Scalar modes in extended hybrid metric-Palatini gravity: Weak field phenomenology

Abstract

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.

Gravitational Landau damping for massive scalar modes

“…That guarantees the existence of the imaginary part for dielectric functions, stemming from the integration around poles and evaluable with the residue theorem, ultimately responsible for the appearing of the damping coefficient (25). We note, however, that condition v p < 1 is not a priori ensured and its attainability has to be indeed verified by calculating ω r from (24).…”

“…Although this effect is conceptually very relevant, it is in reality very small and associated to peculiar wavelengths. More intriguing, therefore, is to look at the large number of modified theories of gravity which allow for the emergence of an additional massive scalar mode, as for instance Horndeski gravity [15][16][17][18], hybrid metric-Palatini approaches [19][20][21][22][23][24][25] or massive gravity [26][27][28][29]. In this case, in fact, together with a breathing polarization in the transverse plane, scalar fields are responsible for a longitudinal polarization as well, that we expect could interact with particles of the medium.…”

Spherically Symmetric Black Holes and Wormholes in Hybrid Metric-Palatini Gravity

“…Lastly, it is of interest to extend the present study to the so-called extended HMPG with functions f (R, R) of two curvatures [12,13], this work is in progress.…”

“…A detailed exposition of HMPG and its achievements may be found in the reviews [10,11]. An even more general theory, containing an arbitrary function of two variables, R and R, is presented in [12], see a recent further work on this theory in [13]. This paper is devoted to a study of exact static, spherically symmetric solutions of HMPG in the simplest case where F (R) ∝ R.This version of HMPG is equivalent to a scalar-tensor theory (STT) of gravity [4,10] with zero scalar field potential V (φ), and this theory coincides with GR with a source in the form of a phantom conformally invariant scalar field.…”

Thick brane structures in generalized hybrid metric-Palatini gravity

“…The linear hybrid metric-Palatini theory can be further generalized, where the gravitational action depends on a general function of both the metric and Palatini curvature scalars [40]. This extension has also received attention with a plethora of applications, namely, cosmological solutions [41,42], weak-field phenomenology [43], wormhole [44] and black hole [45] solutions. In this work, we consider the possibility to study braneworld structures in the generalized hybrid metric-Palatini gravity.…”