How do astronomers know that the universe is expanding?

Question

I understand how they can tell how big a planet is or how hot a star is. The thing I don’t understand is how do they know the universe is expanding? We cant see the edge of the universe also things are moving away and towards us. So how do they know its expanding? Also, what is it expanding to? For example, at the edge of the universe, is there a wall or is there complete nothingness? Like what is it expanding towards?

Edit: hopefully I used the right flair. I wanted to use “Astronomy” but there isn’t a flair for that

In: Physics

Answer 1

The doppler effect is better understood visually so here a little video showing it.

[https://www.youtube.com/watch?v=h4OnBYrbCjY](https://www.youtube.com/watch?v=h4OnBYrbCjY)

All waves behave that way, and you propably experienced it in your life. Like the sound of a formular car passing by.

[https://www.youtube.com/watch?v=hpvuu5MfaSk](https://www.youtube.com/watch?v=hpvuu5MfaSk)

Or the sound of an emergency vehicle passing by.

[https://www.youtube.com/watch?v=FG2im7UP3FI](https://www.youtube.com/watch?v=FG2im7UP3FI)

In both case for the person inside the car, the sound doesn’t change. The frequency stay the same since they are not moving relatively to the source of the sound. But for people outside, the frequency increase or decrease depending if the source is going away from you or toward you.

Same happen to a star. If the star is going away from us or toward us the frequency of the light will shift. And as you can see in the [Light Spectrum](https://science.nasa.gov/science-pink/s3fs-public/styles/background_image_file_size/public/thumbnails/image/visible-wave.jpg?itok=piKCDAWL), the frequency change how the color look to us. For light we call that a red or blue shift, because that’s the extreme on the visible light spectrum. If the star is moving toward us, we see a blue shift, if the star is going away from us, we see a red shift.

If we look at all the stars we can in the universe, we see that the Galaxies very close to us are moving toward us, because of gravity. This is why Andremoda will collide with our Galaxy in the distant future. But if you look at ANY other stars, they are all moving away from us. Even more importantly, the further away the star is from us, the bigger the red shift we see, aka the faster they are moving away from us. This is the universe expanding.