Surprises in Lorentzian path-integral of Gauss-Bonnet gravity

TL;DR

This paper analyzes the Lorentzian path-integral of Gauss-Bonnet gravity in a mini-superspace model, revealing how the universe transitions from Euclidean to Lorentzian phases and uncovering surprising conditions where initial states become irrelevant.

Contribution

It provides an exact computation of the transition amplitude in Gauss-Bonnet gravity with Neumann boundary conditions, using Picard-Lefschetz methods to analyze the universe's phase transition.

Findings

Small universe is Euclidean with exponential wave-function growth

Wave-function peaks and becomes oscillatory, indicating time emergence

Special relation between cosmological constant and Gauss-Bonnet coupling makes initial conditions irrelevant

Abstract

In this paper we study the Lorentzian path-integral of Gauss-Bonnet gravity in the mini-superspace approximation in four spacetime dimensions and investigate the transition amplitude from one configuration to another. Past studies motivate us on imposing Neumann boundary conditions on initial boundary as they lead to stable behaviour of fluctuations. The transition amplitude is computed exactly while incorporating the non-trivial contribution coming from the Gauss-Bonnet sector of gravity. A saddle-point analysis involving usage of Picard-Lefschetz methods allow us to gain further insight of the nature of transition amplitude. Small-size Universe is Euclidean in nature which is shown by the exponentially rising wave-function. It reaches a peak after which the wave-function becomes oscillatory indicating an emergence of time and a Lorentzian phase of the Universe. We also notice an interesting hypothetical situation when the wave-function of Universe becomes independent of the initial conditions completely, which happens when cosmological constant and Gauss-Bonnet coupling have a particular relation. This however doesn't imply that the initial momentum is left arbitrary as it needs to be fixed to a particular value which is chosen by demanding regularity of Universe at an initial time and the stability of fluctuations.