  We study the effect of a bubble wall on the entanglement entropy of a free
massive scalar field between two causally disconnected open charts in de Sitter
space. We assume there is a delta-functional wall between the open charts. This
can be thought of as a model of pair creation of bubble universes in de Sitter
space. We first derive the Euclidean vacuum mode functions of the scalar field
in the presence of the wall in the coordinates that respect the open charts. We
then derive the Bogoliubov transformation between the Euclidean vacuum and the
open chart vacua that makes the reduced density matrix diagonal. We find that
larger walls lead to less entanglement. Our result may be regarded as evidence
of decoherence of bubble universes from each other. We also note an interesting
relationship between our results and discussions of the black hole firewall
problem.
