Timeline for Shape of the universe?
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| Nov 6, 2013 at 8:30 | history | edited | Manishearth | CC BY-SA 3.0 |
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| Jan 15, 2011 at 13:33 | comment | added | Luboš Motl | Third question: No, there is no "center outside the Universe" for a Universe that looks like a 3-sphere. The right way to interpret the balloon analogy is that the rubber is everything there is. There is no space outside the rubber. In the real world, there is room away from the rubber but that's just a misleading artifact of the analogy. The Universe is only the rubber. Saying that a physical point is "outside the Universe" is a contradiction because the Universe is defined as everything that physically exists. | |
| Jan 15, 2011 at 13:31 | comment | added | Luboš Motl | Second question: no, if the Universe is globally flat, then it must be either infinite, or it could be compactified on a torus - the coordinates could simply be periodic separately (which is very, very unlikely) - but at any rate, it has no center. Just like the sphere (the surface of a ball), an infinite Universe doesn't have to have any center. Just imagine that an infinite plane is the limit of a very large sphere. This is a key point that the balloon analogy is meant to convey, and you should really take the analogy seriously in this respect. | |
| Jan 15, 2011 at 13:29 | comment | added | Luboš Motl | First question: yes, if the energy density exceeds the critical density, the Universe - I mean the spatial slice at a given time $t$ - curves back on itself, into a sphere. And vice versa, it's hyperbolic when the density is lower than the critical density. Within the error margin, the experimental density is still equal to the critical one. | |
| Jan 14, 2011 at 21:18 | comment | added | TROLLHUNTER | Am I correct in saying that thh energy density determines weather or not the universe curves back on itself? If the universe is globally flat, then its either infinite in some direction, or it has a center that could lie within this universe? And if it is curved like a 3-sphere, then it has a center outside the universe? Can it be globally flat without beeing infinite? | |
| Jan 14, 2011 at 19:29 | history | edited | Luboš Motl | CC BY-SA 2.5 |
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| Jan 14, 2011 at 19:23 | history | answered | Luboš Motl | CC BY-SA 2.5 |