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Originally Posted by tatwood106
And no I don't think the universe is a sphere, current theory says it is either flat or saddle shaped. Both of which would make it finite. Unimaginably LARGE but still finite and expanding.
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The open universes (flat or saddle shaped) are generally considered to be infinite in spatial extent. This is not necessarily provable as we can never see beyond our local observable universe, but it is deductively true assuming the rest of the universe is pretty much like our local universe. This is commented on here:
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Is the Universe really infinite or just really big?
We have observations that say that the radius of curvature of the Universe is bigger than 70 billion light years. But the observations allow for either a positive or negative curvature, and this range includes the flat Universe with infinite radius of curvature. The negatively curved space is also infinite in volume even though it is curved. So we know empirically that the volume of the Universe is more than 20 times bigger than volume of the observable Universe. Since we can only look at small piece of an object that has a large radius of curvature, it looks flat. The simplest mathematical model for computing the observed properties of the Universe is then flat Euclidean space. This model is infinite, but what we know about the Universe is that it is really big.
Frequently Asked Questions in Cosmology
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And your question about what the universe is expanding into is commented on at the same site:
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What is the Universe expanding into?
This question is based on the ever popular misconception that the Universe is some curved object embedded in a higher dimensional space, and that the Universe is expanding into this space. This misconception is probably fostered by the balloon analogy which shows a 2-D spherical model of the Universe expanding in a 3-D space. While it is possible to think of the Universe this way, it is not necessary, and there is nothing whatsoever that we have measured or can measure that will show us anything about the larger space. Everything that we measure is within the Universe, and we see no edge or boundary or center of expansion. Thus the Universe is not expanding into anything that we can see, and this is not a profitable thing to think about. Just as Dali's Corpus Hypercubicus is just a 2-D picture of a 3-D object that represents the surface of a 4-D cube, remember that the balloon analogy is just a 2-D picture of a 3-D situation that is supposed to help you think about a curved 3-D space, but it does not mean that there is really a 4-D space that the Universe is expanding into.
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Do these quotes make sense? Is there anything that can be elaborated on?
~modest
