Academic literature on the topic 'Horndeski action'

We study the quantum cosmology of a quadratic fR theory with a FRW metric, via one of its equivalent Horndeski type actions, where the dynamic of the scalar field is induced. The classical equations of motion and the Wheeler-DeWitt equation, in their exact versions, are solved numerically. There is a free parameter in the action from which two cases follow: inflation + exit and inflation alone. The numerical solution of the Wheeler-DeWitt equation depends strongly on the boundary conditions, which can be chosen so that the resulting wave function of the universe is normalizable and consistent with Hermitian operators.

Abstract In this work we study the cosmological perturbations in Bahamonde-Dialektopoulos-Levi Said (BDLS) theory, i.e. the teleparallel analog of Horndeski gravity. In order to understand the evolution of structure in a cosmological model, it is necessary to study its cosmology not only in the background but also perturbatively. Both Horndeski and its teleparallel analog have been analyzed a lot in the literature, but in order to study them quantitatively, we need to know their cosmological perturbations. That is why, we study here the scalar-vector-tensor decomposition of the theory and we also express the so-called alpha parameters in terms of the arbitrary functions of the theory, that designate the deviation from the ΛCDM model. We have explored tensor, vector and scalar perturbation of the action up to second order, which drastically opens up new possibilities on searches in the parameter space of scalar-tensor theories in the context of observations.

Abstract Horndeski's vector-tensor (HVT) gravity is described by a Lagrangian in which the field strength fμν = ∂μAν-∂νAμ of a vector field Aμ interacts with a double dual Riemann tensor Lμναβ in the form βLμναβ Fμν Fαβ , where β is a constant. In Einstein-Maxwell-HVT theory, there are static and spherically symmetric black hole (BH) solutions with electric or magnetic charges, whose metric components are modified from those in the Reissner-Nordström geometry. The electric-magnetic duality of solutions is broken even at the background level by the nonvanishing coupling constant β. We compute a second-order action of BH perturbations containing both the odd- and even-parity modes and show that there are four dynamical perturbations arising from the gravitational and vector-field sectors. We derive all the linear stability conditions associated with the absence of ghosts and radial/angular Laplacian instabilities for both the electric and magnetic BHs. These conditions exhibit the difference between the electrically and magnetically charged cases by reflecting the breaking of electric-magnetic duality at the level of perturbations. In particular, the four angular propagation speeds in the large-multipole limit are different from each other for both the electric and magnetic BHs. This suggests the breaking of eikonal correspondence between the peak position of at least one of the potentials of dynamical perturbations and the radius of photon sphere. For the electrically and magnetically charged cases, we elucidate parameter spaces of the HVT coupling and the BH charge in which the BHs without naked singularities are linearly stable.

A five-dimensional braneworld cosmological model in general scalar–tensor action that is comprised of various Horndeski Lagrangians is considered. The Friedmann equations in the case of strongly and weakly coupled [Formula: see text] Horndeski Lagrangians have been obtained. The strongly coupled [Formula: see text] model produces the Cardassian term [Formula: see text] with [Formula: see text], which can serve as an alternative explanation for the accelerated expansion phase of the universe. Furthermore, the latest combined observational facts from BAO, CMB, SNIa, [Formula: see text] and [Formula: see text] value observation suggest that the [Formula: see text] term lies quite close to the constrained value. On the other hand, the weakly coupled [Formula: see text] case has several new correction terms which are omitted in the braneworld Einstein–Hilbert model, e.g. the cubic [Formula: see text] and the dark radiation–matter interaction term [Formula: see text]. Furthermore, this model provides a cosmological constant constructed from the bulk scalar field, requires no brane tension and supports the big bang nucleosynthesis (BBN) constraint naturally.