How do scientists know the universe is expanding?

Question

How do scientists know the universe is expanding?

In: 3

Answer 1

The first thing that suggested an expanding universe is that everything is moving away from us. We can tell how something very far away is moving relative to us by the color of its light.

Have you heard of the Doppler effect? This is what makes a siren traveling towards you higher in pitch, and a siren traveling away from you lower in pitch.

A similar thing happens to light. Things traveling towards us look more blue, and things traveling away look more red. This is too subtle to see in everyday life, but we can detect it. If the objects are moving *really* fast, we can even see it with the naked eye.

We know what objects like stars should look like. By their size and composition, we know if they should be a specific shade of yellow for example. Finding out their [size](http://spiff.rit.edu/classes/phys440/lectures/size/size.html) and [composition](https://astronomy.com/magazine/ask-astro/2019/06/how-do-scientists-determine-the-chemical-compositions-of-the-planets-and-stars) is another story, but suffice to say we can learn this with careful study. We can also tell their [distance](http://web.mnstate.edu/colson/est/est3a4.html).

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So, knowing the what color we expect stars to be, what do we see when we look up at the sky?

We see that all the stars are redder than they should be. *All* of them. In every direction. What’s more, the further away they are from us, the redder they are. They’re all moving away, and further away ones are moving away faster.

That’s strange. If the universe were static, that’s not at all what we’d expect to see.

It is what we’d expect to see if the universe were expanding away from us. But why us? Are we special?

No, it’s simpler to assume that the universe is expanding everywhere equally. Aliens looking at the sky from their planet would see the same thing. Everything is moving away from everything else, i.e. expanding like the surface of an inflating balloon.

Obviously there’s a lot more evidence than this, but this is ELI5.