DISPLACEMENT TO THE RED ONE

The electromagnetic waves, like for example the light, are characterized by his wavelength, that is to say, for the distance that comes up between two consecutive undulatory combs. An any wave, be or not electromagnetic, it can be expressed by a certain wavelength but then it is possible to see affected by multitude of processes that do that the recipient receives it with a different wavelength.
In case of the sound, when it changes the wavelength there falters the tone perceived (sharp or serious character). In case of the light, the changes of wavelength bear modifications in the color.
The displacement to the red one is only a change in the wavelength of the electromagnetic radiation. An electromagnetic wave expressed with a certain wavelength (a certain color) is received by a wavelength (color) different. When this alteration implies a reddening of the tone of the light or, in general, a lengthening of the wavelength, one speaks about displacement towards the red one. Although the displacement towards the red one should be the most popular, it is necessary to insist that also it is possible that there takes place a reduction of the wavelengths of the radiation: in this case one speaks about displacement to the blue.
In case of the electromagnetic radiation in general, and much in particular if it is a question of the light, this change of wavelength can owe to three different physical processes: to which the issuer and the recipient move away between themselves (effect Doppler), to which the issuer is submitted to a gravitational field more intense than the recipient (displacement to the red gravitational one) or to the expansion of the universe (displacement to the red cosmological one). When issuing and receiving they approach, when the recipient experiences a more intense gravitational field or when the universe contracts, then the opposite effect, the displacement takes place to the blue. The displacement to the red one is represented by the letter z. The variable z adopts positive values when it is a question of a displacement to the red one and negatives if it is a question of a displacement to the blue.
Since it has been indicated, the displacement to the red one (or) can have three well differentiated physical causes to the blue. But there is one of them who stands out for his importance and for the frequency with which it appears in physical contexts, and especially in astronomical problems: the effect Doppler.
The change is known as an effect Doppler in the length of a wave as a result of the movement of the issuer with regard to the recipient. We observe this effect numerous times in the daily life. When a car us approaches at big speed, we perceive that the sound of the engine (a wave, after all) is sharper than when it moves away from us. This perception owes to the fact of which when the car approaches, the expressed sonorous waves seem to join and diminishes his length, while the opposite effect takes place when the car removes, situation in which the waves seem to separate, what does that his length increases.
This effect is very important in astrophysics, where he acquires relevancy applied to the electromagnetic waves, especially to the case of the light. When an object that expresses light, like a star or a galaxy, approaches us (or we to the object), we see his compressed light waves, with minor wavelength than the correspondent to the emission: the color moves towards the blue. If the issuing body moves away from us (or we of the issuing body), then we see that his light moves the red one, his waves get longer. The effect returns more intense major all that is the relative speed between the issuer and the recipient, which allows to use the effect Doppler to calculate the speed of the stars with regard to us.
This effect receives his name of the Austrian physicist Christian Doppler, and it was key in the discovery of the expansion of the universe for Edwin Hubble. Nevertheless, it is necessary to clarify that the displacements to the red one of which so much it is a question in cosmology do not owe to the effect Doppler, but to an independent effect, that of the displacement to the red cosmological one related to the expansion of the universe, and not with the displacement of the galaxies in strict sense in the bosom of the space.

Glossary: "100 basic concepts of Astronomy”