4.2 Formation of the Universe
The Doppler Effect
The Big Bang Theory suggests that the universe is continually expanding. Measurements of the Doppler Effect support this theory. The Doppler Effect is the change in frequency (and wavelength) of a source when it is moving relative to an observer. If the source is moving towards an observer, the wavelength squashes up and the frequency increases. When the object is moving away from the observer, the wavelength spreads out and the frequency decreases. This can be witnessed in everyday life when you hear the siren of an ambulance or police car moving towards you then away from you. The siren sounds higher in pitch (higher frequency) when the vehicle comes towards you and then seems lower in pitch (lower frequency) when it moves away from you.
