Quantum (quadratic) gravity: replacing the massive tensor ghost with an inverted harmonic oscillator-like instability

TL;DR

This paper proposes a new interpretation of quadratic gravity where the problematic massive tensor ghost is replaced by a well-behaved inverted harmonic oscillator instability, offering a consistent quantum framework and new observational predictions.

Contribution

It introduces a novel quantization approach for quadratic gravity that transforms the tensor ghost into a healthy IHO-like mode, avoiding unitarity issues.

Findings

The extra spin-2 mode can be described as an inverted harmonic oscillator.

This mode remains off-shell and decoupled from matter, avoiding unitarity violations.

New bounds on primordial gravitational waves and tensor-to-scalar ratio are derived.

Abstract

The quadratic theory of gravity is the unique renormalizable theory of quantum gravity in 4 dimensions, as proved by K. S. Stelle in 1977. Over the decades, the theory has been understood to contain a massive tensor ghost, and several attempts have been made to evade its adverse effects by proposing new quantization prescriptions and interpretations. In this paper, we show that the additional spin--2 of quadratic gravity can be turned into a healthy inverted harmonic oscillator (IHO)-like instability, which can be quantized consistently with direct-sum quantum field theory (DQFT), which incorporates geometric superselection sectors. Such modes possess a well-defined quantum description yet do not admit a particle interpretation and are not part of the asymptotic spectrum, being characterized by hyperbolic evolution and spacelike momentum support. We argue that, as a consequence, the extra spin--2 degree of freedom remains off-shell and effectively decoupled from ordinary matter fields, avoiding unitarity violations in observable processes. We argue that this IHO instability is a prevalent feature of fundamental physics, whether it concerns quantum fields on curved spacetimes or the Higgs $\mathbb{Z}_2$ symmetry breaking in the Standard Model of particle physics. Thus, we demonstrate that our new understanding of quadratic gravity offers a fundamental pathway to quantum gravity and a safe beginning for the Universe. Furthermore, we derive key observational predictions of this construction in the view of primordial gravitational waves with new bounds on the tensor-to-scalar ratio and the parity asymmetric features on the large angular scales.