift - distance relationship using Type Ia supernovae have suggested that in comparatively recent times the expansion rate of the Universe has begun to accelerate . </P>

When is the sun's light redshifted when viewed from earth