Effective Non-Minimal Quadratic Gravity and the Classical Cut-Off

TL;DR

This paper explores the classical cut-off limits of quadratic gravity as an effective field theory, considering non-minimal scalar couplings, symmetry breaking, and (Anti-)de Sitter backgrounds to extend the theory's validity.

Contribution

It introduces a method to determine the maximum classical cut-off in quadratic gravity, incorporating non-minimal scalar fields and cosmological constant effects.

Findings

Maximum classical cut-off depends on spin-2 ghost pathology.

Symmetry breaking influences the effective theory parameters.

(Anti-)de Sitter backgrounds can extend the cut-off limit.

Abstract

In this paper we consider Quadratic Gravity as a low energy effective field theory of some unknown UV-complete theory of gravity. Using the spin-2 ghost pathology that occurs in Quadratic Gravity, we derive maximum value for the classical cut-off of the low energy effective field theory in various configurations. We then add on higher order, bottom-up, effective terms. At each order we allow the introduction of non-minimally coupled scalar field terms, and we calculate the vacuum expectation value of the theory under an assumed $\mathbb{Z}_2$ symmetry breaking of the scalar field. This symmetry breaking leads us to introduce a constant in the action which is required to ensure that the physical spin-2 degree of freedom is massless in the flat space approximation, an implicit assumption we make throughout the work. We then consider the case in which we do not fine tune the introduced constant, which leads to an (Anti-)de Sitter solution about which we can then expand. The sign of the chosen cosmological constant is the major factor in determining whether the geometry is de Sitter or Anti-de Sitter. Then expanding about this geometry allows us to increase the theoretical classical cut-off as compared to the flat space case by an amount that is proportional to the free cosmological constant parameter.