Redshift is a term commonly used in astronomy and physics to refer to the phenomenon by which electromagnetic radiation's observed wavelength increases, thereby causing an apparent decrease in the observed frequency. Generally, redshift occurs for one (or both) of two reasons:
1. The distance between the source and the observer of the electromagnetic radiation is increasing, thus causing an apparent increase in wavelength. This is commonly referred to as the Doppler effect. The effect can be easily observed in day-to-day life, causing, among other things, the shift in pitch of a siren when an emergency vehicle passes by.
2. The observer is in a stronger gravitational field than the source, and the wavelength's increase can be attributed to gravitational time dilation as predicted by Albert Einstein in his theory of special relativity.
There is, naturally, an opposite effect observed when the change in wavelength is a decrease, and this is commonly referred to as blueshift.
z is the symbol designating the amount of redshift. The most distant known galaxies and quasars have values of z = 6 or greater.
- Glossary of terms Beyond Einstein, NASA