> Anders Sandberg writes:
>
> >Actually, both is equivalent (at least in the simpler cosmological
> >models; I'm not sure this holds for more complex models). An open
> >universe is spatially infinite and will expand infinitely, a closed
> >universe is finite and will contract to a point after a certain time.
>
> I'm not too sure about a closed universe being finite - or to be more
> specific existing in a finite space. From my understanding, a way
> around the problem of there being boundaries to the closed universe is
> to say that "in a closed universe, space is infinite, but the matter
> within it only expands to a finite point".
Nononono! A closed universe doesn't have a boundary, but its total
volume is finite. It is not a ball of stars in a huge emptiness.
The spatial geometry of a closed Friedman universe is equivalent to a
3-sphere, while the geometry of an open universe is a
3-pseudosphere. General relativistic cosmology does not assume
anything "outside" the spacetime manifold - it is simply not
necessary.
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