You're probably familiar with the sound of a siren (or a car horn, or a rumbling semi) as it approaches and then passes you. The sound starts low and quiet, then begins to rise in pitch as it gets closer, then 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 responsible for one of the most astounding scientific discoveries in human history: That the universe is expanding at an accelerating rate.
Related Video: How Fast Is the Universe Expanding?
How the Doppler Effect Works
What Does This Have to Do With the Universe?
Written by Austin Jesse Mitchell March 6, 2019
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