Redshift | Wikipedia audio article | Wikipedia audio article
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Date: 18/11/2018

This is an audio version of the Wikipedia Article:
Redshift | Wikipedia audio article

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- Socrates



SUMMARY
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In physics, redshift happens when light or other electromagnetic radiation from an object undergoes an increase in wavelength.
"Redshifting" does not mean that the light is actually red, or actually becomes red. The term "red" refers to the fact that in human terms, longer wavelengths are found at the red end of the visible spectrum. Whether or not the light is visible, a "redshift" means an increase in wavelength, equivalent to lower frequency and lower photon energy, in accordance with, respectively, the wave and quantum theories of light. A gamma ray perceived as an X-ray, or initially visible light perceived as radio waves would be typical examples of redshifting in astronomy. The opposite of a redshift is a "blueshift", where light experiences a shortening of wavelength, or increase in energy. Blueshifts are generally seen when a light-emitting object moves toward an observer or when electromagnetic radiation moves into a gravitational field. However, redshift is a more common term and sometimes blueshift is referred to as negative redshift.
There are three main causes of red and blue shifts in astronomy and cosmology: 1) Objects move apart or closer together in space. These shifts are an example of the Doppler effect, familiar in the change of apparent pitches of sirens and frequency of the sound waves emitted by speeding vehicles, or whenever a light source moves away from an observer. 2) Space itself expands, causing objects to become more separated (even if they have not changed their positions in space). This is known as the cosmological redshift. All sufficiently distant light sources (generally more than a few million light years away) show redshift corresponding to the rate of increase in their distance from Earth, known as Hubble's Law. 3) Finally, gravitational redshift is a relativistic effect observed due to strong gravitational fields, which distort spacetime and in effect,  exert a force on light and other particles.
Knowledge of redshifts and blueshifts has been applied to develop several terrestrial technologies such as Doppler radar and radar guns. Redshifts are also seen in the spectroscopic observations of astronomical objects. Its value is represented by the letter z.
A special relativistic redshift formula (and its classical approximation) can be used to calculate the redshift of a nearby object when spacetime is flat. However, in many contexts, such as black holes and Big Bang cosmology, redshifts must be calculated using general relativity. Special relativistic, gravitational, and cosmological redshifts can be understood under the umbrella of frame transformation laws. There exist other physical processes that can lead to a shift in the frequency of electromagnetic radiation, including scattering and optical effects; however, the resulting changes are distinguishable from true redshift and are not generally referred to as such (see section on physical optics and radiative transfer).