In the old days before dark energy we used to say that a flat universe,
i.e. critical density, would expand at a decreasing rate with the rate
of expansion tending to zero.
Now we know about dark energy. The rate of expansion is accelerating.
But CMBR observations tell us that the universe is pretty much flat. How
do flatness, critical density and fate of the universe relate to each
other in a universe with dark energy?