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Redshifts are attributable to the Doppler effect, familiar in the changes in the apparent pitches of sirens and frequency of the sound waves emitted by speeding vehicles ; an observed redshift due to the Doppler effect occurs whenever a light source moves away from an observer.
Cosmological redshift is seen due to the expansion of the universe, and sufficiently distant light sources ( generally more than a few million light years away ) show redshift corresponding to the rate of increase of their distance from Earth.
Finally, gravitational redshifts are a relativistic effect observed in electromagnetic radiation moving out of gravitational fields.
Conversely, a decrease in wavelength is called blueshift and is generally seen when a light-emitting object moves toward an observer or when electromagnetic radiation moves into a gravitational field.

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