Inter-universal entanglement
  Quantum information theory and the multiverse are two of the greatest outcomes of the XX century physics. The consideration of entanglement between the quantum states of two or more universes in a multiverse scenario provides us with a completely new paradigm that opens the door to novel approaches for traditionally unsolved problems in cosmology. More precisely, the problems of the cosmological constant, the arrow of time and the choice of boundary conditions, among others. It also encourages us to adopt new points of view about major philosophical ideas. In this chapter, we shall present the main features that may characterize inter-universal entanglement and it will be addressed the customary problems of cosmology from the new perspective that the quantum multiverse scenario supplies us with.   In summary, the appropriate boundary condition that has to be imposed on the quantum state of the whole multiverse allows us to interpret it as made up of entangled pairs of universes. Then, a quantum thermodynamical description of single universes can be given and it can be shown that it may induce observable effects in the energy properties of the Universe. The effects that the boundary condition of the multiverse has on the vacuum energy and the arrow of time of single universes are also studied. As a consequence of inter-universal entanglement, the former might be discriminated from observational data and the latter would favor the growth of cosmic structures that increase the amount of local entropy mainly in the very early phase of the universe. All these characteristics of inter-universal entanglement would eventually impel us to develop the concept of the physical multiverse, one for which the theory could be not only fallible but also indirectly observed. 
Salvador J. Robles-P\'erez
gr-qc hep-th