Framework Of Brans-Dicke Theory Research Articles

A Study of Evolution of Cosmological Parameters Based on a Dark Energy Model in the Framework of Brans-Dicke Gravity

The objective of the present study is to find the characteristics of evolution of a homogeneous and isotropic universe in the framework of Brans-Dicke (BD) theory of gravity. FLRW space-time, with zero spatial curvature, has been used to obtain BD field equations. Scale factor and Hubble parameter have been obtained from an ansatz for the deceleration parameter, assumed on the basis of its property of signature flip indicating a change of phase from deceleration to acceleration. Validation of the model has been achieved by a suitable parametrization of that ansatz. Expressions for energy density, pressure, equation of state (EoS) parameter, cosmological constant, gravitational constant have been derived and depicted graphically. The gravitational constant is found to decrease with time at a gradually decreasing rate. The Hubble parameter, deceleration parameter and energy density decrease with time, which is in agreement with many other studies. The value of the EoS parameter at the present epoch is negative, and it becomes more negative with time. The cosmological constant increases very rapidly in the early universe from negative to smaller negative values, becoming positive finally, with a much slower change thereafter. A cosmographic and a geometrical analysis have been carried out. It is observed that a gradual transition takes place from a regime of quintessence to phantom dark energy. An important finding of this study is that the signature flip of the deceleration parameter takes place almost simultaneously with the signature flip of the cosmological constant, implying a connection between accelerated expansion and dark energy, which is represented here by the cosmological constant. Unlike the common practice of using arbitrary units, proper SI units for all measurable quantities have been used. This theoretical investigation provides the reader with a simple method to formulate models in the framework of BD theory.

Bianchi type-V transitioning model in Brans–Dicke theory with observational constraints

In this paper, we have examined the viability of the Bianchi type-V universe in Brans–Dicke (BD) theory of gravitation. We have discussed the interacting and non-interacting scenarios between dark matter (DM) and dark energy (DE) of the derived universe within the framework of BD theory. CCA technique has been applied to constrain the model parameters using 46 values of observational Hubble data (OHD), Pantheon data (the latest compilation of SNIa with 40 binned in the redshift range [Formula: see text] and their combined datasets. We establish an exact solution of the field equations to derive the dynamics of the derived universe and the obtained results are found to agree with the observations. We also noted a distinctive change in the sign of the deceleration parameter from positive to negative, as well as the presence of a transition red-shift exists. Using various observational data points, the evolution trajectories for [Formula: see text] diagnostic planes are shown to understand the geometrical behavior of the Bianchi-V model. Some physical properties of the universe are also discussed. It is also worth noting that the conclusions of the cosmological parameter are consistent with modern observational data.

Matter creation cosmology in Brans-Dicke theory: Observational tests and thermodynamic analysis

A matter-dominated model with gravitationally induced matter creation is proposed in the framework of Brans-Dicke theory. We obtain the main cosmological functions such as the scale factor of the Universe, the Hubble expansion rate, and the deceleration parameter analytically. We explore the viability of the model to explain the present accelerated expansion of the Universe. In this scenario, the present cosmic acceleration is supposed to be driven only by the negative creation pressure associated with the matter component. The evolution of such a model is tested by statistical analysis of the latest SNe, OHD, and BAO probes. We study and plot the trajectories of the evolution of the Universe with the best estimated values of the model parameters. It is observed that the expanding Universe begins with a big bang followed by a smooth transition from the decelerated phase to the accelerated phase. The ages of the Universe obtained by $\mathrm{SNe}+\mathrm{OHD}$ and $\mathrm{SNe}+\mathrm{OHD}+\mathrm{BAO}$ in this model are in good agreement with the age predicted by the $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ model. We analyze the model with a statefinder diagnostic and find that the model is different from the $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ model but approaches $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ in the future. The model shows quintessence behavior.

