Gravitational constrained instantons

We find constrained instantons in Einstein gravity with and without a cosmological constant. These configurations are not saddle points of the Einstein-Hilbert action, yet they contribute to non-perturbative processes quantum gravity. In some cases we expect that give dominant contribution from spacetimes certain fixed topologies. With negative constant, these metrics describe wormholes connecting two asymptotic regions. many examples such wormhole for symmetric establish their perturbative stability. Euclidean versions encode energy level statistics AdS black hole microstates. de Sitter flat space settings new homogeneous isotropic bounce big bang/crunch cosmologies.