We explore the cosmological implications of quantum entanglement between two causally disconnected universes in the multiverse. We first consider two causally separated de Sitter spaces with a state which is initially entangled. We derive the reduced density matrix of our universe and compute the spectrum of vacuum fluctuations. We then consider the same system with an initially non-entangled state. We find that due to quantum interference scale dependent modulations may enter the spectrum for the case of initially non-entangled state. This gives rise to the possibility that the existence of causally disconnected universes may be experimentally tested by analyzing correlators in detail.
Kanno, S. (2015). Cosmological implications of quantum entanglement in the multiverse. Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics, 751, 316–320. https://doi.org/10.1016/j.physletb.2015.10.050