An Analysis of the Accelerated Expansion of the Universe in the Framework of Brans-Dicke Theory of Gravitation

An Analysis of the Accelerated Expansion of the Universe in the Framework of Brans-Dicke Theory of Gravitation

Time Dependence of the Proportions of Matter and Dark Energy of the Universe in the Framework of Brans-Dicke Theory

Time Dependence of the Proportions of Matter and Dark Energy of the Universe in the Framework of Brans-Dicke Theory

Holographic Dark Energy in Brans-Dicke Theory with Logarithmic Form of Scalar Field

In this paper, an interacting holographic dark energy model with Hubble horizon as an infra-red cut-off is considered in the framework of Brans-Dicke theory. We assume the Brans-Dicke scalar field as a logarithmic form ϕ = ϕ 0 l n(α + β a), where a is the scale factor, α and β are arbitrary constants, to interpret the physical phenomena of the Universe. The equation of state parameter w h and deceleration parameter q are obtained to discuss the dynamics of the evolution of the Universe. We present a unified model of holographic dark energy which explains the early time acceleration (inflation), medieval time deceleration and late time acceleration. It is also observed that w h may cross the phantom divide line in the late time evolution. We also discuss the cosmic coincidence problem. We obtain a time-varying density ratio of holographic dark energy to dark matter which is a constant of order one ( $r\sim \mathcal {O}(1)$ ) during early and late time evolution, and may evolve sufficiently slow at present time. Thus, the model successfully resolves the cosmic coincidence problem.

Effects of anisotropy on interacting ghost dark energy in Brans-Dicke theories

By interacting ghost dark energy (ghost DE) in the framework of Brans-Dicke theory, a spatially homogeneous and anisotropic Bianchi type I Universe has been studied. For this purpose, we use the squared sound speed $c_s^2$ whose sign determines the stability of the model. As well as we probe observational constraints on the ghost dark energy models as the unification of dark matter and dark energy by using the latest observational data. In order to do so, we focus on observational determinations of the Hubble expansion rate (namely, the expansion history) $H(z)$. After that we evaluate the evolution of the growth of perturbations in the linear regime for both ghost DE and Brans-Dicke theory and compare the results with standard FRW and $\Lambda$CDM models. We display the effects of the anisotropy on the evolutionary behavior the ghost DE models where the growth rate is higher in this models. Eventually the growth factor for the $\Lambda$CDM Universe will always fall behind the ghost DE models in an anisotropic Universe

Bulk Viscous Cosmological Model in Brans-Dicke Theory with New Form of Time Varying Deceleration Parameter

We have presented FRW cosmological model in the framework of Brans-Dicke theory. This paper deals with a new proposed form of deceleration parameter and cosmological constant Λ. The effect of bulk viscosity is also studied in the presence of modified Chaplygin gas equation of state (p=Aρ-B/ρn). Furthermore, we have discussed the physical behaviours of the models.

Cyclic multiverses

Using the idea of regularisation of singularities due to the variability of the fundamental constants in cosmology we study the cyclic universe models. We find two models of oscillating and non-singular mass density and pressure ("non-singular" bounce) regularised by varying gravitational constant $G$ despite the scale factor evolution is oscillating and having sharp turning points ("singular" bounce). Both violating (big-bang) and non-violating (phantom) null energy condition models appear. Then, we extend this idea onto the multiverse containing cyclic individual universes with either growing or decreasing entropy though leaving the net entropy constant. In order to get an insight into the key idea, we consider the doubleverse with the same geometrical evolution of the two "parallel" universes with their physical evolution (physical coupling constants $c(t)$ and $G(t)$) being different. An interesting point is that there is a possibility to exchange the universes at the point of maximum expansion -- the fact which was already noticed in quantum cosmology. Similar scenario is also possible within the framework of Brans-Dicke theory where varying $G(t)$ is replaced by the dynamical Brans-Dicke field $\phi(t)$ though these theories are slightly different.

On holographic dark energy in non-isotropic Universe with Brans-Dicke theory

A spatially homogeneous and anisotropic Bianchi type I Universe has been studied with the interacting holographic dark energy (HDE) in the framework of Brans-Dicke theory. At first, we obtain the equation of state parameter, the deceleration parameter and the evolution equation of the interacting HDE in BD theory. We find that, in this case, $\omega_{\varLambda}$ can cross the phantom line ( $\omega_{\varLambda}>-1$ ) and eventually the Universe approaches a de-Sitter phase of expansion ( $\omega_{\varLambda}\rightarrow-1$ ). Then, we extend our study to the case of stability of the interacting HDE in BI Universe according to BD theory. In case of we find that the squared sound speed of the interacting HDE is either positive or negative. This implies that the interacting HDE in BD theory can lead to a stable Universe favored by observations at the present time in an anisotropic Universe.

