An embarrassing question: what were the boundaries of the universe

[houhou.trad]

I say always that the universe has expanded, from an initially-hot and dense state, until it get its current forme...
But it will be very natural to ask the question: at the first instances of its expansion, what were the boundaries of this universe, and what were the exterior of this boundaries..?
Can someone explain me this?

 

[narrator]

A similar question was asked in the "https://www.physicsforums.com/showthread.php?t=550977"" thread.
As I understand it, the universe has always been infinite, even at the beginning, and has been expanding since it was about 6 billion years old.

 

[cephron]

We're actually not sure if it's infinite or not (according to Marcus, and the FAQ, the current data fits both infinite and finite models, pretty much equally well), but:

-If the universe is infinite, then it was already infinite at the point of the Big Bang; there was never a time that it was finite.

-If the universe is finite, it still does not (and never did) have boundaries. What happens when you reach the edge of the universe? Well, you never reach an edge; try this and you'll only end up back where you started (rather like as if you tried to sail to the edge of the earth).

-In either case, it is expanding. One may wonder: if the infinite universe is true, how would an already infinite space expand further? Well, all the things in it just keep getting further away from each other.

 

[houhou.trad]

Thank you narrator and cephron, I think the idea is now somewhat more clear.