Eli5 if the universe is everything but it is constantly expanding, what is it expanding into?
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“Expansion” is a mathematical model.
Imagine a 2D ant living on the surface of a spherical balloon. The ant exists on the surface and can only move around on the surface. To the ant, there is no such thing as the direction “into the balloon” or “out from the surface of the balloon”.
If the balloon is inflated, all of the points in the balloon move away from all of the others (when you measure distance along curves over the surface which connect the points). In every experiment the ant can perform, it seems as though his universe is expanding, even though he has no concept of the space it is “expanding into”.
Similarly, we do experiments and the results are well described by a model which “embeds” the universe we live in into a hypothetical space with one additional dimension, with the “surface” we live in expanding within that hypothetical space.
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