I've heard that a flat Universe is one where there is enough density in the Universe to prevent a big crunch or ever-accelerating expansion, and that the fate of such a Universe is that the expansion slows down to zero as time goes to infinity. I have also heard that the Universe is accelerating in its expansion, and yet it is widely believed that the Universe is flat. How is this possible? If it is accelerating in its expansion, shouldn't it be open? Thanks!!