Massive, massless and ghost modes of gravitational waves from higher-order gravity

TL;DR

This paper explores higher-order gravity theories revealing additional massive and ghost gravitational wave modes, and assesses their detectability with current and future interferometric detectors like LISA.

Contribution

It introduces a formalism to analyze extra polarization modes in higher-order gravity and evaluates their potential detection in stochastic gravitational wave backgrounds.

Findings

Massive and ghost modes are present alongside standard gravitons.

Additional polarization modes could be detectable by LISA.

The formalism extends cross-correlation analysis for stochastic backgrounds.

Abstract

We linearize the field equations for higher order theories that contain scalar invariants other than the Ricci scalar. We find that besides a massless spin-2 field (the standard graviton), the theory contains also spin-0 and spin-2 massive modes with the latter being, in general, ghost modes. Then, we investigate the possible detectability of such additional polarization modes of a stochastic gravitational wave by ground-based and space interferometric detectors. Finally, we extend the formalism of the cross-correlation analysis, including the additional polarization modes, and calculate the detectable energy density of the spectrum for a stochastic background of the relic gravity waves that corresponds to our model. For the situation considered here, we find that these massive modes are certainly of interest for direct detection by the LISA experiment.