9 Is the universe finite or infinite?

The current consensus, since the discovery that the expansion is accelerating, is that the universe will expand forever. But we’re still no closer to knowing for sure whether the universe as a whole is infinite – the proximity to the critical density threshold discussed earlier means that the notion of a closed, finite universe is not entirely ruled out. If the universe were closed, what would it look like? And what does it actually mean for a universe to be ‘closed’?

Einstein’s theory of general relativity, which gives us the mathematical tools to make calculations about how the universe is expanding, also tells us that space can be curved. In this view, the gravitational pull of one object on another is due to the curvature of space, not a mysterious force acting at a distance. Einstein also showed that the whole universe could be curved due to the matter within it, and the curvature will determine whether the universe is finite or infinite.