Brans-Dicke Research Articles

Study of Yang–Mills condensate dark energy in the framework of generalized Brans–Dicke theory

The prediction about the accelerating expansion of the universe introduced the concept of dark energy. Since then, a plethora of work has been done to explain the origin and nature of dark energy. This work deals with the possibility of obtaining cosmic acceleration of a new kind of dark energy, viz., Yang–Mills condensate dark energy in generalized Brans–Dicke theory with variable or constant [Formula: see text] (Brans–Dicke parameter) and vanishing and non-vanishing self-interacting potential. We assume the scale factor and scalar field in power law form. Then we investigate the above model to determine whether the universe is accelerating. We obtain a redshift-dependent equation of state parameter, which describes the phase transition of the universe. We notice that the equation of the state parameter meets the [Formula: see text] cold dark matter (CDM) limit in the far future. Then we study the [Formula: see text] analysis to get the thawing or freezing region. We study the stability of the model by studying the square speed of sound. Last, the nature of the scalar field and the potential are analyzed for four cases.

Brans–Dicke cosmology with cosmological term [formula omitted

In this paper, we study the dynamics of a flat Friedmann–Lemaître–Robertson–Walker (FLRW) cosmological model by considering varying vacuum energy density (VED) in Brans–Dicke (BD) cosmology. For this purpose, we consider the well-motivated VED of the form Λ(H)=c0+3νH2, where c0 and ν are dimensionless constants. We first adopt a theoretical method to find the exact solutions for various cosmological parameters of two models, namely ΛRG1 and ΛRG2. In ΛRG1 model, the scale factor evolves as a power-law expansion which gives the deceleration parameter a constant value. Hence, this type of model does not show the transition phase. The second model ΛRG2 describes the transition phase from deceleration to acceleration. In the second part, we perform two joint likelihood analysis in order to find the constrain on the main free parameters of the ΛRG2 model using the latest observational data sets including SNe Pantheon, H(z) data, BAO/CMB data and local H0 by SH0ES. Performing the two different combination of datasets, we find that the model shows prior decelerated epoch followed by late time accelerated epoch. We also compare the decaying VED with traditional Λ cosmology, which help us to define the evolution of the VED model. The results show that varying VED model in BD theory is consistent with data and the cosmic evolutions are in good agreement with the concordance ΛCDM and BD with constant Λ models. The AIC and BIC selection criteria are also discussed.

Rip behaviour in Brans–Dicke theory

In this work, we present little rip and pseudo rip scenarios in the framework of the generalised Brans–Dicke (GBD) theory where an evolving BD parameter along with a self-interacting potential is considered. The GBD field equations are derived for an anisotropic space time to provide a more general approach to the cosmic expansion. The evolutionary behaviour of the Brans–Dicke (BD) scalar field, dynamical Brans–Dicke parameter and the self interacting potential are studied. It is not possible to separate out the dynamical Brans–Dicke parameter into distinct evolving and non-evolving part for the rip scenarios as is obtained for constant deceleration parameter. While the little rip behaviour requires a positive value for the GBD parameter, a small negative value around −1.33 favours a pseudo rip behaviour.

Homogeneous Cosmological Models in Weyl’s Geometrical Scalar–Tensor Theory

In this paper, we consider homogeneous cosmological solutions in the context of the Weyl geometrical scalar–tensor theory. Firstly, we exhibit an anisotropic Kasner type solution taking advantage of some similarities between this theory and the Brans–Dicke theory. Next, we consider an isotropic model with a flat spatial section sourced by matter configurations described by a perfect fluid. In this model, we obtain an analytical solution for the stiff matter case. For other cases, we carry out a complete qualitative analysis theory to investigate the general behaviour of the solutions, presenting some possible scenarios. In this work, we do not consider the presence of the cosmological constant nor do we take any potential of the scalar field into account. Because of this, we do not find any solution describing the acceleration of the universe.

Dynamical equivalence of the varying G and varLambda cosmology with the Brans–Dicke theory

We investigate the dynamical equivalence of the varying G and varLambda cosmology with the Brans–Dicke theory. We show that there exist a point transformation which connects the field equations for a spatially flat FLRW geometry. We conclude that the two theories share the same integrability properties and solution trajectories.

Properties of Spherically Symmetric Black Holes in the Generalized Brans-Dicke Modified Gravitational Theory.

