Starts With A Bang #8: What is Dark Energy?

How will the Universe end? Will it recollapse in on itself, ending in a Big Crunch? Will it expand forever, ending in a Big Freeze? Or will it tear itself apart, ending in a Big Rip? With the discovery of dark energy, we finally think we know, but there's always more science to be done, more possibilities to consider, and new evidence to look for to help point the way.