How do we know the universe is ever expanding?
In: Physics
Doppler effect. When the source of a wave is travelling away from you, the waves stretch out causing a shift down. That’s why the sound of a car horn changes as it drives past – the sound waves are getting longer and therefore the pitch is changing.
The same thing happens with light waves – if the source is travelling away from you, the wavelength of light stretches until it drops below the visible spectrum. If this didn’t happen, then you wouldn’t see individual stars at night – the night sky would be full of stars from all over the universe.
So this guy named Hubble did a whole bunch of measurements of distant stars and galaxies including how far they are form us and how fast they’re moving. Okay, that’s not exactly true, a bunch of the velocity measurements came from a guy named Slipher. When you look at the data he collected you see [this relationship](https://www.pnas.org/content/pnas/112/11/3173/F1.medium.gif). That graph shows that the further something is from us, the faster it’s moving away from us. There’s really no special magic here. It’s just a matter of measuring the data and looking at it.
Also, the guy that found this out? He got a fancy telescope named after him. You’ve probably heard of it. Unfortunately, there’s not also a Slipher telescope.
When a source is moving away from a viewer the light emitted gets a slightly reddish color due to light red light moving slightly slower. If a source is moving toward a viewer, the light gets a bluish tint. We have observed a general redshift in light from other celestial bodies. There is other evidence too in quantum mechanics that has further support for this, but it’s quite complicated and I don’t fully understand it myself.
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