Scalar-tensor Model Of Dark Energy Research Articles

The speed of gravitational waves and power-law solutions in a scalar-tensor model

One of the most relevant solutions in any cosmological model concerning the evolution of the universe is the power-law solution. For the scalar-tensor model of dark energy with kinetic and Gauss Bonnet couplings, it is shown that we can conserve the power-law solution and at the same time meet the recent observational bound on the speed of gravitational waves. In the FRW background the anomalous contribution to the speed of gravitational waves, coming from the kinetic and Gauss-Bonnet couplings, cancel each other for power-law solutions. It is shown that by simple restriction on the model parameters we can achieve a non-time-dependent cancellation of the defect in the velocity of the gravitational waves. The model can realize the cosmic expansion with contributions from the kinetic and Gauss-Bonnet couplings of the order of O(1) to the dark energy density parameter. The results are valid on the homogeneous FRW background and the limitations of the approach are discussed.

Dark Energy After GW170817: Dead Ends and the Road Ahead.

Multimessenger gravitational-wave (GW) astronomy has commenced with the detection of the binary neutron star merger GW170817 and its associated electromagnetic counterparts. The almost coincident observation of both signals places an exquisite bound on the GW speed |c_{g}/c-1|≤5×10^{-16}. We use this result to probe the nature of dark energy (DE), showing that a large class of scalar-tensor theories and DE models are highly disfavored. As an example we consider the covariant Galileon, a cosmologically viable, well motivated gravity theory which predicts a variable GW speed at low redshift. Our results eliminate any late-universe application of these models, as well as their Horndeski and most of their beyond Horndeski generalizations. Three alternatives (and their combinations) emerge as the only possible scalar-tensor DE models: (1)restricting Horndeski's action to its simplest terms, (2)applying a conformal transformation which preserves the causal structure, and (3)compensating the different terms that modify the GW speed (to be robust, the compensation has to be independent on the background on which GWs propagate). Our conclusions extend to any other gravity theory predicting varying c_{g} such as Einstein-Aether, Hořava gravity, Generalized Proca, tensor-vector-scalar gravity (TEVES), and other MOND-like gravities.

Dark energy from Gauss-Bonnet and nonminimal couplings

We consider a scalar-tensor model of dark energy with Gauss-Bonnet and non-minimal couplings. Exact cosmological solutions were found in absence of potential, that give equations of state of dark energy consistent with current observational constraints, but with different asymptotic behaviors depending on the couplings of the model. A detailed reconstruction procedure is given for the scalar potential and the Gauss-Bonnet coupling for any given cosmological scenario. Particularly, we consider conditions for the existence of a variety of cosmological solutions with accelerated expansion, including quintessence, phantom, de Sitter, Little Rip. For the case of quintessence and phantom we have found a scalar potential of the Albrecht-Skordis type, where the potential is an exponential with a polynomial factor.

QUINTESSENTIAL AND PHANTOM POWER-LAW SOLUTIONS IN SCALAR TENSOR MODEL OF DARK ENERGY

We consider a scalar-tensor model of dark energy with kinetic and Gauss–Bonnet (GB) couplings. We study the conditions for the existence of quintessential and phantom power-law expansion, and also analyze these conditions in the absence of a potential (closely related to string theory). A mechanism to avoid the Big Rip singularity in various asymptotic limits of the model has been studied. It was found that the kinetic and GB couplings might prevent the Big Rip singularity in a phantom scenario. The autonomous system for the model has been used to study the stability properties of the power-law solution, and the center manifold analysis was used to treat zero eigenvalues.

Exact solutions in a scalar-tensor model of dark energy

We consider a model of scalar field with non minimal kinetic and Gauss Bonnet couplings as a source of dark energy. Based on asymptotic limits of the generalized Friedmann equation, we impose restrictions on the kinetic an Gauss-Bonnet couplings. This restrictions considerable simplify the equations, allowing for exact solutions unifying early time matter dominance with transitions to late time quintessence and phantom phases. The stability of the solutions in absence of matter has been studied.

Cosmic expansion and growth histories in Galileon scalar-tensor models of dark energy

We study models of late-time cosmic acceleration in terms of scalar-tensor theories generalized to include a certain class of nonlinear derivative interaction of the scalar field. The nonlinear effect suppresses the scalar-mediated force at short distances to pass solar-system tests of gravity. It is found that the expansion history until today is almost indistinguishable from that of the $\ensuremath{\Lambda}\mathrm{CDM}$ model or some (phantom) dark energy models, but the fate of the universe depends clearly on the model parameter. The growth index of matter density perturbations is computed to show that its past asymptotic value is given by $9/16$, while the value today is as small as 0.4.

