What is the fate of the universe?

The fate of the universe strongly depends on a factor of unknown value: Ω, a measure of the density of matter and energy throughout the cosmos. If Ω is greater than 1, then space-time would be "closed" like the surface of an enormous sphere. If there is no dark energy, such a universe would eventually stop expanding and would instead start contracting, eventually collapsing in on itself in an event dubbed the "Big Crunch." If the universe is closed but there is dark energy, the spherical universe would expand forever.

Alternatively, if Ω is less than 1, then the geometry of space would be "open" like the surface of a saddle. In this case, its ultimate fate is the "Big Freeze" followed by the "Big Rip": first, the universe's outward acceleration would tear galaxies and stars apart, leaving all matter frigid and alone. Next, the acceleration would grow so strong that it would overwhelm the effects of the forces that hold atoms together, and everything would be wrenched apart.

If Ω = 1, the universe would be flat, extending like an infinite plane in all directions. If there is no dark energy, such a planar universe would expand forever but at a continually decelerating rate, approaching a standstill. If there is dark energy, the flat universe ultimately would experience runaway expansion leading to the Big Rip.

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