Creation of wormholes by quantum tunnelling in modified gravity theories

We study the process of quantum tunnelling in scalar-tensor theories in which the scalar field is nonminimally coupled to gravity. In these theories gravitational instantons can deviate substantially from sphericity and can in fact develop a neck-a feature prohibited in theories with minimal coupling. Such instantons with necks lead to the materialization of bubble geometries containing a wormhole region. We clarify the relationship of neck geometries to violations of the null energy condition, and also derive a bound on the size of the neck relative to that of the instanton.