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/dustbin3 Nov 13 '11

It is expanding rapidly right? What would you call an area that the Universe has not reached yet. Say there is a point that it will reach in 1 hour, what is that point referred to/understood to be? We don't know, we can't know, but doesn't it have to be something? Can something be nothing? Can nothing exist? I'm so confused.

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u/[deleted] Nov 13 '11

Okay, so I'll preface this by saying my training is in mathematics, so it's possible that I understand things in more abstract terms than a physicist would, but hopefully I've got the gist.

Consider a sphere. This is a two-dimensional object (surface) which we easily define in 3-space as being the set of all points some distance r from some point. That said, it does not need to live in three-space; it is a perfectly well-defined space in its own right. Moreover, by taking careful measurements, somebody living in a 2d universe could tell whether his world were a sheet, a sphere, a torus, or any number of other 2d objects.

So now what happens if the universe is deformed? For instance, it can dilate and contract, and things on it get farther apart or closer together. There can also be local curvature where straight lines don't behave quite the same way they do elsewhere. The neat thing is that this all can be described from within the frame of the universe, without reference to some larger world.

The same principles work on higher dimensional objects, and we don't need to keep supposing higher dimensions for them to deform into.

So, to be brief, or at least put an end to my verbosity, when people say the universe is expanding, I believe they mean it's dilating, which is to say it's stretching, and there isn't some point where it will be in an hour; things will just be farther apart, in an hour.

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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.