Model In Scalar-tensor Theory Research Articles

Effective-one-body Hamiltonian in scalar-tensor gravity at third post-Newtonian order

We determine the general local-in-time effective-one-body (EOB) Hamiltonian for massless Scalar-Tensor (ST) theories at third post-Newtonian (PN) order. Starting from the Lagrangian derived in [Phys. Rev. D 99, 044047 (2019)], we map it to the corresponding ordinary Hamiltonian describing the two-body interaction in ST theories at 3PN level. Using a canonical transformation, we then map this onto an EOB Hamiltonian so as to determine the ST corrections to the 3PN-accurate EOB potentials $(A,B,Q_e)$ at 3PN. We then focus on circular orbits and compare the effect of the newly computed 3PN terms, also completed with finite-size and nonlocal-in-time contributions, on predictions for the frequency at the innermost stable circular orbit. Our results will be useful to build high-accuracy waveform models in ST theory, which could be used to perform precise tests against General Relativity using gravitational wave data from coalescing compact binaries.

Kantowski Sachs macroscopic body cosmological models in scalar tensor theories of gravitation

In this paper, we have obtained the field equations in scalar-tensor theories of gravitation proposed by Brans-Dicke [5] and Saez-Ballester [28] with the aid of Kantowski Sachs Space time in the presence of macroscopic body. Both theories' field equations can be solved definitively by using transformation. The present discussion explores into the expression involving energy density, energy flow vector, stress tensor, and specific physical features of both of these models.

Gravitational wave interactions in Λ3 models of dark energy

We argue that cubic order interactions between two scalar gravitons and one tensor graviton are ubiquitous in models of dark energy where the strong coupling scale is Λ3. These interactions can potentially provide efficient decay channels for gravitational waves. They can also lead to gradient instabilities of the scalar perturbations in the presence of large amplitude gravitational waves, e.g. those detected by LIGO/Virgo. In contrast with models in scalar-tensor theories, there is an infinite number of higher order interactions in generic Λ3 models, which make it difficult to predict the fate of these instabilities inferred from cubic order interactions.

Loop quantum cosmological dynamics of scalar-tensor theory in the Jordan frame

The effective dynamics of scalar-tensor theory (STT) in the Jordan frame is studied in the context of loop quantum cosmology with holonomy corrections. After deriving the effective Hamiltonian from the connection dynamics formulation, we obtain the holonomy-corrected evolution equations of STT on spatially flat Friedmann-Robterson-Walker background, which exhibit some interesting features unique to the Jordan frame of STT. In particular, the linear term of the cosine function appearing in the equations could lead to dynamics much different from the classical theory in the low-energy limit. In the latter part of this paper, we choose a particular model in STT---the Brans-Dicke theory to specifically illustrate these features. It is found that in Brans-Dicke theory the effective evolution equations can be classified into four different cases. Exact solutions of the Friedmann equation in terms of the internal time are obtained in these cases. Moreover, the solutions in terms of the proper time describing the late time evolution of the Universe are also obtained under certain approximation; in two cases the solutions coincide with the existing solutions in classical Brans-Dicke theory while in the other two cases the solutions do not.

Discrete cosmological models in the Brans–Dicke theory of gravity

We consider the problem of building inhomogeneous cosmological models in scalar-tensor theories of gravity. This starts by splitting the field equations of these theories into constraint and evolution equations, and then proceeds by identifying exact solutions to the constraints. We find exact, closed form expressions for geometries that correspond to the initial data for cosmological models containing regular arrays of point-like masses. These solutions extend similar methods that have recently been applied to Einstein’s equations, and provides sufficient initial conditions to perform numerical integration of the evolution equations. We use our new solutions to study the effects of inhomogeneity in cosmologies governed by scalar-tensor theories of gravity, including the spatial inhomogeneity allowed in Newton’s constant. Finally, we compare our solutions to their general relativistic counterparts, and investigate the effect of changing the coupling constant between the scalar and tensor degrees of freedom.

Quasi-Newtonian Cosmological Models in Scalar-Tensor Theories of Gravity

In this contribution, classes of shear-free cosmological dust models with irrotational fluid flows will be investigated in the context of scalar-tensor theories of gravity. In particular, the integrability conditions describing a consistent evolution of the linearised field equations of quasi-Newtonian universes are pre-sented. We also derive the covariant density and velocity propagation equations of such models and analyse the corresponding solutions to these perturbation equations.

Rapidly rotating neutron stars with a massive scalar field—structure and universal relations

We construct rapidly rotating neutron star models in scalar-tensor theories with a massive scalar field. The fact that the scalar field has nonzero mass leads to very interesting results since the allowed range of values of the coupling parameters is significantly broadened. Deviations from pure general relativity can be very large for values of the parameters that are in agreement with the observations. We found that the rapid rotation can magnify the differences several times compared to the static case. The universal relations between the normalized moment of inertia and quadrupole moment are also investigated both for the slowly and rapidly rotating cases. The results show that these relations are still EOS independent up to a large extend and the deviations from pure general relativity can be large. This places the massive scalar-tensor theories amongst the few alternative theories of gravity that can be tested via the universal I-Love-Q relations.