The many problems faced by the theory of general relativity (GR) have always motivated us to explore the modified theory of GR. Considering the importance of studying the black hole (BH) entropy and its correction in gravity physics, we study the correction of thermodynamic entropy for a kind of spherically symmetric black hole under the generalized Brans-Dicke (GBD) theory of modified gravity. We derive and calculate the entropy and heat capacity. It is found that when the value of event horizon radius r+ is small, the effect of the entropy-correction term on the entropy is very obvious, while for larger values r+, the contribution of the correction term on entropy can be almost ignored. In addition, we can observe that as the radius of the event horizon increases, the heat capacity of BH in GBD theory will change from a negative value to a positive value, indicating that there is a phase transition in black holes. Given that studying the structure of geodesic lines is important for exploring the physical characteristics of a strong gravitational field, we also investigate the stability of particles' circular orbits in static spherically symmetric BHs within the framework of GBD theory. Concretely, we analyze the dependence of the innermost stable circular orbit on model parameters. In addition, the geodesic deviation equation is also applied to investigate the stable circular orbit of particles in GBD theory. The conditions for the stability of the BH solution and the limited range of radial coordinates required to achieve stable circular orbit motion are given. Finally, we show the locations of stable circular orbits, and obtain the angular velocity, specific energy, and angular momentum of the particles which move in circular orbits.

Study of a Minimally Deformed Anisotropic Solution for Compact Objects with Massive Scalar Field in Brans–Dicke Gravity Admitting the Karmarkar Condition

In the present paper, we focused on exploring the possibility of providing a new class of exact solutions for viable anisotropic stellar systems by means of the massive Brans–Dicke (BD) theory of gravity. In this respect, we used the decoupling of gravitational sources by minimal geometric deformation (MGD) (e−η=Ψ+βh) for compact stellar objects in the realm of embedding class-one space-time to study anisotropic solutions for matter sources through the modified Einstein field equations. For this purpose, we used the ansatz for Ψ relating to the prominent, well-known and well-behaved Finch–Skea model via Karmarkar condition, and the determination scheme for deformation function h(r) was proposed via mimic requirement on radial pressure component: θ11(r)=pr(r) and matter density: θ00(r)=ρ(r) for the anisotropic sector. Moreover, we analyzed the main physical highlights of the anisotropic celestial object by executing several physical tests for the case θ11(r)=pr(r). We have clearly shown how the parameters α, β and ωBD introduced by massive BD gravity via the MGD approach incorporating the anisotropic profile of the matter distribution have an immense effect on many physical parameters of compact bodies such as LMC X-4, LMC X-4, Her X-1, 4U 1820-30, 4U 1608-52, SAX J1808.4–658 and many others that can be fitted.

Thermodynamics of Brans–Dicke–BTZ black holes coupled to conformal-invariant electrodynamics

Abstract The field equations of Brans–Dicke conformal-invariant theory in (2+1)-dimensions are highly nonlinear and difficult to solve directly. They are related to those of Einstein–dilaton theory, where the solutions can be obtained easily, by use of a mathematical tool known as the conformal transformation. The exact solutions of three-dimensional Brans–Dicke theory, which are obtained from their Einstein-dilaton counterparts, give two novel classes of conformal-invariant black holes. When the scalar potential is absent (or is considered constant) in the action, it has been shown that the exact solution of this theory is just the conformal-invariant BTZ black hole with a trivial constant scalar field. This issue corresponds to the four-dimensional Brans–Dicke–Maxwell theory discussed in Ref. [R.-G. Cai, Y. S. Myung, Phys. Rev. D 56, 3466 (1997)]. The Brans–Dicke conformal-invariant black holes’ thermodynamic quantities have been calculated by use of the appropriator methods, and it has been shown that they satisfy the first law of black hole thermodynamics in its standard form. The thermal stability of Brans–Dicke black holes has been studied by use of the canonical ensemble method and noting the signature of the black holes’ heat capacity.

Role of Relativistic Charged Perfect Fluid in Bianchi type-III Space-time in Brans-Dicke theory of gravitation

In this paper, we investigate the role of relativistic charged perfect fluid in Bianchi type-III cosmological model in Brans-Dicke theory of gravitation. Solutions of the model are obtained by volumetric exponential expansion, power law expansion and power law relation between scalar field and the scale factor . Some physical and kinematical properties of the model also are studied.

Cosmic and Thermodynamic Consequences of Kaniadakis Holographic Dark Energy in Brans–Dicke Gravity

In this manuscript, we investigate the cosmological and thermodynamic aspects of the Brans-Dicke theory of gravity for a spatially flat FRW universe. We consider a theoretical model for interacting Kaniadakis holographic dark energy with the Hubble horizon as the infrared cutoff. We deal with two interaction scenarios (Q1 and Q2) between Kaniadakis holographic dark energy and matter. In this context, we study different possible aspects of cosmic evolution through some well-known cosmological parameters such as Hubble (H), deceleration (q), jerk (j), and equation of state (ωd). For both interaction terms, it is observed that the deceleration parameter exhibits early deceleration to the current accelerating universe and also lies within the suggested range of Planck data. The equation of state parameter shows quintessence behavior (for the first interaction term) and phantom-like behavior (for the second interaction term) of the universe. The jerk parameter represents consistency with the ΛCDM model for both interaction terms. In the end, we check the thermodynamic behavior of the underlying model. It is interesting to mention here that the generalized second law of thermodynamics holds for both cases of interaction terms.

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.

