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Post by Progenitor A on Sept 26, 2010 13:16:42 GMT 1
If the universe is everything, and scientists say that the universe is expanding, what is it expanding into?
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Post by eamonnshute on Sept 26, 2010 13:22:57 GMT 1
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Post by Progenitor A on Sept 26, 2010 13:34:11 GMT 1
If the universe is not expanding into something, does that mean it is expanding into nothing? If I were to go to the very limits of the universe's expansion what would I see as I looked byond the limit of expansion? |
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Post by eamonnshute on Sept 26, 2010 13:47:11 GMT 1
The universe does not have a limit, it does not have an edge, it is infinite. Think of an infinite flat sheet of rubber - points on the rubber get further apart, but the sheet still occupies exactly the same space.
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Post by olmy on Sept 26, 2010 13:52:40 GMT 1
If the universe is not expanding into something, does that mean it is expanding into nothing? No. At least, not in the sense that there is some 'nothingness' that it is expanding into and not if we take the 'classical' General Relativity view of things. There is no need for an 'outside' at all. In fact 'outside' space doesn't really make any sense, done it? We are dealing with concepts that defy intuition here. If I were to go to the very limits of the universe's expansion what would I see as I looked byond the limit of expansion? There is no limit. It is either infinite in extent or 'closed' - like the surface of the Earth. |
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Post by Progenitor A on Sept 26, 2010 13:56:54 GMT 1
The universe does not have a limit, it does not have an edge, it is infinite. Think of an infinite flat sheet of rubber - points on the rubber get further apart, but the sheet still occupies exactly the same space. If it does not have a limit then what is this talk of a diameter (or is it radius) of 46billion light years - that's a lot less than infinity isn't it? |
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Post by olmy on Sept 26, 2010 14:16:53 GMT 1
If it does not have a limit then what is this talk of a diameter (or is it radius) of 46billion light years - that's a lot less than infinity isn't it? That would be the observable universe. |
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Post by speakertoanimals on Sept 27, 2010 14:09:11 GMT 1
The only examples of curvature or expansion that we can see are extrinsic -- we have a curved 2D surface IN 3D, or an expanding 2D surface IN 3D (like the surface of a balloon).
Now the mathematical leap here is between realising a surface is curved because you can measure, say, a radius of curvature for the surface in 3D (extrinsic curvature), and a surface being curved because you can measure it is curved by just staying strictly within the surface (like drawing circles IN the surface, and seeing what ahhpens to circumference with the radius) -- the latter being intrinsic curvature.
The point about a cylinder is that it is extrinsically curved, but from the point of view of someone living in the surface, it is FLAT, just with a non-trivial topology.
Which leads to the realisation that curvature cam be described purely in terms of the surface, and what you measure in the surface, it doesn't need an outside in which to curve, nor does it need an outside in which to expand.
And since all that we can measure is in the space, not outside, our physics had better just talk about the intrinsic stuff, rather than the unseeable extrinsic stuff.
There may be an outside (extra dimensions where gravity leaks out etc), but they are not necessary, just from the point of view of curvature or expansion.
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