Brans-Dicke Scalar Field Research Articles

Thermodynamics analysis of non-interacting Barrow holographic dark energy in Brans-Dicke theory

In the present paper, we study Barrow holographic dark energy in Brans-Dicke theory of gravity for flat Frieddman-Lemaitre-Robertson-Walker metric. We have assumed power law form of Brans-Dicke scalar field in terms of scale factor . The thermodynamics analysis of the model is discussed. It is observed that the model satify generalized second law of thermodynamics for all values of the parameter in past, present as well as in the future universe.

Inflationary model in Brans–Dicke theory

In this paper, we present a model of inflation in the original form of Brans–Dicke (BD) theory without any modification and without adding any dark energy candidate. We consider a log function of the scale factor in the BD scalar field to discuss the inflationary era of the universe. We obtain the expressions for the deceleration parameter and the equation of state parameter, and plot their graphs against the cosmic scale factor. We observe that the universe starts with decelerated expansion and experiences an early time phase transition from a decelerated phase to an accelerated phase. After a very short period, the accelerated expansion ends in a decelerated expansion, showing the inflationary era of the cosmic evolution. To analyze the model on thermodynamic ground, we consider generalized second law of thermodynamics and observe that the law is satisfied within the model. Further, we study the stability of the model using the squared sound speed method. We plot the squared sound speed against cosmic time t and discuss the effects of the parameters on the stability of the model. We observe that for suitable values of the model parameters our model is stable.

Reconsidering bulk viscosity in Brans–Dicke theory

In this paper, we reconsider the concept of bulk viscosity in Brans–Dicke theory to study accelerated expansion of the universe in a flat Friedmann–Robertson–Walker metric. In recent works on bulk viscosity in Brans–Dicke theory, the power-law form of Brans–Dicke scalar field has been taken to explain recent phase transition of the universe, however, power-law form confronts constant deceleration parameter problem. Therefore, we consider recently proposed well-motivated logarithmic form of Brans–Dicke scalar field to discuss bulk viscous model in Brans–Dicke theory. We obtain the values of deceleration parameter and equation of state parameter for the model, and plot their graphs against the redshift parameter z. The plots show that the model is able to explain the recent phase transition of the universe and equation of state parameter shows the behavior of dark energy. Further, we apply two important analysis, namely, [Formula: see text] analysis and thermodynamic analysis to examine our model. The [Formula: see text] analysis shows that our model belong to thawing region and shows similarities with quintessence model. All the trajectories start from the point [Formula: see text] with different negative values of w having quintessence behavior. The thermodynamic analysis shows that the model satisfies generalized second law of thermodynamics.

Casimir wormholes in Brans–Dicke theory

In recent years there has been a growing interest in the field of wormhole physics in the presence of Casimir effect. As this effect provides negative energy density, it can be utilized as an ideal candidate for the exotic matter required for creating a traversable wormhole. In the context of modified theories of gravity such as Brans–Dicke (BD) theory (Brans and Dicke 1961 Phys. Rev. 124 925), wormhole geometries have been vastly investigated. However, the scientific literature is silent on the issue of BD wormholes in the presence of Casimir energy. Our aim in the present study is to seek for static spherically symmetric solutions representing wormhole configurations in BD theory with Casimir energy as the supporting matter. The Casimir setup we assume comprises two electrically neutral, infinitely large parallel planes placed in a vacuum. We then consider the Casimir vacuum energy density of a scalar field in such a configuration with Dirichlet and mixed boundary conditions. In the former case the corresponding Casimir force is attractive and in the latter this force is repulsive. We present exact zero tidal force wormhole solutions as well as those with non vanishing redshift function for both types of Casimir energies. The conditions on wormhole solutions along with the weak (WEC) and null (NEC) energy conditions put constraints on the values of BD coupling parameter. These constraints are also subject to the value of BD scalar field at the throat and the throat radius. We therefore find that BD wormholes in the presence of Casimir energy can exist without violating NEC and WEC (for the repulsive Casimir force). Finally, we examine the equilibrium condition for stability of the obtained solutions using Tolman–Oppenheimer–Volkoff equation.

Gauss–Bonnet source of Brans–Dicke scalar field and accelerated expansion of universe

In this work, using a different theoretical approach, we study a homogeneous and isotropic cosmological spacetime filled with a perfect fluid. Using the framework of Brans–Dicke (BD) and Gauss–Bonnet (GB) theories and including a scalar potential, we obtain the analytical solution of the field equations and find the field functions and the relevant expansion parameter of the spacetime. We assume that the BD and GB media can interact with each other, and hence that there is an energy transfer between these two components. Using this approach, we assume that the BD scalar field is related to intrinsic properties of the GB medium. We introduced a power-law scalar field [Formula: see text] and found the appropriate dynamical functions for the different powers p. We found that the expansion rate of spacetime also behaves as a power-law, and the accelerating expansion is always satisfied for this model. We also analyze the stability for physically acceptable critical points for a given scale factor.

