  This paper suggests a model for a bubble universe using buildable false
vacuum bubbles. We study the causal structures of collapsing false vacuum
bubbles using double-null simulations. False vacuum bubbles violate the null
energy condition and emit negative energy along the outgoing direction through
semi-classical effects. If there are a few collapsing false vacuum bubbles and
they emit negative energy to a certain region, then the region can be
approximated by a negative energy bath, which means the region is homogeneously
filled by negative energy. If a false vacuum bubble is generated in the
negative energy bath and the tension of the bubble effectively becomes negative
in the bath, then the bubble can expand and form an inflating bubble universe.
This scenario uses a set of assumptions different from those in previous
studies because it does not require tunneling to unbuildable bubbles.
