Eli5 if the universe is everything but it is constantly expanding, what is it expanding into?

Question

Eli5 if the universe is everything but it is constantly expanding, what is it expanding into?

In: 22

Answer 1

Okay so… TL;DR – its the space between stuff thats expanding.

Say I put four cups on a carpet in a line, and the carpet has a pattern. We super carefully cut the middle bits btween the carpets, stitch on a new bit, and slide it so its flat again. The cups all stay at their same positions on the carpet but each cup is now further apart. If there was a little camera on each cup, it would see the other cups moving away from it but itself not moving.

The mind kindof breaks when we get to infinites but make this infinite cups and infinite carpet and now you’ve got the universe – the cups are stars and the carpet is space.

The reason why we believe this is the case is because when we look at stars they are all a little bit red. This means they are moving away from us. But not only that there doesn’t seem to be like a ‘every star is moving in one direction’ type thing, instead the further away a star is the faster its moving away (and more red it is). One of the things scientists prefer to do is assume that there is nothing special about us (the stars don’t find humans yucky and want to move away) so we assume every star is moving away from every other. We are the centre of the observative universe (science speak for “what we can see”) but not The Universe. If rewind time we get everything cramming together like a really tight train ride – but theres still an infinite amount of stuff in an infinite space its just we’ve “cut off all the in between bits of carpet”. And there you’ve reached the big bang.

We can’t work out what happens when there is less carpet than cup, when you’ve cut away so much that each cup is on the same bit of carpet as another cup. And thats why we say the universe started when it did, because before it, nothing makes sense.

This is not the only theory by any means and is probably a bit of a whack interpretation – but its my best shot at understanding and explaining one of the more popular ones.