Generalized ghost dark energy model in Brans–Dicke theory with logarithmic scalar field

We study generalized ghost dark energy model in Brans–Dicke theory within the framework of flat Friedmann–Lemaitre–Robertson–Walker Universe. We assume the well-motivated logarithmic form of Brans–Dicke scalar field in terms of the scale factor to find the observational parameters such as equation of state parameter [Formula: see text] and deceleration parameter q. It is observed that the equation of state parameter crosses the phantom divide line [Formula: see text] for a suitable range of model parameters. We observe that the deceleration parameter is time-dependent which shows the recent phase transition from decelerated to accelerated expansion of the Universe. Moreover, the model shows that the Universe will go through two future phase transitions, recent accelerated expansion to decelerated expansion and again decelerated expansion to accelerated expansion. We have also plotted the trajectories of the model in the [Formula: see text] plane for different values of parameters and explored the freezing and thawing regions. Further, we apply the sound squared speed method to check the stability of the model and found that the model is stable for suitable range of parameters.

Classical and quantum cosmology for two scalar field Brans–Dicke type theory: a Noether symmetry approach

The paper deals with a cosmological model containing two scalar fields which can be considered as an extension of the Brans–Dicke scalar field model. Due to highly coupled non linear field equations, Noether symmetry analysis has been imposed and as a result the Lagrangian as well as the field equations become much simple in form to have the classical solutions. The relevant cosmological parameters are analyzed graphically. Finally, quantum cosmology has been studied by constructing the Wheeler–DeWitt equation and the solution of this second order partial differential equation has been done using this symmetry analysis.

An Investigation into LRS Bianchi I Universe in Brans-Dicke Theory

In this study, the homogenous and anisotropic locally rotationally symmetric (LRS) Bianchi type-I universe filled with the bulk viscous and the string cloud matter has been investigated in the Brans-Dicke (BD) scalar-tensor theory. The modified Einstein field equations of the constructed model have been solved by using the relation of the scale factors $A=B^m$ and considering the deceleration parameter to be $q=m-1$. It is found that the string does not survive for the model due to the obtained zero string energy density ($\rho_s=0$) and agrees with some studies in recent years. Moreover, it is possible to say that string matter may be turned into phantom energy over time, depending on the obtained negative rest energy density of the matter. When BD scalar field $\Phi$ is assumed constant, the attained solutions are reduced to General Relativity (GR) solutions for the static vacuum. Thus, the constructed model has not allowed the reduction of the BD solutions to GR solutions, including the matter distributions. In addition, some expansion models for choosing a value of constant $m$ have been obtained and provided. All the results have been analyzed in detail.

Evolution of Generalized Brans–Dicke Parameter within a Superbounce Scenario

We studied a superbounce scenario in a set up of the Brans–Dicke (BD) theory. The BD parameter was considered to be time-dependent and was assumed to evolve with the Brans–Dicke scalar field. In the superbounce scenario, the model bounced at an epoch corresponding to a Big Crunch provided the ekpyrotic phase continued until that time. Within the given superbounce scenario, we investigated the evolution of the BD parameter for different equations of state. We chose an axially symmetric metric that has an axial symmetry along the x-axis. The metric was assumed to incorporate an anisotropic expansion effect. The effect of asymmetric expansion and the anisotropic parameter on the evolving and non-evolving parts of the BD parameter was investigated.

Modified Brans–Dicke cosmology with minimum length uncertainty

We consider a modification of the Brans-Dicke gravitational Action Integral inspired by the existence of a minimum length uncertainty for the scalar field. In particular, the kinetic part of the Brans-Dicke scalar field is modified such that the equation of motion for the scalar field be modified according to the quadratic Generalized Uncertainty Principle (GUP). For the background geometry, we assume the homogeneous and isotropic Friedmann--Lema\^{\i}tre--Robertson--Walker metric. We investigate the dynamics and the cosmological evolution for the dynamical variables of the theory and we compare the results with the unmodified Brans-Dicke theory. It follows that in consideration because of the additional degrees of freedom in the energy-momentum tensor the dynamical variables describe various aspects of the cosmological history. This is one of the first studies on the effects of GUP in a Machian gravitational theory.

