Cycles of interactions in multi-gravity theories

TL;DR

This paper investigates multi-gravity theories with cyclic interactions, demonstrating that multi-metric versions contain ghost instabilities, unlike the ghost-free multi-vielbein models, impacting their theoretical consistency and applications.

Contribution

It provides the first explicit demonstration of ghost instabilities in cyclic multi-metric theories and explains their absence in multi-vielbein formulations.

Findings

Ghosts are present in multi-metric theories with cycles, shown by higher derivative terms.

Multi-vielbein theories avoid ghosts due to different structural properties.

Presence of ghosts affects the viability of dimensional deconstruction approaches.

Abstract

In this paper we study multi-gravity (multi-metric and multi-vielbein) theories in the presence of cycles of interactions (cycles in the so-called `theory graph'). It has been conjectured that in multi-metric theories such cycles lead to the introduction of a ghost-like instability, which, however, is absent in the multi-vielbein version of such theories. In this paper we answer this question in the affirmative by explicitly demonstrating the presence of the ghost in such multi-metric theories in the form of dangerous higher derivative terms in the decoupling limit Lagrangian; we also explain why these terms are absent in the vielbein version of these theories. Finally we discuss the ramifications of our result on the dimensional deconstruction paradigm, which would seek an equivalence between such theories and a truncated Kaluza-Klein theory, and find that the impediment to taking the continuum limit due to a low strong-coupling scale is exacerbated by the presence of the ghost, when these theories are constructed using metrics.