Gravitational Landau damping for massive scalar modes

Abstract

We establish the possibility of Landau damping for gravitational scalar waves which propagate in a non-collisional gas of particles. In particular, under the hypothesis of homogeneity and isotropy, we describe the medium at the equilibrium with a Jüttner–Maxwell distribution, and we analytically determine the damping rate from the Vlasov equation. We find that damping occurs only if the phase velocity of the wave is subluminal throughout the propagation within the medium. Finally, we investigate relativistic media in cosmological settings by adopting numerical techniques.

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

“…We stress that the observation of the multi-messenger signals associated to GW170817-GRB170817A [4,122] put severe constraints on gravitational waves speed propagation in vacuum, restricting the form of the terms actually feasible of (29) (see [19,[123][124][125] for more details and implications in cosmological settings). With respect to [89] we are not considering the coupling with the matter (see the discussion in section III or [81]), since we are only interested in the phenomenology of gravitational wave polarizations on the sphere of test masses, i.e. we are neglecting at all the backreaction due to the displacement of the medium particles.…”

“…The number of radiating degrees of freedom and the dispersion suffered by the macroscopic radiation allow to establish an effective analogy with the propagation of massive gravitational waves, as described by the Fierz-Pauli theory [147], the linear approximation of the full theory of massive gravity which has been introduced only in recent years [148]. In the second work [89] it is analyzed the interaction between gravitational waves from Horndeski theories and a non-collisional medium of massive particles. The kinetic treatment enforced outlines the presence of Landau damping for the scalar massive mode, when an inequality between theory parameters and physical quantities describing the medium is satisfied.…”

“…Furthermore, we will study the interaction of gravitational waves with a material medium, having the morphology of a molecular array or plasma-like features. In this respect, we will introduce some new subtle concepts like "Macroscopic Gravity Theory" [88] or "Gravitational Plasma" [89]. In more detail, regarding the investigation of the gravitational modes in Horndeski gravity, we review and partially extend the gauge-invariant formulation of the linear theory [90], in order to unequivocally identify the physical degrees of freedom.…”

“…by setting the mass of the particles, their volume density and the temperature, it is possible to give a quantitative estimate of the damping suffered by scalar waves of a certain spatial size. For instance, considering the interaction between the cosmological dark matter medium and scalar radiation at a redshift z ≈ 2000, we calculate from our formulae an estimated maximum damping of roughly ω i 10 −16 Hz (see [89] for details). It is interesting to notice that in [151] a similar value for the damping rate has been directly inferred from observational data, i.e.…”

Polarization Modes of Gravitational Waves in Palatini Horndeski theory

“…And in the massless limit, only the breathing mode survives. In the metric Horndeski theory, the probability of Landau damping for gravitational scalar waves [28] and the polarization modes of gravitational waves under arbitrary background [29] were also studied. Different from general relativity, Horndeski theory in Palatini formalism is different from that in metric one.…”