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If the universe is expanding, what is it expanding into? In other words how does one define the universe?

By definition, Universe includes everything therefore there is no question of its expanding into something. An example which is easy to grasp is that of surface of a balloon. If we were two dimensional creatures restricted to surface of a balloon then we will see the "Universe" (balloon) expand. This example does not convey the whole richness of possible universe, as there can be universes which are infinite in spatial extent which can stretch as time goes on. No matter how much stretching the universe goes through, it can not have a size larger than infinite!

It seems that the Universe is infinite in extent. If it is not then the most likely form is that of surface of a sphere but in higher dimensions (Surface of a sphere in 4 dimensions). In that case, it is finite without a boundary - just like the surface of a football or that of Earth.

Jasjeet S Bagla

No one has a picture from a telescope that shows the end. So, no one really knows. As far as we know for the present, it does not end.

Samuel P Bowen

Space and time exist within the universe. The universe does not exist within space.

Nathan Unterman

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