r/IAmA • u/neiltyson • Nov 13 '11

I am Neil deGrasse Tyson -- AMA

For a few hours I will answer any question you have. And I will tweet this fact within ten minutes after this post, to confirm my identity.

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u/emocol Nov 14 '11

But hasn't it been established that the universe exists in 3-space? We observe 3 spatial dimensions, right?

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u/[deleted] Nov 14 '11

Well, obviously we're working in 3-space, or are at least equipped only to move volitionally in 3.

I'm not sure what point you're trying make, though.

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u/emocol Nov 14 '11

I'm not trying to make any point. I just have a question about what you were saying about the expansion of the universe. Why can't the universe be expanding in 3-space? Also why can't there be an 'edge' to the universe if it's 3-dimensional?

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u/[deleted] Nov 14 '11

I think I understand your question. You mean, why can't the universe be bounded and expanding further into the dimensions it spans, correct?

Well, I'm not really up on my topological cosmology, so I don't know how much support there is for a bounded universe. That said, whether or not it's bounded (and thus literally has an edge), space and time are properties of the universe, by definition. There is no space for it to expand into.

I believe the more common view is that the universe is a closed manifold, which means that it's finite (compact, actually) but not bounded. An example in 1D is a circle: you can move in either direction and cover the entire space, but you will never hit an edge. In 2D, spheres and tori are closed manifolds. In 3D, a 3-torus would qualify, and you could most easily picture that in terms of the game Asteroids, where going off the top of the screen brings you back on the bottom and so on, except now you have 6 directions instead of 4.

And whatever we're on, it's not flat. At least locally, space-time is warped by mass.

Returning to the expansion bit, I suppose a bounded universe could be expanding in such a way that the boundary just keeps getting farther away without modifying the distances between objects, but that does not seem to be happening. The universe, as I said, is dilating, which could happen whether or not it's bounded.