Physical instabilities and the phase of the Euclidean path integral

Abstract: We compute the phase of the Euclidean gravity partition function on manifolds of the form $S^p \times M_q$. We find that the total phase is equal to the phase in pure gravity on $S^p$ times an extra phase that arises from negative mass squared fields that we obtain when we perform a Kaluza-Klein reduction to $S^p$. The latter can be matched to the phase expected for physical negative modes seen by a static path observer in $dS_p$. In the case of $S^p \times S^q$ the answer can be interpreted in terms of a computation in the static patch of $dS_p$ or $dS_q$. We also provide the phase when we have a product of many spheres. We clarify the procedure for determining the precise phase factor. We discuss some aspects of the interpretation of this phase.