Multiverse in an inverted island

We study the redundancies in the global spacetime description of the eternally inflating multiverse using the quantum extremal surface prescription. We argue that a sufficiently large spatial region in a bubble universe has an entanglement island surrounding it. Consequently, the semiclassical physics of the multiverse, which is all we need to make cosmological predictions, can be fully described by the fundamental degrees of freedom associated with certain finite spatial regions. The island arises due to mandatory collisions with collapsing bubbles, whose big crunch singularities indicate redundancies of the global spacetime description. The emergence of the island and the resulting reduction of independent degrees of freedom provides a regularization of infinities which caused the cosmological measure problem.

Automated summary

By employing the quantum extremal surface prescription, we studied the redundancies in the global spacetime description of the eternally inflating multiverse and found that a quantum entanglement "island" surrounds sufficiently large spatial regions in bubble universes. This results in the semiclassical physics of the multiverse being fully described by the fundamental degrees of freedom associated with certain finite spatial regions. The emergence of the island due to collisions with collapsing bubbles leads to a reduction of independent degrees of freedom, providing a regularization of infinities that resolves the cosmological measure problem.