Cosine problem in EPRL/FK spinfoam model

TL;DR

This paper investigates the classical limits of the EPRL/FK spinfoam model of quantum gravity with matter, revealing multiple effective actions including Einstein-Hilbert and antigravity, due to cosine-like asymptotics.

Contribution

It demonstrates the emergence of multiple classical limits in the EPRL/FK model caused by cosine asymptotics and discusses potential methods to select the physically relevant one.

Findings

Multiple classical effective actions including Einstein-Hilbert and antigravity.

Cosine-like asymptotics lead to multiple classical limits.

Discussion of strategies to eliminate unwanted classical solutions.

Abstract

We calculate the classical limit effective action of the EPRL/FK spinfoam model of quantum gravity coupled to matter fields. By employing the standard QFT background field method adapted to the spinfoam setting, we find that the model has many different classical effective actions. Most notably, these include the ordinary Einstein-Hilbert action coupled to matter, but also an action which describes antigravity. All those multiple classical limits appear as a consequence of of the fact that the EPRL/FK vertex amplitude has cosine-like large spin asymptotics. We discuss some possible ways to eliminate the unwanted classical limits.