Quantum Gravity

I'm reading a book about Quantum Gravity. The idea of gravitons forming around open superstrings, with one end having a surplus -  to form what would become an electromagnetic dipole makes sense to me, but i am confused about the concept of the universe begining from a 'white hole'.  How can a wormhole form inside a black hole that allows gravitons to escape and create a big bang? Surely a wormhole would need something to attach both ends to, and an area outside a known universe would not exist - still it makes you wonder.