When astronomers talk about the ‘shape’ of the Universe, they are talking about something more specific than whether it is a sphere or a cube, for example. Einstein’s General Theory of Relativity allows three shapes, or geometries, for the Universe. These are ‘flat’, ‘closed’ or ‘open’. It’s difficult to envisage these shapes in terms of the Universe, but they can be compared to a sheet of paper (flat), a sphere (closed) or a saddle (open). The shape of the Universe determines whether it will expand forever (or eventually collapse), and whether it’s finite or infinite. Which shape it is depends on its total density and its rate of expansion.