How do we know the universe is constantly expanding?
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Have you ever heard a train horn or siren get higher-pitched as it approached then lower-pitched as it went by? Well, that’s the Doppler effect.
What’s happening there is that the sound gets compressed, making it sound higher-pitched, as the thing making it moves towards you and stretching the sound, making it sound lower-pitched, as it moves away. [Here’s a cute little animation from Wikipedia.](https://upload.wikimedia.org/wikipedia/commons/9/90/Dopplerfrequenz.gif)
Light works the same way, but it turns bluer when compressed and redder when stretched. This is called redshift. When we look out into space, pretty much everything is redshifting. While I suppose it’s possible this is due to galaxies trying to clear the room after sniffing the cosmic fart that is humanity, it’s more likely that the galaxies are just spreading out to fill extra room.
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