Quantum scalar field in quantum gravity: the propagator and Lorentz invariance in the spherically symmetric case

TL;DR

This paper investigates the quantum scalar field propagator within a loop quantum gravity framework in spherical symmetry, examining Lorentz invariance and the effects of polymerization on the quantum state and propagators.

Contribution

It extends previous work by verifying the robustness of the quantum state under scalar field polymerization and analyzes Lorentz invariance in the context of quantum gravity with scalar fields.

Findings

Polymerized scalar field states remain consistent with the master constraint.

Propagators in flat spacetime show small Lorentz invariance violations with real clocks.

Results suggest minimal Lorentz violation effects in the low energy limit.

Abstract

We recently studied gravity coupled to a scalar field in spherical symmetry using loop quantum gravity techniques. Since there are local degrees of freedom one faces the "problem of dynamics". We attack it using the "uniform discretization technique". We find the quantum state that minimizes the value of the master constraint for the case of weak fields and curvatures. The state has the form of a direct product of Gaussians for the gravitational variables times a modified Fock state for the scalar field. In this paper we do three things. First, we verify that the previous state also yields a small value of the master constraint when one polymerizes the scalar field in addition to the gravitational variables. We then study the propagators for the polymerized scalar field in flat space-time using the previously considered ground state in the low energy limit. We discuss the issue of the Lorentz invariance of the whole approach. We note that if one uses real clocks to describe the system, Lorentz invariance violations are small. We discuss the implications of these results in the light of Horava's Gravity at the Lifshitz point and of the argument about potential large Lorentz violations in interacting field theories of Collins et. al.