タイトル：
Entanglement between two disjoint universes

アブストラクト：
We use the replica method to compute the entanglement entropy
of a universe without gravity entangled in a thermofield-double-like
state with a disjoint gravitating universe. Including wormholes between
replicas of the latter gives an entropy functional which includes an ``
island" on the gravitating universe. We solve the back-reaction
equations when the cosmological constant is negative to show that this
island coincides with a causal shadow region that is created by the
entanglement in the gravitating geometry. At high entanglement
temperatures, the island contribution to the entropy functional leads to
a bound on entanglement entropy, analogous to the Page behavior of
evaporating black holes. We also apply the formalism to black holes in
de Sitter space, and find similar islands.