How do we know the universe is constantly expanding?

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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.

In a nutshell the doppler effect. Just like sound, light is a wave, when a wave is coming at you the pitch sounds higher, with light it blue shifts. When moving away from you light red shifts.

They use type 1a supernova as a consistent light source across the universe to measure this. That is because type 1a supernova are the brightest type of supernova and they are consistently the same luminosity. So known light source plus red or blue shift and a bunch of math I don’t fully understand and they can calculate with great precision how far away the supernova is and how fast its moving away from us.

Technically what is expanding is the space itself, picture it as a balloon being inflated and we are just a dot in the surface of the balloon. The larger the balloon, the more separation between us the dot, and another dot or star.

We know the space is expanding because when the stars move further apart from each other, like the dots in the balloon, they change their light to become more red. If a star is blue, then the light can turn yellow, then orange then red, this is called the red shift.

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.