Screening mechanism and late-time cosmology: Role of a Chameleon–Brans–Dicke scalar field

ABSTRACT We discuss a way in which the geometric scalar field in a Brans–Dicke (BD) theory can evade local astronomical tests and act as a driver of the late-time cosmic acceleration. This requires a self-interaction of the BD scalar as well as an interaction with ordinary matter. The scalar field in this construct acquires a density-dependent effective mass much like a Chameleon field. We discuss the viability of this setup in the context of Equivalence Principle, Fifth Force, and Solar System tests. The cosmological consistency is adjudged in comparison with observational data from recalibrated light curves of type Ia supernova (JLA), the Hubble parameter measurements (OHD) and the Baryon Acoustic Oscillation (BAO). We deduct that the astrophysical constraints indeed favour the existence of a mild scalar-matter interaction in the Jordan Frame.

Reconsidering holographic dark energy in Brans\u2013Dicke theory

In this paper, we reconsider the concept of holographic dark energy in the framework of Brans–Dicke theory in the formalism of the flat Friedmann–Lemaitre–Robertson–Walker metric. Firstly, we demonstrate how the assumption phi propto a^n, where phi and a stand for the Brans–Dicke scalar field and scale factor, respectively, naturally leads to a constant deceleration parameter, irrespective of the energy content of the universe. Secondly, we consider interacting holographic dark energy with Hubble horizon as IR cut-off, and find the value of the Hubble parameter and corresponding value of the scale factor. Further, we find the value of the Brans–Dicke scalar field phi for the obtained value of the Hubble parameter and holographic dark energy. We obtain the corresponding value of the deceleration parameter and show that it can explain the phase transition of the universe. Moreover, statefinder diagnostics has been applied to compare the model with existing models. On the other hand, we consider the viscous behavior of holographic dark energy and show that the viscous holographic dark energy can play the role of interacting holographic dark energy as it is able to explain the phase transition of the universe. Further, we find the value of the Brans–Dicke scalar field phi for this viscous holographic dark energy. In this model also, we apply the statefinder diagnostic.

Determination of time evolution of the gravitational constant in the framework of Brans-Dicke Theory: An easy wa

The present article demonstrates a very simple mathematical way to determine the time-dependence of the dynamical gravitational constant () in the framework of the Brans-Dicke theory of gravity. Brans-Dicke field equations, for a matter-dominated, pressure-less and spatially flat universe with homogeneous and isotropic space-time, have been used for this formulation. The gravitational constant () is the reciprocal of the Brans-Dicke scalar field (). Using a simple ansatz, which represents the Brans-Dicke scalar field () as a function of time, the possible values of a constant parameter (constituting the ansatz) have been calculated with the help of the field equations, using the values of some cosmological parameters at the present time. The values of that parameter (belonging to the ansatz) lead to the conclusion that the scalar field () decreases and consequently the gravitational constant () increases with time. The value of the relative time-rate of change of the gravitational constant (i.e., ) has also been estimated and this quantity has been found to be independent of time. Time-dependence of and has been depicted graphically for all values of the parameter belonging to the ansatz. The novel features of this study are that the gravitational field equations did not have to be solved, unlike other studies, to arrive at the results and the mathematical scheme for calculations is extremely easy in comparison to other recent studies in this regard.

New Conservation Laws and Exact Cosmological Solutions in Brans–Dicke Cosmology with an Extra Scalar Field

The derivation of conservation laws and invariant functions is an essential procedure for the investigation of nonlinear dynamical systems. In this study, we consider a two-field cosmological model with scalar fields defined in the Jordan frame. In particular, we consider a Brans–Dicke scalar field theory and for the second scalar field we consider a quintessence scalar field minimally coupled to gravity. For this cosmological model, we apply for the first time a new technique for the derivation of conservation laws without the application of variational symmetries. The results are applied for the derivation of new exact solutions. The stability properties of the scaling solutions are investigated and criteria for the nature of the second field according to the stability of these solutions are determined.

Anisotropic stars in Brans-Dicke gravity

In the present work we show application of Brans-Dicke theory of gravitation, as an alternative to Einstein’s general relativity, in modeling anisotropic compact stellar objects. To make the field equations solvable we consider the space-time under Class 1 embedding condition. The numerical solution of wave equation for Brans-Dicke scalar field Φ has been well fitted with well known function proposed by Bruckman and Kazes (Phys.Rev.D 16:261,1977). The solutions set seems viable as far as observational data are concerned.

