http://www.sciencenews.org/view/generic/id/43571/title/Honing_the_Hubble_constant

Revised value supports finding that dark energy does not vary with time

Dark energy is a mysterious repulsive force that causes the universe to expand at an increasing rate.

The question of whether or not dark energy varies with time has profound significance for the fate of the universe. For instance, if the stuff grows stronger with time — akin to stepping on the cosmic accelerator — the entire universe could end in a Big Rip, with every last atom torn asunder. A constant dark energy would end in a lonely universe, with every galaxy fleeing from each other so rapidly that a Milky Way astronomer some 30 billion years from now would look out and see nothing beyond our galaxy’s own stars. If dark energy drops to zero, the universe might end in a Big Crunch, with gravity’s tug creating a giant implosion.

-- Ron Cowen