String Theory: Phantom Energy and Rogue Universes
In string theory, the assumption is that the separate, parallel universes don’t normally interact with each other, but some approaches over the years have called this into question. One of the most recent is a 2008 paper in the journal Physics Review D by Eduardo Guendelman and Nobuyuki Sakai, in which they examine the idea of bubble universes expanding without the need for a big bang.
To make the equations work, Guendelman and Sakai had to introduce a repulsive phantom energy, which is possibly similar to dark energy. They found two types of stable solutions:
The child universe, which is isolated from the parent universe (essentially a universe inside a black hole)
A rogue universe, which is not isolated from the parent universe
This second kind of universe is troublesome, because as it begins to go through its inflation cycle, it does so by devouring the space-time of the parent universe. The parent universe is swept away as the rogue universe expands in its place — and it does so faster than the speed of light, so there’s no warning.
Fortunately, there’s no evidence that this phantom energy actually exists, or, if it does, it’s possible that it exists in the form of dark energy (or inflation energy), which means that we may be one of these rogue universes ourselves. As our universe expands, it may be devouring some other, larger universe!