Remarks on generalized scalar-tensor models of dark energy

The generalized scalar-tensor models with Lagrangian $F(\ensuremath{\phi},R)\ensuremath{-}U(\ensuremath{\phi})(\ensuremath{\nabla}\ensuremath{\phi}{)}^{2}$ are considered. It is shown that the phantom-divide-line crossing and the deceleration to acceleration transition generally occur in these models. Two specific examples, the coupled quintessence model and the Brans-Dicke model, are considered. For the first example, it is shown that for the models with $\ensuremath{\xi}>3/16$, the $\ensuremath{\omega}=\ensuremath{-}1$ transition exists. This is verified numerically for some special cases. For the Brans-Dicke model, it is shown that the transition does not occur, a result which can be verified by using the exact solution of this model. Finally the contribution of quantum effects on these phenomena is investigated. It is shown that for some special cases where the $\ensuremath{\omega}=\ensuremath{-}1$ transition is classically forbidden, the quantum effects can induce transition. The $\ensuremath{\xi}=1/6$ of the coupled quintessence model is an example of this. The quantum effects are described via the account of conformal anomaly.

The growth of matter perturbations in f(R) models

We consider the linear growth of matter perturbations on low redshifts in some f(R) dark energy (DE) models. We discuss the definition of dark energy (DE) in these models and show the differences with scalar-tensor DE models. For the f(R) model recently proposed by Starobinsky we show that the growth parameter γ0≡γ(z = 0) takes the value γ0 ≃ 0.4 for Ωm,0 = 0.32 and γ0 ≃ 0.43 for Ωm,0 = 0.23, allowing for a clear distinction from ΛCDM. Though a scale-dependence appears in the growth of perturbations on higher redshifts, we find no dispersion for γ(z) on low redshifts up to z ∼ 0.3, γ(z) is also quasi-linear in this interval. At redshift z = 0.5, the dispersion is still small with Δγ ≃ 0.01.As for some scalar-tensor models, we find here too a large value for γ′0≡(dγ/dz)(z = 0), γ′0 ≃ −0.25 for Ωm,0 = 0.32 and γ′0 ≃ −0.18 for Ωm,0 = 0.23. These values are largely outside the range found for DE models in General Relativity (GR). This clear signature provides a powerful constraint on these models.

Constraints on scalar-tensor models of dark energy from observational and local gravity tests

We construct a family of viable scalar-tensor models of dark energy (DE) which possess a phase of late-time acceleration preceded by a standard matter era, while at the same time satisfying the local gravity constraints (LGC). The coupling $Q$ between the scalar field and the nonrelativistic matter in the Einstein frame is assumed to be constant in our scenario, which is a generalization of $f(R)$ gravity theories corresponding to the coupling $Q=\ensuremath{-}1/\sqrt{6}$. We find that these models can be made compatible with local gravity constraints even when $|Q|$ is of the order of unity through a chameleon mechanism, if the scalar-field potential is chosen to have a sufficiently large mass in the high-curvature regions. We show that these models generally lead to the divergence of the equation of state of DE, which occurs at smaller redshifts as the deviation from the $\ensuremath{\Lambda}\mathrm{CDM}$ model becomes more significant. We also study the evolution of matter density perturbations and employ them to place bounds on the coupling $|Q|$ as well as model parameters of the field potential from observations of the matter power spectrum and the cosmic microwave background (CMB) anisotropies. We find that, as long as $|Q|$ is smaller than the order of unity, there exist allowed parameter regions that are consistent with both observational and local gravity constraints.

The growth of matter perturbations in some scalar–tensor DE models

We consider asymptotically stable scalar–tensor dark energy (DE) models for which the equation of stateparameter wDE tends to zero in the past. The viable models are of the phantom type today: however, thisphantomness is milder than in general relativity if we take into account the varyinggravitational constant when dealing with the SNIa data. We study further the growth ofmatter perturbations and we find a scaling behaviour on large redshifts which couldprovide an important constraint. In particular, the growth of matter perturbations on largeredshifts in our scalar–tensor models is close to the standard behaviour , while it is substantially different for the best-fit model in general relativity for the sameparametrization of the background expansion. As for the growth of matter perturbations onsmall redshifts, we show that in these models the parameter can take absolute values much larger than in models inside general relativity. Assuming a constantγ when γ′0 is large would lead to a poor fit of the growth functionf. This provides another characteristic discriminative signature for these models.

No realistic wormholes from ghost-free scalar-tensor phantom dark energy

It is proved that no wormholes can be formed in viable scalar-tensor models of dark energy admitting its phantom-like ($w < -1$) behaviour in cosmology, even in the presence of electric or magnetic fields, if the non-minimal coupling function $f(\Phi)$ is everywhere positive and the scalar field $\Phi$ itself is not a ghost. Some special static, spherically symmetric wormhole solutions may exist if $f(\Phi)$ is allowed to reach zero or to become negative, so that the effective gravitational constant becomes negative in some region making the graviton a ghost. If $f$ remains non-negative, such solutions require severe fine tuning and a very peculiar kind of model. If $f < 0$ is allowed, it is argued (and confirmed by previous investigations) that such solutions are generically unstable under non-static perturbations, the instability appearing right near transition surfaces to negative $f$.