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Questions related to the phenomenon whereby electromagnetic radiation (such as visible light) generated by an object moving away from an observer will have increase in wavelength (i.e. shifted toward the red end of the spectrum) once it reaches the observer.

According to the Wikipedia entry on redshift.

In physics, redshift happens when light or other electromagnetic radiation from an object moving away from the observer is increased in wavelength, or shifted to the red end of the spectrum. In general, whether or not the radiation is within the visible spectrum, "redder" means an increase in wavelength – equivalent to a lower frequency and a lower photon energy, in accordance with, respectively, the wave and quantum theories of light.

Redshift also occurs when light leaves a gravitational potential (gravitational redshift) or due to the expansion of the universe (cosmological redshift). In the latter case, it indicates the change in the scale factor between the lights emission and observation.