Counting ghosts in the "ghost-free" non-local gravity

TL;DR

This paper investigates a non-local quantum gravity theory claiming to be ghost-free at the classical level, but reveals it contains numerous unphysical states at the quantum level, including complex poles.

Contribution

It demonstrates that the non-local gravity theory, despite being ghost-free classically, possesses an infinite number of unphysical quantum states.

Findings

Presence of infinite massive unphysical states at the quantum level

Most unphysical states correspond to complex poles

Classical ghost-free property does not extend to quantum regime

Abstract

In the recently proposed non-local theory of quantum gravity one can avoid massive tensor ghosts at the tree level by a special choice of the non-local form factor between the two Ricci tensors. We show that at the quantum level this theory has an infinite amount of massive unphysical states, mostly corresponding to complex poles.