The constraint algebra in Smolins' $G\rightarrow 0$ limit of 4d Euclidean Gravity

TL;DR

This paper investigates the constraint algebra in Smolin's G→0 limit of 4d Euclidean gravity, proposing modifications to quantum dynamics that enable anomaly-free, diffeomorphism covariant multiple Hamiltonian constraint products.

Contribution

It introduces structural modifications to the quantum Hamiltonian constraint to achieve a well-defined continuum limit of multiple constraint products in the model.

Findings

Supports the continuum limit action on an appropriate state domain

Yields anomaly-free commutators between Hamiltonian constraints

Maintains diffeomorphism covariance

Abstract

Smolin's generally covariant $G_{\mathrm{Newton}}\rightarrow0$ limit of 4d Euclidean gravity is a useful toy model for the study of the constraint algebra in Loop Quantum Gravity. In particular, the commutator between its Hamiltonian constraints has a metric dependent structure function. While a prior LQG like construction of non-trivial anomaly free constraint commutators for the model exists, that work suffers from two defects. First, Smolin's remarks on the inability of the quantum dynamics to generate propagation effects apply. Second, the construction only yields the action of a single Hamiltonian constraint together with the action of its commutator through a continuum limit of corresponding discrete approximants; the continuum limit of a product of 2 or more constraints does not exist. Here, we incorporate changes in the quantum dynamics through structural modifications in the choice of discrete approximants to the quantum Hamiltonian constraint. The new structure is motivated by that responsible for propagation in an LQG like quantization of Paramaterized Field Theory and significantly alters the space of physical states. We study the off shell constraint algebra of the model in the context of these structural changes and show that the continuum limit action of multiple products of Hamiltonian constraints is (a) supported on an appropriate domain of states (b) yields anomaly free commutators between pairs of Hamiltonian constraints and (c) is diffeomorphism covariant. Many of our considerations seem robust enough to be applied to the setting of 4d Euclidean gravity.