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.

We investigate the nature of additional scalar degrees of freedom contained in extended hybrid metric-Palatini gravity, outlining the emergence of two coupled dynamical scalar modes. In particular, we discuss the weak field limit of the theory, both in the static case and from a gravitational waves perspective. In the first case, performing an analysis at the lowest order of the parametrized post-Newtonian structure of the model, we stress the settling of Yukawa corrections to the Newtonian potential. In this respect, we show that one scalar field can have long range interactions and used in the principle for mimicking dark matter effects. Concerning the gravitational waves propagation, instead, we demonstrate that is possible to have well-defined physical degrees of freedom, provided by suitable constraints on model parameters. Moreover, the study of the geodesic deviation points out the presence of breathing and longitudinal polarizations due to these novel scalar waves, which on peculiar assumptions can give rise to beating phenomena during their propagation.

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We analyze the propagation of gravitational waves in a medium containing bounded subsystems ("molecules"), able to induce significant Macroscopic Gravity effects.We establish a precise constitutive relation between the average quadrupole and the amplitudes of a vacuum gravitational wave, via the geodesic deviation equation. Then we determine the modified equation for the wave inside the medium and the associated dispersion relation. A phenomenological analysis shows that anomalous polarizations of the wave emerge with an appreciable experimental detectability if the medium is identified with a typical galaxy. Both the modified dispersion relation (wave velocity less than the speed of light) and anomalous oscillations modes could be detectable by the incoming LISA or pulsar timing arrays experiments, having the appropriate size to see the concerned wavelengths (larger than the molecular size) and the appropriate sensitivity to detect the expected deviation from vacuum General Relativity.The Einsteinian theory of gravity offers a predictive tool to investigate the Universe structure on very different spatial scales, from the Hubble flow to the Solar system morphology [1][2][3][4].However, Einstein field equation in matter is approached by considering the space-time metric and the physical sources as continuous fields, having a differentiable (at least of class C 2 ) profile. Nonetheless, as it turns out looking at the real morphology of astrophysical systems [5][6][7][8], this notion of continuum is valid only on an average sense. In fact, the matter sources are typically characterized by a discrete nature, for instance, stars or galaxies are merely point-like sources, when treated on a scale much larger than their typical size. As a consequence, also the space-time geometry and the associated metric tensor acquire a discrete nature: clumpiness of the sources induces irregularities in the Einsteinian manifold.Therefore, an appropriate treatment of the implementation of Einstein equation to real astrophysical systems requires a suitable procedure for averaging both matter and geometry. It is easy to realize how the definition of an averaging procedure of the space-time be a non-trivial task, mainly due to the non-linearity of the gravitational interaction: the average of the Einstein tensor is not the Einstein tensor in the averaged metric, but a complicate set of correlation functions comes out [9][10][11].Another subtle question, strictly connected with the above considerations, is the existence of bounded subsystems within a matter medium, for instance the presence of binary systems and open clusters within the galaxy [12]. Such subsystems behave as real "gravitational molecules" and when the gravitational field interact with them, their structure is altered with a consequent gravitational backreaction. Thus, we see how it is, in general, necessary to deal with "Macroscopic Gravity" physics, in close analogy to what happens in the case of the electromagnetism within matter [13][14][15]. For relevant analyses of m...

In the context of the metric-affine Chern-Simons gravity endowed with projective invariance, we derive analytical solutions for torsion and nonmetricity in the homogeneous and isotropic cosmological case, described by a flat Friedmann-Robertson-Walker metric. We discuss in some details the general properties of the cosmological solutions in the presence of a perfect fluid, such as the dynamical stability and the emergence of big bounce points, and we examine the structure of some specific solutions reproducing de Sitter and power law behaviours for the scale factor. Then, we focus on first-order perturbations in the de Sitter scenario, and we study the propagation of gravitational waves in the adiabatic limit, looking at tensor and scalar polarizations. In particular, we find that metric tensor modes couple to torsion tensor components, leading to the appearance, as in the metric version of Chern-Simons gravity, of birefringence, characterized by different dispersion relations for the left and right circularized polarization states. As a result, the purely tensor part of torsion propagates like a wave, while nonmetricity decouples and behaves like a harmonic oscillator. Finally, we discuss scalar modes, outlining as they decay exponentially in time and do not propagate.