Could the universe contract to a single point?
If the Big Bang suddenly meant the universe expanded incredibly quickly, is there any concern that the universe could contract equally quickly and unexpectedly back to an infinitely dense point again?
Theoretical astrophysicist Katie Mack answered this question from listener Matt...
Katie - We're pretty sure the universe is not going to contract again. So right now we see the expansion of the universe, and the way we see the expansion of the universe is that we see distant galaxies moving farther apart from each other and moving farther away from us. And so we see that there's this uniform expansion throughout space. And for a long time, it wasn't clear if that would continue forever, or if the expansion would slow down and stop and turn around because the expansion was set off by the Big Bang, but all of the gravity of everything in the universe is kind of pulling together and that sort of slows down the expansion and puts on the brakes. And so for a long time, we were measuring the expansion and trying to figure out how quickly it was slowing down to see if it was slowing down enough that it would stop and re-collapse, or if it would just keep going but never actually stop.
When those measurements were completed in the 1990s, it turned out that it was not slowing down. It was actually speeding up, and the expansion of the universe is now accelerating. And we don't know exactly why that is. We attribute it to something we call dark energy. We don't know what dark energy is, but it looks like it might just be a property of space - that there's a certain expansion, a sort of stretchiness in space itself, that kind of pushes out. And if that's the case, if that's what the expansion is accelerating because of, then it's not going to stop and turn around, it's going to keep going and keep expanding forever and the universe will just get bigger and bigger and bigger and emptier and emptier and emptier as the space expands. But there's some caveats there. One of them is that we don't know what the dark energy is. So we don't know for sure that it is just a property of space that'll stay the same forever. It might be something that changes over time. It might be something that will get more powerful. It might turn around and make the universe start coming together again, we don't really know.