Causal Structure for Generalized Spinfoams

TL;DR

This paper explores the incorporation of causality into the generalized EPRL spinfoam model, proposing a new causal structure definition, a causal vertex amplitude, and analyzing its implications for semiclassical limits and future research directions.

Contribution

It introduces a formal definition of causal structure on 2-complexes, a causal vertex amplitude, and analyzes its asymptotic behavior within the generalized EPRL spinfoam framework.

Findings

Defined causal structure on arbitrary 2-complexes.

Introduced a causal vertex amplitude generalizing previous models.

Analyzed asymptotic behavior and implications for semiclassical analysis.

Abstract

We investigate the role of causality in the generalized EPRL spinfoam model introduced by Kaminski, Kisielowski, and Lewandowski (EPRL-KKL). We first propose a definition of causal structure on an arbitrary 2-complex and analyze how causal orientations can be assigned to its 1-skeleton and 2-skeleton. We identify a criterion that determines when an orientation of the 2-skeleton induces a consistent causal structure on the 1-skeleton. This characterization naturally involves tools from graph theory and linear algebra over the Galois field $\mathbb{F}_2$. Using these results, we introduce a causal vertex amplitude that generalizes previous proposals by Bianchi--Dussaud and Bianchi--Chen--Gamonal. We study its asymptotic behavior and revisit the interpretation of the two critical points appearing in the semiclassical analysis of the EPRL and EPRL-KKL spinfoam models. Finally, we discuss several directions in which the causal amplitude introduced here may provide new insights for future developments, like the cosine problem and the presence of irregular light cone structures.