How can the Universe be flat and accelerating in its expansion?

*FaerieLight*

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!!

ImaLooser

"that the fate of such a Universe is that the expansion slows down to zero as time goes to infinity."

This is not correct. The expansion of an open universe could continue indefinitely. At this time it seems to be accelerating, which was quite unexpected.

George Jones

I think that you have been looking at pre-1998 references that don't take into account dark energy/cosmological constant. When there is no dark energy or cosmological constant, a spatially flat universe is right on the boundary between: 1) closed and open; 2) expanding forever or eventually collapsing. Without dark energy (cosmological constant), the expansion of the universe decelerates.

With dark energy, a spatially flat universe is still on the boundary between closed and open, but if there is enough dark energy, the expansion of the universe accelerates, and the universe can expand forever, even when it is closed.