A hypothetical hole or tunnel in the fabric of spacetime. Standard cosmological theories are based on the assumption that spacetime is smooth and simply connected. To give a three-dimensional analogy, spacetime is assumed to be like a sphere rather than a torus; a sphere is said to be simply connected, whereas a torus is not. In quantum cosmology it is thought that, on scales of the order of 10−35 m, spacetime has a very complicated, multiply connected structure in which ‘tunnels’ and ‘handles’ supply short cuts between apparently distant points. In principle, sufficiently large wormholes might allow one to travel to a distant part of the Universe much more quickly than light and, in some situations, to travel in time. The existence of wormholes is, however, highly speculative.