How do scientists know the universe is expanding?
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Answer: have you ever heard a train drive by with its horn going? It sounds different one it passes you, the tone changes. This is the doppler effect.
Well, light does the same thing, only thing is instead of sound you can observe a shift in color, red vs blue shift and know if the object is moving away or closer to you.
When stars move away the light they emit shifts a certain color, when they move closer they shift to a different color. Boom.
TLDR: we can see it expanding
When light moves long distances it gets stretched out or compressed. This is called the Doppler effect, and it’s the same thing that causes an ambulance siren to raise and lower in pitch and is comes closer and then passes you.
For light this causes it to blue-shift or red-shift. Light from things moving towards us turns more blue, and stuff that’s moving away from us more Red.
Edwin Hubble (for whom the Hubble space telescope is named) catalogue distanced stars and Galaxies looking for Blue shift and Red Shift and discovered to his amazement that virtually EVERYTHING and in every direction is red shifted. He and other scientists concluded that the only possible explanation for this is that the universe is expanding and everything is moving further apart.
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.
If it was stationary then the entire night sky would be bright because every star’s light would be reaching us from the beginning of time. If the universe is expanding that means the light has longer and longer to reach us so we have a black sky.
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