  In the present article we point out a conceptual issue of the Third
Quantization formalism for Canonical Quantum Gravity. Based on earlier results
on interuniversal entanglement, the picture of noninteracting
universe-antiuniverse pairs is incomplete. In particular, the variation of the
entanglement entropy with the scale factor, understood as a measure of time,
implies either an interaction between pairs of universes or universes with a
heat bath, i.e. an external field. We propose a simple solution to the problem
by adding a new quantum field to the formalism. Correspondingly, particles of
this field mediate interactions between universes -- depending on the model at
tree level or one loop -- and may further provide a heat bath in the
background.
