Generally, we work from the assumption that there is one universe, and that our galaxy is a component of this one singular universe that emerged from the Big Bang. But the multiverse notion suggests there are infinite (or at least numerous) universes beyond our own, existing as alternative realities. Imagine an endless roll of bubble wrap; our universe (and everything in it) would be one tiny bubble, and all the other bubbles would be other universes that are equally vast. In his book The Hidden Reality, Greene maps out nine types of parallel universes within this hypothetical system.
