PT Journal
AU Moretti, F
   Bombacigno, F
   Montani, G
TI The Role of Longitudinal Polarizations in Horndeski and Macroscopic Gravity: Introducing Gravitational Plasmas
SO Universe
JI Universe
PY 2021
BP 496 - 28pp
VL 7
IS 12
DI 10.3390/universe7120496
LA English
DE gravitational waves; gauge-invariant method; Landau damping; macroscopic gravity
AB 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 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 with the background field play the role of cold ions in electromagnetic plasma.
ER