"Quintessence is a dynamic, time-evolving, and spatially dependent form of energy with negative pressure sufficient to drive the accelerating expansion [...] Whereas the cosmological constant is a very specific form of energy." -Robert Caldwell, inventor of the Big Rip scenario
Our Universe began with a period of cosmic inflation: where energy intrinsic to space itself caused an extremely rapid, exponential expansion. This stretched the Universe flat, gave it the same properties, temperature and spectrum of fluctuations everywhere, and then gave rise to the hot Big Bang. And our Universe is ending in a blaze of dark energy, driving the Universe apart with an extremely slow (but accelerating) exponential expansion, that will stretch the Universe flat, and give it the same properties and temperature everywhere.
Would it be possible that these two phenomena aren't only related, but that dark energy could result in the Universe recycling itself, creating a new hot Big Bang after a long enough period of time? Realistically, there are two possible ways for this to happen, although the data presently don't favor either one.
Go get the whole story on this edition of Ask Ethan! (And my apologies that you won't get any new Starts With A Bang until Monday; I'm away at a conference in Memphis now!)