Primordial Gravitational Waves in Quadratic Gravity

TL;DR

This paper investigates primordial gravitational waves within quadratic gravity, revealing how the tensor spectrum is suppressed and consistent with inflationary predictions, while addressing the ghost problem inherent in fourth-order theories.

Contribution

The study reformulates quadratic gravity into a second-order form and calculates the tensor power spectrum, demonstrating suppression factors and restoring slow-roll consistency conditions.

Findings

Tensor spectrum suppression factor depends on Hubble rate and ghost mass

Restoration of slow-roll consistency condition in quadratic gravity

Discussion of ghost problem implications in the theory

Abstract

Quadratic gravity is a fourth-order (in derivatives) theory that can serve as an attractive upgrade to the standard description of gravity provided by General Relativity, thanks to its renormalizability and its built-in description of primordial inflation. We bring quadratic gravity into a second-order form by introducing an auxiliary tensor field and we consider the primordial tensor fluctuations (gravitational waves) in the theory around a Friedmann-Lema\^itre-Robertson-Walker background. After a canonical quantization of the perturbations, we calculate the tensor power spectrum in quasi de Sitter spacetime. We find that the spectral index $n_t$ and the amplitude $A_t$ of the tensor power spectrum are both suppressed by the factor $(1 + 2{\bf H}^2_*/m_\text{gh}^2)^{-1}$, where ${\bf H}_*$ is the Hubble rate at horizon exit and $m_\text{gh}$ is the mass of the spin-two ghost. This restores the slow-roll consistency condition familiar from single-field inflation models, where the tensor-to-scalar ratio $r$ is equal to $-8n_t$ in the lowest nontrivial order in the slow-roll approximation. We also discuss the well-known issue of the ghost problem in fourth-order theories and how it pertains to the results at hand.