Redshift occurs when an object emitting electromagnetic radiation recedes from an observer. The light detected appears "redder" than it should be.
An object (often referred to as the source) will emit electromagnetic radiation of a specific wavelength. But as the source moves away from the observer the wavelength appears to increase (since each peak is emitted further away from the previous peak as the object recedes). Similarly, while the wavelength increases (gets redder) the frequency, and therefore the energy, decreases.
The faster that an object recedes, the greater the redshift. This phenomenon is due to the doppler effect.
Some of the most common applications of the doppler effect (both redshift and blueshift) are police radar guns, as well as doppler weather radar.
The Expansion of the Universe
In the early 1900s it was believed that the entire Universe was encased in our galaxy, the Milky Way. But measurements made of other galaxies, then believed to be simply nebulae in our own galaxy, showed that these objects were, in fact, outside of the Milky Way.
Furthermore, redshifts (and in some cases blueshifts) were measured for these galaxies as well as their distances. The observer, Edwin Hubble, made the startling discovery that the further away a galaxy was the greater its redshift.
This correlation is now known as Hubble's Law, and defines the expansion of the Universe. Specifically, that the further away objects are from us, the faster they are receding. (This is true in the broad sense, there are local galaxies, for instance, that are moving towards us due to the motion of our "local group".)
Other Uses in Astronomy
- Motion of the Milky Way: By measuring the doppler shift objects in the Milky Way we can determine how they are moving relative to us. Doing this on a large scale gives us a sense of how the entire galaxy is moving around the galactic center.
- High Redshift Galaxies: One of the fields of astronomy garnering the most interest is the study of galaxies (and other objects, such as gamma-ray bursts) at the edge of the known Universe. These objects have a very high redshift, and in effect give us a snapshot of the state of the Universe some 13.7 billion years ago. At these distances, Hubble's Law break's down, as the Universe not only appears to be expanding, but accelerating. The source of this effect is dark energy - an as-yet-unknown part of the Universe that is driving this acceleration at large astronomical distances.
