The role of longitudinal polarizations in Horndeski and macroscopic gravity: Introducing gravitational plasmas

TL;DR

This paper explores how longitudinal gravitational modes in Horndeski gravity interact with matter, leading to phenomena like gravitational plasmas and Landau damping, expanding understanding of gravitational wave propagation in media.

Contribution

It introduces the concept of gravitational plasma, analyzing longitudinal modes and their damping in Horndeski gravity, with a focus on matter interactions and effective massive gravitons.

Findings

Longitudinal modes induce additional polarizations in gravitational waves.

Effective massive gravitons can emerge from molecular media interactions.

Landau damping can occur in the gravitational sector, leading to gravitational plasma behavior.

Abstract

We discuss some general and relevant features of longitudinal gravitational modes in Horndeski gravity and their interaction with matter media. Adopting a gauge-invariant formulation we clarify how massive scalar and vector fields can induce additional transverse and longitudinal excitations, resulting in breathing, vector and longitudinal polarizations. We review, then, the interaction of standard gravitational waves with a molecular medium, outlining the emergence of effective massive gravitons, induced by the net quadrupole moment due to the molecule deformation. Finally, we investigate the interaction of the massive mode in Horndeski gravity with a noncollisional medium, showing that Landau damping phenomenon can occur in the gravitational sector as well. That allows us to introduce the concept of "gravitational plasma'', where inertial forces associated to the background field play the role of cold ions in electromagnetic plasma.