Big crunch versus the Cosmological Constant

[zheng89120]

I am no expert on cosmology. But from what I have heard, the big crunch is the scenario that will most likely happen to our universe in the (far) future. Yet the cosmological constant makes it so that our universe is currently expanding and acceleratingly so. So the question is (I know this may not be that hard to imagine), how is the fact that our universe will de-accelerate and contract in time to be reconciled with the fact that it is expanding from the cosmological constant. Thanks.

 

[Chronos]

It appears the big crunch is less likely than the big chill, based on current cosmological data. It appears expansion was slowing until about the time Earth formed. Since then, it appears to be accelerating.

 

[bapowell]

So the question is (I know this may not be that hard to imagine), how is the fact that our universe will de-accelerate and contract in time to be reconciled with the fact that it is expanding from the cosmological constant. Thanks.

You are exactly right! As Chronos says, the accelerated expansion appears to be leading the universe to a rather different fate than a big crunch. Heat death, in which the universe eventually reaches thermal equilibrium, is the ultimate fate of all open and flat universes. Throw in a cosmological constant and it becomes possible to attain heat death even in closed models. Of course, the universe could throw us another curve ball at some point in the future. We currently don't understand the nature of the dark energy that's driving the accelerated expansion, in particular, we don't know whether it's a true constant or whether it has a time dependance. If it is time dependent, it might decay at some point and then the fate of the universe will be left in the hands of its curvature: if it's closed, we get a big crunch; otherwise, heat death.