Propagation of scalar and tensor gravitational waves in Horndeski theory

Gravitational waves travel through the distributions of matter and dark energy during propagation. For this reason, gravitational waves emitted from binary compact objects serve as a useful tool especially to probe the nature of dark energy. The geometrical optics approximation is a conventional way of investigating wave propagation. However, the approximation becomes less accurate as the wavelength approaches the curvature radius of the background, which can occur in generic situations. In this paper, we suggest a formulation for higher-order corrections of the geometrical optics expansion, applied to Horndeski theory which accommodates many dark energy models. At the level of the background, assuming that the derivative of the scalar field is non-vanishing and timelike, we choose the time slices to coincide with the contours of the scalar field. This choice of the background time slices is advantageous as the sound cones of both scalar and tensor gravitational waves are upright with respect to the background time slices whenever the scalar field behaves as a perfect fluid. We then analyze the equations of motion for scalar and tensor components of gravitational waves at the leading and next-to-leading order in the geometrical optics expansion, deriving the evolution equations for their amplitudes under certain conditions. In particular, for Generalized Brans-Dicke theories, we find a simple description of equations for gravitational waves in terms of an effective metric.

New agegraphic dark energy in Brans–Dicke theory with sign changeable interaction for flat universe

New agegraphic dark energy in Brans–Dicke theory with sign changeable interaction for flat universe

Constraining the time-varying vacuum energy models in Brans-Dicke theory

Constraining the time-varying vacuum energy models in Brans-Dicke theory

Primordial black holes and scalar-induced gravitational waves from the generalized Brans-Dicke theory

The power spectrum of the scalar-tensor inflation with a quadratic form Ricci scalar coupling function Ω(ϕ) = 1 - 2ϕ/ϕ c + (1 + δ 2)(ϕ/ϕ c )2 can be enhanced enough to produce primordial black holes and generate scalar-induced gravitational waves. The masses of primordial black holes and the frequencies of scalar-induced gravitational waves are controlled by the parameter ϕ c , and their amplitudes are determined by the parameter δ. Primordial black holes with stellar masses, planetary masses, and masses around 10-12 M ⊙ are produced and their abundances are obtained from the peak theory. The frequencies of the corresponding scalar-induced gravitational waves are around 10-9 Hz, 10-6 Hz, and 10-3 Hz, respectively. The primordial black holes with masses around 10-12 M ⊙ can account for almost all of the dark matter, and the scalar-induced gravitational waves with frequencies around 10-9 Hz can explain the NANOGrav 12.5 yrs signal.

Spherically symmetric viscous cosmology in Brans–Dicke theory

Spherically symmetric viscous cosmology in Brans–Dicke theory

Electron mass variation from dark sector interactions and compatibility with cosmological observations

We investigate the model where electrons and dark matter interact with dark energy through the rolling of a scalar field which comes from extra dimensional theories such as the braneworld theory and Brans-Dicke theory. In this model, dark energy couples to dark matter and electrons, which leads to larger values of the mass energies of dark matter and electrons in the early universe. We also fit our model to the cosmological data. By analyzing the data from Planck, baryon acoustic oscillation (BAO), light curves (Pantheon), and type-Ia supernovae (SH0ES), it can be seen that the Hubble tension is relieved in our model and the coupling parameter prefers a nonzero value with a significance of over $2\ensuremath{\sigma}$.

Dynamics of self-gravitating systems in non-linearly magnetized chameleonic Brans–Dicke gravity

We study the effects of magnetic fields of non-linear electrodynamics in chameleonic Brans-Dicke theory under the existence of anisotropic spherical fluid. In particular, we explore dissipative and non-dissipative self-gravitating systems in the quasi-homologous regime with the minimal complexity constraint. As a result, under the aforementioned circumstances, several analytic solutions are found. Furthermore, by analyzing the dynamics of a dissipative fluid, it is demonstrated that a void covering the center can satisfy the Darmois criteria. The temperature of the self gravitating systems is also investigated.

A comparison between the Jordan and Einstein frames in Brans-Dicke theories with torsion

In recent years, gravitational models motivated by quantum corrections to gravity which introduce higher order terms like \(R^{2}\) or terms in which the Riemann tensor is not symmetric have been studied by several authors in the form of a general Brans-Dicke type model containing the Ricci scalar, the Holst term and the Nieh-Yan invariant. In this paper we focus on the less explored Jordan frame of such theories and in the comparison between both this frame and the Einstein one. Furthermore, we discuss the role of the transformation of the torsion under conformal transformations and show that the transformation proposed in this paper (extended conformal transformation) contains a special case of the projective transformation of the connection used in some of the papers that motivated this work. We discuss the role and advantages of the extended conformal transformation and show that this new approach can have interesting consequences by working with different variables such as the metric and torsion. Moreover, we study the stability of the system via a dynamical analysis in the Jordan frame, this in order to analyze whether or not we have the fixed points that can be later identified as the inflationary attractor and the unstable fixed point where inflation could take place. Finally we study the scale invariant case of the general model in the Jordan frame. We find out that both the scalar spectral index and the tensor-to-scalar ratio are in agreement with the latest Planck results.

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.