Non-perturbative materialization of ghosts

In theories with a hidden ghost sector that couples to visible matter throughgravity only, empty space can decay into ghosts and ordinary matter by gravitonexchange. Perturbatively, such processes can be very slow provided that thegravity sector violates Lorentz invariance above some cut-off scale. Here, weinvestigate non-perturbative decay processes involving ghosts, such as thespontaneous creation of self-gravitating lumps of ghost matter, as well aspairs of Bondi dipoles (i.e., lumps of ghost matter chasing after positiveenergy objects). We find the corresponding instantons and calculate theirEuclidean action. In some cases, the instantons induce topology change or havenegative Euclidean action. To shed some light on the meaning of suchpeculiarities, we also consider the nucleation of concentrical domain walls ofordinary and ghost matter, where the Euclidean calculation can be compared withthe canonical (Lorentzian) description of tunneling. We conclude thatnon-perturbative ghost nucleation processes can be safely suppressed inphenomenological scenarios.Comment: 43 pages, 12 figures. v2: reference adde