You're probably familiar with the sound of a siren (or a car horn, or a rumbling semi truck) as it approaches and then passes you. The sound starts low and quiet, then begins to rise in pitch as it gets closer, then gradually drops in pitch as it passes and moves down the road. This change in pitch is known as the Doppler effect, and it may surprise you to learn that it's the same phenomenon that tells astronomers our universe is expanding.
The Doppler Effect
Watch a demonstration of how this effect works.
How We Know The Universe Is Expanding
Hear about how redshift led to this discovery.
Written by Ashley Hamer October 16, 2016
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