Redshift is the phenomenon that the frequency of light when observed, under certain circumstances, can be lower than the frequency of light when it was emitted at the source. This usually occurs when the source moves away from the observer, as in the Doppler effect. More specifically, the term redshift is used for the observation that the spectrum of light emitted by distant galaxies is shifted to lower frequencies (towards the red end of the spectrum, hence the name) when compared to the spectrum of closer stars. This is taken as evidence that galaxies are moving away from each other, that the universe is expanding and that it started in a Big Bang.
In general, redshift (and blueshift, the observation of higher frequency light than emitted) is quantified by
1. Movement of the source. If the source of the light is moving away from the observer, then redshift (z > 0) occurs; if the source moves towards the observer, then blueshift (z < 0) occurs. This is true for all waves and is explained by the Doppler effect. If the source moves away from the observer with velocity v and this velocity is much smaller than the speed of light c, then the redshift is approximately given by
3. Gravitational effects. The theory of general relativity holds that light moving through strong gravitational fields experiences a red- or blueshift. This is known as the Einstein shift. The effect is very small but measurable on Earth using the Mossbauer effect. However it is significant near a black hole and as an object approaches the event horizon, the red shift becomes infinite. Gravitational redshift was offered as an explanation of the redshift of quasars in the 1960s, although this is not widely accepted now.
The redshift observed in astronomy can be measured because the emission and absorption spectra for atoms are distinctive and well known. When analyzing light from distant galaxies, one observes absorption and emission features which appear shifted to lower frequencies. Furthermore, farther away objects generally exhibit larger redshifts. Since the redshift of light can be measured easily and precisely, astronomers often use it to indicate both distance and age of observed events (since both are much harder to determine precisely). The largest observed redshifts, corresponding to the biggest distances and largest ages, are those of the cosmic microwave background radiation; their numerical values are about z = 1100.
For galaxies more distant than the Local Group, but within a thousand megaparsecs or so, the redshift is proportional to the galaxy's distance, a fact discovered by Edwin Hubble and known as Hubble's law. Since the redshift is thought to be a result of the movement of the source (or expansion of space), this means that the farther away a galaxy is from us, the faster it moves away from us.
For more distant galaxies, the relationship between current distance and observed redshift becomes more complex. When one sees a distant galaxy, one is seeing the galaxy as it was sometime in the past, when its receding velocity was different from what it is now, and the influences on the velocity of the galaxy from gravitational deceleration and perhaps the cosmological constant become significant. However if one assumes that the expansion of the universe is uniform then there is still a linear relationship between current distance and current receding speed, also known as Hubble's law. This assumption appears reasonable since it follows from the Copernican principle that there are no special places in the universe.
This picture of the expanding universe, if extrapolated back in time, yields a "singularity", a point in time when all distances in the universe were zero. This describes the Big Bang theory. It is believed that a yet unknown theory of quantum gravity would take over before the distances become zero. One recent and perplexing observation is that the expansion of the universe appears to be accelerating, see accelerating universe.