A Theoretical Study of the Cosmic Expansion in the Framework of Brans-Dicke Theory

On the basis of an assumption of inter-conversion of matter and dark energy, a theoretical model of cosmic expansion has been formulated, in the framework of Brans-Dicke (BD) theory. Empirical expressions of scale factor (�� ) and scalar field parameter (�� ) have been used. The scale factor is such that it generates a signature flip of the deceleration parameter with time. A function of time �� (�� ), defined in terms of the density of matter (�� ), is used to account for the non-conservation of matter. This function is proportional to the matter content of the universe. Using an empirical scalar field parameter, in terms of scale factor, the functional form of �� (�� ) has been determined from the BD field equations. The present study reveals that �� (�� ) decreases with time, indicating a conversion of matter into dark energy. Based on this function, the time variation of the density of matter (�� ) has been determined. Expressions regarding the proportions of matter and dark energy of the universe have been derived. The time dependence of gravitational constant (�� ), �� �� and the BD dimensionless parameter ��(�� ) have been explored. The functional dependence of BD parameter ��(�� ) upon the scalar field (�� ) has been formulated. The study based on the present model establishes a correlation between the transformation of matter content into dark energy and the change of cosmological parameters connected to cosmic expansion, without employing any particular model of interaction between scalar field and matter. Keywords: Cosmology, Brans-Dicke parameter, Scalar field, Conversion of matter into dark energy, Time varying gravitational constant, Accelerated expansion of the universe.

Bianchi type I Universe and instability of new agegraphic dark energy in Brans-Dicke theories

In this paper, we consider the new agegraphic dark energy (NADE) in a Bianchi type-I metric (which is a spatially homogeneous and anisotropic) in the framework of Brans-Dicke theory. For this purpose, we use the squared sound speed $v_{s}^{2}$ whose sign determines the stability of the model. We explore the stability of this model in the presence/absence of interaction between dark energy and dark matter in both flat and non-isotropic geometry. The equation of state and the deceleration parameter of the new agegraphic dark energy in a anisotropic Universe is obtained. We show that the combination of Brans-Dicke field and new agegraphic dark energy can accommodate $\omega_{\varLambda}=-1$ crossing for the equation of state of noninteracting dark energy. When an interaction between dark energy and dark matter is taken into account, the transition of $\omega_{\varLambda}$ to phantom regime can be more easily accounted when the Einstein field equations is being resort. In conclusion, we find evidences that the new agegraphic dark energy in BD theory can not lead to a stable Universe favored by observations at the present time. The anisotropy of the Universe decreases and the Universe transits to an isotropic flat FRW Universe accommodating the present acceleration.

A Study of Transition of the Expansion of the Universe from a Phase of Deceleration to Acceleration through a Conversion of Matter into Dark Energy in the Framework of Brans-Dicke Theory

The present study is based on a generalized form of Brans-Dicke (BD) theory where, the dimensionless BD parameter is regarded as a function of the scalar field, which is reciprocal of the gravitational constant. The field equations have been solved by incorporating an empirical function f(t) in the expression representing the conservation of matter. This function f(t) has been chosen to account for a conversion of matter (both dark and baryonic) into some other form, possibly dark energy, which is known to be responsible for the accelerated expansion of universe. The requirement of a signature flip of the deceleration parameter (q), which is evident from other studies, sets the boundary conditions to be satisfied by the function f(t), leading to the formulation of its time dependence. A simple empirical relation was initially assumed to represent the time dependence of f(t), and the constants in this expression have been determined from these boundary conditions. The BD parameter has been found to have a negative value throughout the range of study. The dependence of BD parameter upon the scalar field has been depicted graphically. A smooth transition of the universe, from a decelerated to an accelerated phase of expansion, is found to occur due to a conversion of matter into dark energy. The gravitational constant is found to be increasing with time.