Evolution of FLRW universe in Brans-Dicke gravity theory

This study aims to examine the dynamics of flat Friedmann-Lemaitre-Robertson-Walker (FLRW) model of universe with time varying cosmological constant $\Lambda (t)$ in Brans-Dicke theory of gravity. The authors find a solution of the field equations using simple parametrization of scale factor $a(t)$ i.e. $a(t)=\exp \{(\alpha t+\beta )^{p}\}$ , and power law relation between $a(t)$ and Brans-Dicke scalar field $\phi $ . The derived cosmological model shows transition from early cosmic deceleration to present cosmic acceleration. Such feature of cosmic expansion is in agreement with recent empirical observations. The behaviour of cosmographic parameters such as Hubble parameter, deceleration parameter, jerk, snap and lerk parameters is examined graphically. It is observed that model behaves like $\Lambda CDM$ model in late cosmic evolution. We also use statefinder and Om diagnostic tools to differentiate various dark energy models from $\Lambda CDM$ model. Moreover, model parameter $p$ and present value of deceleration parameter $q_{0}$ is also estimated using Hubble data set and Pantheon Type 1a supernova data set.

Reply to “Comment on ‘Brans-Dicke scalar field cosmological model in Lyra’s geometry’ ”

The present study is a response to the raising comments on Brans-Dicke scalar field cosmological models in Lyra's geometry [A. K. Yadav, Phys. Rev. D 10, 108301 (2020)] by Dr. Anil Kumar Yadav. We have corrected the manuscripts and remove the contradictions observed by the comment author. Now, the present cosmological model is a transit phase model with Brans-Dicke coupling constant $\ensuremath{\omega}l\ensuremath{-}1.33$ while the equation of state parameter (EoS) lies in the range $0\ensuremath{\le}\ensuremath{\gamma}l0.33$. The Brans-Dicke scalar field is responsible candidate for the acceleration in expansion of the universe.

Comment on “Brans-Dicke scalar field cosmological model in Lyra’s geometry”

In the present study, we comment on Brans-Dicke scalar field cosmological model in Lyra's geometry [Maurya \& Zia, Phys. Rev. D \textbf{100}, 023503 (2019)]. In this comment, we investigate that there is no acceleration in the model proposed by the authors of Phys. Rev. D \textbf{100}, 023503 (2019). Therefore, despite the claims to the contrary the Brans-Dicke Scalar Field Cosmological Model in Lyra's Geometry with high Brans-Dicke (BD) coupling parameter $\omega$ and constant $\beta$ can not produce late time acceleration in the universe.

Quark-hadron phase transition in DGP including BD brane * * T. G. has been supported financially by Vice Chancellorship of Research and Technology, University of Kurdistan under research Project (98/11/2724). A. M. has been supported financially by Vice Chancellorship of Research and Technology, University of Kurdistan under research Project (98/10/34704)

A Dvali–Gabadadze–Porrati (DGP) brane-world model with perfect fluid brane matter including a Brans-Dicke (BD) scalar field on brane was utilized to investigate the problem of the quark-hadron phase (QHP) transition in early evolution of the Universe. The presence of the BD scalar field arises with several modified terms in the Friedmann equation. Because the behavior of the phase transition strongly depends on the basic evolution equations, even a small change in these relations might lead to interesting results about the time of transition. The phase transition is investigated in two scenarios, namely the first-order phase transition and smooth crossover phase transition. For the first-order scenario, which is used for the intermediate temperature regime, the evolution of the physical quantities, such as temperature and scale factor, are investigated before, during, and after the phase transition. The results show that the transition occurs in about a micro-second. In the following part, the phenomenon is studied by assuming a smooth crossover transition, where the lattice QCD data is utilized to obtain a realistic equation for the state of the matter. The investigation for this part is performed in the high and low-temperature regimes. Using the trace anomaly in the high-temperature regime specifies a simple equation of state, which states that the quark-gluon behaves like radiation. However, in the low-temperature regime, the trace anomaly is affected by discretization effects, and the hadron resonance gas model is utilized instead. Using this model, a more realistic equation of state is found in the low-temperature regime. The crossover phase transition in both regimes is considered. The results determine that the transition lasts around a few micro-seconds. Further, the transition in the low-temperature regime occurs after the transition in the high-temperature regime.

Bouncing scenario in Brans–Dicke theory

A bouncing scenario is studied in the framework of generalized Brans–Dicke theory. In order to have a dark energy (DE) driven late time cosmic acceleration, we have considered a unified dark fluid simulated by a linear equation of state (EoS). The evolutionary behavior of the DE equation of parameter derived from the unified dark fluid has been discussed. The effect of the bouncing scale factor on the Brans–Dicke parameter, self-interacting potential and the Brans–Dicke scalar field is investigated.