  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 demonstrate that the entanglement wedge of the non-gravitating
universe grows with the entanglement temperature until, eventually, the
gravitating universe can be entirely reconstructed from the non-gravitating
one.
