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
When a car drives past you playing music, you know how the pitch changes? That’s called the Doppler Effect. The sound waves get scrunched up as the thing’s coming towards you, and stretched out as it’s moving away. A similar thing happens when light-emitters (like stars & galaxies) move away from us. This is known as Red Shift. Astronomers can use this phenomenon to determine that space itself is expanding. In all directions, the further away objects are from us, the redder their shift, the faster they are moving away.
They not to think of the space expanding into something. Draw a picture on a balloon. Then blow the balloon up. Every part of the picture is expanding away from every other part, but the only surface that exists for drawings to live on remains the balloon. The universe takes this concept one dimensions higher. 3d space is like the surface of a 4d balloon. The balloon is expanding in *time.* Yes, if you were somehow able to travel at arbitrarily high speeds forever, you would end up back where you started eventually.
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