Simply put, red shift is the shifting of radiation towards the red end of the electromagnetic spectrum by acceleration, gravity, or otherwise.
General Relativity predicts that the wavelength of radiation passing through a gravitational field will be shifted towards redder regions of the spectrum. It is more difficult for a photon to escape a gravitational field than to travel in empty space, but unlike people, photons cannot lose energy by slowing down. As it struggles out of a gravitational field, a photon loses energy by decreasing its frequency, and thus reddening in color. Experiments have measured red shift in the
Accelerational red shift is similar to gravitational red shift (as they should be, since relativity stated the equality of the two). When an object accelerates away from you, the wavelengths get stretched longer and longer, making the radiation redder.
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