  Assuming a toroidal space with finite volume, we derive analytically the full
one-loop vacuum energy for a scalar field tunnelling between two degenerate
vacua, taking into account discrete momentum. The Casimir energy is computed
for an arbitrary number of dimensions using the Abel-Plana formula, while the
one-loop instanton functional determinant is evaluated using the Green's
functions for the fluctuation operators. The resulting energetic properties are
non-trivial: both the Casimir effect and tunnelling contribute to the Null
Energy Condition violation, arising from a non-extensive true vacuum energy. We
discuss the relevance of this mechanism to induce a cosmic bounce, requiring no
modified gravity or exotic matter.
