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At 08:06 AM 10/10/01 -0700, Wes Davis wrote:the
I recently attended a lecture on GR.
I learned of recent data indicating that
the universe is flat. How does this affect
the size of the universe? Is the universe
finite, infinite, or is this not a sensible question?
There are several different questions in there.
First of all: Note that "flat" refers to intrinsic curvature. This is
only sort of curvature that makes sense when talking about theuniverse...
but when visualizing models of the universe, you have to be careful notto
be confused by any extrinsic curvature.
In particular, take a flat pieceup
of paper. It really is flat, i.e. zero intrinsic curvature. Curl it
into a cylinder. It still has zero intrinsic curvature. Flatlanders
living on the cylinder can't measure anything non-flat about it, unless
they manage to circumnavigate it.
1) A flat universe could well be infinite.We
2) A flat universe could be like a cylinder, or a torus, that has zero
intrinsic curvature everywhere but nevertheless closes back on itself.
would AFAICT be unable to distinguish this from case (1), unless wemanaged
to circumnavigate the universe.
3) A lot of the time when people talk about infinite or non-infinitesize.
universes, they are talking about the !time! duration not the spatial
curvature
Consider a universe with enough mass to give itself a positive
(like a sphere) as opposed to having zero or negative curvature (like aBig
sombrero). According to standard theories, such a universe will have
enough gravity to arrest and reverse the Hubble expansion, leading to a
Crunch at some finite time in the future.just
So, if you buy the theory, a flat universe is special because it is
barely Crunch-free.
OTOH competing theories are a dime a dozen.