Ker Than writes on National Geographic News:
Like part of a cosmic Russian doll, our universe may be nested inside a black hole that is itself part of a larger universe. In turn, all the black holes found so far in our universe — from the microscopic to the supermassive — may be doorways into alternate realities.
According to a mind-bending new theory, a black hole is actually a tunnel between universes — a type of wormhole. The matter the black hole attracts doesn’t collapse into a single point, as has been predicted, but rather gushes out a “white hole” at the other end of the black one, the theory goes.
In a recent paper published in the journal Physics Letters B, Indiana University physicist Nikodem Poplawski presents new mathematical models of the spiraling motion of matter falling into a black hole. His equations suggest such wormholes are viable alternatives to the “space-time singularities” that Albert Einstein predicted to be at the centers of black holes.