A Study of Accelerated Expansion of the Universe and Time Varying Gravitational Constant in the Framework of Brans-Dicke Theory

A simple mathematical model has been proposed on the basis of generalized Brans-Dicke (BD) theory where, the dimensionless BD parameter is regarded as a function of the scalar field. The gravitational constant is reciprocal of this scalar field. The calculations in this model are based on a very simple ansatz , which has been chosen for the purpose of solving the differential equations most conveniently. This function has been chosen to account for an inter-conversion between the matter and dark energy. This inter-conversion is said to be connected to an interaction that causes the accelerated expansion of the universe. The requirement of a signature flip of the deceleration parameter , which is evident from other studies, sets the boundary conditions to be satisfied by the function , leading to the formulation of its time dependence. A simple empirical relation has been assumed to represent the time dependence of , and the constants in this expression have been determined from the boundary conditions. The parameter has been found to have a negative value. The dependence of upon has been shown graphically. The present model shows that a transition of the universe, from decelerated expansion to accelerated expansion, takes place due to a conversion of matter into dark energy. An increase of gravitational constant with time is found in the present study

Some Exact Solutions of Bianchi Type-II String Cosmological Models with Magnetic Field in Brans-Dicke Theory of Gravitation

The spatially homogeneous and totally anisotropic Bianchi type-II cosmological solutions of massive strings have been investigated in the presence as well as absence of the magnetic field in the framework of Brans-Dicke theory of gravitation formulated by Brans-Dicke (Phys. Rev. 124, 925, 1961). The energy conditions of the models have been examined. With the help of Takabayasi’s equation of state some exact solutions of this model have been obtained. The physical and kinematical behaviors of the models have also been discussed.

FRW models with particle creation in Brans-Dicke theory

The thermodynamical effect of particle creation on the early evolution of Friedmann-Robertson-Walker universe is analyzed in the context of open thermodynamical systems within the framework of Brans-Dicke theory. The field equations are solved exactly using the “gamma-law” equation of state p=(γ−1)ρ, where the adiabatic parameter γ depends on the scale factor of the universe. We assume the functional form of γ in such a way that it describes a unified description of two early phases of the evolution of the universe, viz. inflationary phase and radiation-dominated phase. The role of Brans-Dicke scalar and creation of matter particles is investigated during the expansion of the universe. It is found that the expansion of the universe is driven by the particle creation in each phase. The physical and geometrical behaviors of the solutions are discussed in detail.

Power-Law Entropy-Corrected HDE and NADE in Brans-Dicke Cosmology

Considering the power-law corrections to the black hole entropy, which appear in dealing with the entanglement of quantum fields inside and outside the horizon, the holographic energy density is modified accordingly. In this paper we study the power-law entropy-corrected holographic dark energy in the framework of Brans-Dicke theory. We investigate the cosmological implications of this model in detail. We also perform the study for the new agegraphic dark energy model and calculate some relevant cosmological parameters and their evolution. {As a result we find that this model can provide the present cosmic acceleration and even the equation of state parameter of this model can cross the phantom line $w_D=-1$ provided the model parameters are chosen suitably}.

Reconstruction of dark energy and equilibrium thermodynamics in Brans-Dicke theory

Both dark energy and the thermodynamics on apparent horizon in cosmology have been broadly investigated in recent several years. In order to maintain the continuity equation of the total matter in the universe, a new interacting dark energy in the framework of Brans-Dicke theory is proposed. Considering this new interacting dark energy, an equilibrium thermodynamics in Brans-Dicke theory is constructed successfully. Moreover, this new interacting dark energy can be regarded as arising from the “Holographic Dark Energy” models.

Viscous cosmological models with particle creation in Brans-Dicke theory

The effect of bulk viscosity on the evolution of the spatially flat Friedmann-Robertson-Walker models in the context of open thermodynamical systems, which allow particle creation, is analyzed within the framework of Brans-Dicke theory. Particle creation and bulk viscosity have been considered as separated irreversible processes. To accommodate the viscous pressure and creation pressure, which is associated with creation of matter out of gravitational field, the energy-momentum tensor is modified. Dynamical behaviour of the models have been discussed.

Gravitational energy in Brans-Dicke cosmological models

The behaviour of gravitational energy and scalar field during the evolution of the universe within the framework of Brans-Dicke theory has been discussed. With help of the Landau-Lifshitz pseudo-tensor for the flat Friedmann-Robertson-Walker model, it is found that (i) the total energy of the universe is always zero, (ii) the Brans-Dicke scalar field for all Ω >-0 contributes energy to the negative energy of gravitational field and this gets transferred to the vacuum energy which accelerates the expansion of the universe.