Unstable no-boundary fluctuations from sums over regular metrics

Understand

It was recently shown by Feldbrugge et al.

• that the no-boundary proposal, defined via a Lorentzian path integral and in minisuperspace, leads to unstable fluctuations, in disagreement with early universe observations.
• In these calculations many off-shell geometries summed over in the path integral in fact contain singularities, and the question arose whether the instability might ultimately be caused by these off-shell singularities.
• We address this question here by considering a sum over purely regular geometries, by extending a calculation pioneered by Halliwell and Louko.