How do we know the universe is constantly expanding?
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Quick recap on light. Light is a wave (it’s also a particle but you can ignore that part for now)
So when you draw the path light takes to get to you it’s not a straight line, [see this diagram](https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcS2pCbVIwFaOeAqXoMCTI7GBNSKgo_h4rQTQxbA-0lyCPK_zMUOCdpnqlHU&s=10)
Now the distance between 2 peaks (the wavelength) determines the colour we see. Closer together is blue, further apart is red.
If the universe wasn’t expanding then looking at 2 similar stars at much different distances would look the same. But the further away a star is when we look at it, the more red it looks.
This is because as the light travels the universe is expanding and stretching those wavelengths out, making it less blue and more red. The further away a star is the more time the light has been traveling and thus the more stretched out the light is.
The next bit is how do we know this? Well we discovered a type of supernova (specifically type Ia supernova, said as type one-a) that have an exact amount of mass to form (1.4 times the mass of our Sun) which means that when they explode they will always have the same brightness. So using that, whenever we detect one we can determine how far away it is based on how bright it is.
Once we have a bunch of these we got to see the difference in wavelengths based on different distances and directions, and that let us calculate how fast the universe is expanding.
This still has 1 main issue which is even further out of scope for this question, in that there is at least 1 other way to determine the expansion speed and it doesn’t match this one, so the exact speed of the expansion is still being figured out.
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