Closed universe
Closed universe:
A homogeneous, isotropic universe is said to be temporally closed if gravity is
strong enough to eventually reverse the expansion, causing the universe to re-collapse.
It is said to be spatially closed if gravity is strong enough to curve the
space back on itself, forming a finite volume with no boundary. Triangles would
contain more than 180°, the circumference of a circle would be less than n times
the diameter, and a traveller intending to travel in a straight line would
eventually find herself back at her starting point. If Einstein's cosmological
constant is zero, as is frequently assumed, then a universe which is temporally
closed is also spatially closed, and vice versa.
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