Imagine an infinite set of distinct Level I multiverses, each represented by a bubble in Fig. 7.1, some perhaps with different dimensionality and different physical constants. We will refer to this as the Level II multiverse, and it is predicted by most currently popular models of inflation. These other domains are so far away that you would never get to them even if you travelled at the speed of light forever. The reason is that the space between our Level I multiverse and its neighbours is still undergoing inflation, which creates space faster than you can travel through it. In contrast, you could travel to an arbitrarily distant Level I universe, providing the cosmic expansion decelerates. In fact, astronomical evidence suggests that the cosmic expansion is currently accelerating and, if this acceleration continues indefinitely, then even some Level I universes will remain forever separate. However, at least some models predict that our universe will eventually stop accelerating and perhaps even recollapse.
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