Definition Doppler Effect: The shift in frequency of a wave as a consequence of the relative motion between the source and the observer. Cosmological Redshift: The name given to the observation that the spectra of sources of light in virtually all distant galaxies is shifted toward longer wavelength than the wavelengths in spectra of stationary sources. The Cosmological Redshift is usually interpreted as a Doppler shift in the observed light.

In the animation we view a source of waves (a car), first at rest, then in motion. In front and in back of the stationary car, the wavelength (and the corresponding frequency) is the same. If we now put the car in motion, the speed of the waves is unchanged, but the wavelength in front of the car is shortened and the wavelength behind the car is increased because of the motion of the car. This change in wavelength is called the Doppler Effect.

If the waves were sound waves, you would hear a higher pitch (higher frequency) in front of the approaching car and a lower pitch behind as the car moves away from you.

The Doppler Effect for light is our primary evidence for the motion of stars and galaxies relative to us. When the source moves away from us, the wavelength becomes longer and we call it a "redshift" regardless of the actual color of the light. Since the colors of light from virtually all of the galaxies that we observe with our telescopes is "redshifted", we conclude that the galaxies are all moving away from us. This observation is called the "cosmological redshift" and it provides evidence for an expanding universe.

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