Resolving Degeneracies in Complex $\mathbb{R}\times S^3$ and $\theta$-KSW

TL;DR

This paper analyzes the Lorentzian gravitational path integral in 4D Gauss-Bonnet gravity, addressing degeneracies in saddle-point methods and proposing complex deformation of parameters to resolve ambiguities.

Contribution

It introduces a novel approach using complex deformation of $(Gar{h})$ to overcome degeneracies in saddle-point analysis of gravitational path integrals.

Findings

Degeneracies cause ambiguities in saddle relevance and merging of saddles.

Quantum fluctuations partially lift degeneracies, but complex deformation fully resolves type-2 degeneracies.

Anti-linear symmetry underpins type-1 degeneracies, and breaking it via complex deformation resolves these issues.

Abstract

Lorentzian gravitational path integral for the Gauss-Bonnet gravity in $4D$ is studied in the mini-superspace ansatz for metric. The gauge-fixed path-integral for Robin boundary choice is computed exactly using {\it Airy}-functions, where the dominant contribution comes from No-boundary geometries. The lapse integral is further analysed using saddle-point methods to compare with exact results. Picard-Lefschetz methods are utilized to find the {\it relevant} complex saddles and deformed contour of integration, thereby using WKB methods to compute the integral along the deformed contour in the saddle-point approximation. However, their successful application is possible only when system is devoid of degeneracies, which in present case appear in two types: {\bf type-1} where the flow-lines starting from neighbouring saddles overlap leading to ambiguities in deciding the {\it relevance} of saddles, {\bf type-2} where saddles merge for specific choices of boundary parameters leading to failure of WKB. Overcoming degeneracies using artificial {\it defects} introduces ambiguities due to the choice of {\it defects} involved. Corrections from quantum fluctuations of scale-factor overcome degeneracies only partially (lifts {\bf type-2} completely with partial resolution of {\bf type-1}), with the residual lifted fluently by complex deformation of $(G\hbar)$. {\it Anti-linear} symmetry present in various forms in the lapse action is the reason behind all the {\bf type-1} degeneracies. Any form of {\it defect} or {\it deformation} breaking anti-linearity resolves {\bf type-1} degeneracies, indicating complex deformation of $(G\hbar)$ as an ideal choice. Compatibility with the KSW criterion is analyzed after symmetry breaking. Complex deformation of $(G\hbar)$ modifies the KSW criterion, imposing a strong constraint on the deformation if No-boundary geometries are required to be always KSW-allowed.