An Inner Product for 4D Quantum Gravity and the Chern-Simons-Kodama State

TL;DR

This paper investigates the inner product structure in 4D quantum gravity using Ashtekar variables, revealing a non-trivial measure that impacts the normalizability of the Chern-Simons-Kodama state and connecting it to Chern-Simons theory.

Contribution

It derives a non-trivial measure for the gravitational inner product in a specific polarization, enabling non-perturbative analysis and exploring the normalizability of the CSK state.

Findings

The measure is expressed as a determinant of a differential operator.

The CSK state is perturbatively non-normalizable with naive inner product.

A semi-classical evaluation on the three-sphere yields a divergent but potentially regulatable measure.

Abstract

We demonstrate that reality conditions for the Ashtekar connection imply a non-trivial measure for the inner product of gravitational states in the polarization where the Ashtekar connection is diagonal, and we express the measure as the determinant of a certain first-order differential operator. This result opens the possibility to perform a non-perturbative analysis of the quantum gravity scalar product. In this polarization, the Chern-Simons-Kodama state, which solves the constraints of quantum gravity for a certain factor ordering, and which has de Sitter space as a semiclassical limit, is perturbatively non-normalizable with respect to the naive ve inner product. Our work reopens the question of whether this state might be normalizable when the correct non-perturbative inner product and choice of integration contour are taken into account. As a first step, we perform a semi-classical treatment of the measure by evaluating it on the round three-sphere, viewed as a closed spatial slice of de Sitter. The result is a simple, albeit divergent, infinite product that might serve as a regulator for a more complete treatment of the problem. Additionally, our results suggest deep connections between the problem of computing the norm of the CSK state in quantum gravity and computing the Chern-Simons partition function for a complex group.