Dynamical vacuum energy and phantom divide-line cross in scalar cosmology

In this paper, we study cosmological models in scalar tensor theory of gravity a la Brans–Dicke with a dynamical equation of state parameter, a dynamical effective cosmological constant and a Hubble parameter varying like \( H = \varepsilon t^{s} + H_{0} \) with \( \varepsilon \) and \( H_{0} \) are real positive parameters and \( s \) is some pending power index. It is observed that the universe is accelerating in time and it is dominated by a decreasing energy density and a vacuum energy density. However the vacuum energy density is increasing logarithmically at early epoch of time and then decaying toward its present positive value at late-time epoch. For specific choices of the free parameters in the theory, it is observed that the phantom divide line is crossed in the presence of a cosmological constant and the universe expands asymptotically a la de-Sitter at very large time. Additional features are discussed as well.

A New Class of Bianchi Type-I Cosmological Models in Scalar-Tensor Theory of Gravitation and Late Time Acceleration

A new class of a spatially homogeneous and anisotropic Bianchi type-I cosmological models of the universe for perfect fluid distribution within the framework of scalar-tensor theory of gravitation proposed by Saez and Ballester (Phys. Lett. 113:467, 1986) is investigated. To prevail the deterministic solutions we choose the different scale factors which yield time-dependent deceleration parameters (DP) representing models which generate a transition of the universe from the early decelerated phase to the recent accelerating phase. Three different physically viable models of the universe are obtained in which their anisotropic solutions may enter to some isotropic inflationary era. The modified Einstein's field equations are solved exactly and the models are found to be in good concordance with recent observations. Some physical and geometric properties of the models are also discussed.

Higher dimensional effective stiff fluid cosmological models in scalar tensor theory

Five dimensional LRS Bianchi type-I effective stiff fluid cosmological models in scalar tensor theory of gravitation proposed by Saez and Ballester (Phys. Lett. A 113:467, 1986) are constructed. Further, some physical and geometrical features of these models are discussed.

Early Universe in Scalar-Tensor Theory

We consider the flat Robertson–Walker model in scalar-tensor theory proposed by Lau and Prokhovnik. In this model, the field equations are solved by using “gamma-law” form of equation of state p=(γ−1)ρ, where the adiabatic parameter ‘gamma’ (γ) varies continuously as the universe expands. Our aim is to study how the adiabatic parameter γ should vary so that in the course of its evolution the universe goes through a transition from an inflationary to a radiation-dominated phase. A unified one parameter function of γ has been considered to describe the two early phases of evolution of universe. The solutions show the power-law expansion and cosmological constant is found to be positive and decreasing function of cosmic time. The solutions are compatible with the Dirac’s large number hypothesis. The deceleration parameter has been presented in a unified manner in terms of scale factor, which describes the inflation of the model. The nature of singularity and the physical properties have been discussed in details.

Cosmological models in scalar tensor theories of gravity and observations: a class of general solutions

Aims. We study cosmological models in scalar tensor theories of gravity with power-law potentials as models of an accelerating universe. Methods. We consider cosmological models in scalar tensor theories of gravity that describe an accelerating universe and study a family of inverse power-law potentials, for which exact solutions of the Einstein equations are known. We also compare theoretical predictions of our models with observations. For this we use the following data: the publicly available catalogs of type Ia supernovae and high redshift gamma ray bursts, the parameters of large-scale structure determined by the 2-degree Field Galaxy Redshift Survey (2dFGRS), and measurements of cosmological distances based on the Sunyaev-Zel’dovich effect, among others. Results. We present a class of cosmological models that describe the evolution of a homogeneous and isotropic universe filled with dust-like matter and a scalar field that is non minimally-coupled to gravity. We show that this class of models depends on three parameters: V0 – the amplitude of the scalar field potential, � H0 – the present value of the Hubble constant, and a real parameter s that determines the overall evolution of the universe. It turns out that these models have a very interesting feature naturally producing an epoch of accelerated expansion. We fix the values of these parameters by comparing predictions of our model with observational data. It turns out that our model is compatible with the presently available observational data.

Asymptotic behavior of cosmological models in scalar-tensor theories of gravity

We study the qualitative properties of cosmological models in scalar-tensor theories of gravity by exploiting the formal equivalence of these theories with general relativity minimally coupled to a scalar field under a conformal transformation and field redefinition. In particular, we investigate the asymptotic behaviour of spatially homogeneous cosmological models in a class of scalar-tensor theories which are conformally equivalent to general relativistic Bianchi cosmologies with a scalar field and an exponential potential whose qualitative features have been studied previously. Particular attention is focussed on those scalar-tensor theory cosmological models, which are shown to be self-similar, that correspond to general relativistic models that play an important r\^{o}le in describing the asymptotic behaviour of more general models (e.g., those cosmological models that act as early-time and late-time attractors).

Inflationary solutions for anisotropic model in scalar-tensor theory

In this paper we have studied B-D theory and general scalar tensor theory of Gravitation for anisotropic cosmological model in the false vacuum state. The possibility of both exponential inflation and power function inflation are examined for constant or variable coupling parameter ω. Also asymptotic limit of the scalar field and ω are discussed.

GENERAL CLASS OF SCALAR-TENSOR THEORIES: A REVIEW

The general class of scalar-tensor theories is discussed in which, in contrast to Brans-Dicke theory, the coupling parameter ω is not a constant but a function of the scalar field ϕ. Some other generalizations of the Brans-Dicke theory have also been discussed in brief. The physical aspects of these theories have been dealt. The cosmological models in scalar-tensor theories have been discussed in detail. Finally some open problems in these theories have been pointed out.