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How is the Expansion of the Universe Related to the Doppler Effect?
Have you ever wondered why the pitch of a siren drops while an ambulance passes by? Or wondered how scientists discovered that the universe was expanding? Well, believe it or not, the answers to those two questions are deeply related through something called the Doppler effect.
The Doppler effect can be observed for all types of waves, sound, light, even ocean waves. Both sound and light waves travel at a finite speed, though light is significantly faster and is the fastest thing that can move through space.
Born on November 29, 1803, in Salzburg, Austria, Christian Doppler was the person to first person describe how the relative motion of a source and the observer affects observed frequency of sound and light waves. Accordingly, the phenomenon was given the name “the Doppler effect.”
How it all works
Let’s use an example. Waves emitted by an object traveling toward an observer get compressed, resulting in a higher frequency as the source approaches the observer. In contrast, waves emitted by a source traveling away from an observer get stretched out, resulting in a lower frequency. High frequency waves are blue shifted while low frequency waves are redshifted.
Frequency refers to how many cycles occur in a wave in a given measurement of time. One cycle is one oscillation, one crest and trough.
Most people are familiar with the Doppler effect in terms of sound waves. When an ambulance, police car, firetruck, (or anything with a siren) is moving towards you with its siren blasting, the pitch of the siren sound, which is a measure of the siren’s frequency, is high. After the car passes by, the pitch drops because the frequency is lower, and the siren sound is lower. We can attribute this to the Doppler effect, the shift in the frequency of a sound wave produced by a moving source.
When the sound is moving towards you, the waves are compressed, creating a higher frequency and therefore a higher pitch. As the sound passes you and is now moving away from you, the waves are stretched out, creating a lower frequency and therefore a lower pitch. Once you know why this happens, it’s even cooler to hear it in real life.
