On Sun, Oct 26, 2014 at 11:48 PM, Bruce Kellett <[email protected]>
wrote:
>>> it expands for ever even though closed). So you can never see the
>> back of your own head.
>>
>> >> Obviously if it expands forever you could never see the back of your
>> head, and our universe is not only expanding its accelerating; but how
>> could you call such a universe closed?
>>
>
> > It has a finite extent, even if expanding exponentially.
I suppose a universe could be flat and finite at any given time and only in
the infinite future become infinite in extant do to the expansion; or a
flat universe could be infinite at any given time, but either way the
universe would be open not closed.
> in the case under discussion, which is what we mean by a closed universe.
>
A closed universe means that the observed expansion of the universe will
eventually reverse direction, so obviously if a universe expands forever it
will never reverse direction and thus is open.
> The point is that the intrinsic curvature is positive
If the curvature is positive then the universe must be closed and finite.
If the curvature is negative then the universe must be open and infinite.
If the curvature is exactly zero (flat) then the universe could be either
open and finite or open and infinite. My intuition says its unlikely to be
flat, open and finite but I could be wrong. The best experimental evidence
we have right now is that the universe is flat, or if it is curved it's
curved by less than one part in 100,000. So we do know one thing, the
universe is either infinitely larger than the observable universe or just
astronomically larger.
John K Clark
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