Causality and matter propagation in 3d spin foam quantum gravity

TL;DR

This paper develops a causal spin foam model for 3d quantum gravity, incorporating matter fields, and demonstrates its relation to non-commutative quantum field theory and standard QFT in the flat space limit.

Contribution

It introduces a new causal spin foam model for 3d quantum gravity with matter, explicitly constructs the model, and links it to non-commutative quantum field theory.

Findings

Constructed a causal spin foam model for 3d quantum gravity with matter.

Showed the model's amplitudes approximate non-commutative QFT Feynman amplitudes.

Reduced the effective field theory to standard QFT in flat space without gravity.

Abstract

In this paper we tackle the issue of causality in quantum gravity, in the context of 3d spin foam models. We identify the correct procedure for implementing the causality/orientation dependence restriction that reduces the path integral for BF theory to that of quantum gravity in first order form. We construct explicitly the resulting causal spin foam model. We then add matter degrees of freedom to it and construct a causal spin foam model for 3d quantum gravity coupled to matter fields. Finally, we show that the corresponding spin foam amplitudes admit a natural approximation as the Feynman amplitudes of a non-commutative quantum field theory, with the appropriate Feynman propagators weighting the lines of propagation, and that this effective field theory reduces to usual QFT in flat space in the no-gravity limit.