Brans-Dicke Scalar Field Research Articles

Role of a chameleon field in the presence of variable modified Chaplygin gas in Brans–Dicke theory

In this work, we have considered the Friedmann–Robertson–Walker model of the universe for Brans–Dicke (BD) theory with a BD scalar field as a chameleon field. First we have transformed the field equations and conservation equation from Jordan’s frame to Einstein’s frame. We have shown that, in the presence of a variable modified Chaplygin gas (VMCG), the potential function V and another analytic function f always increase with respect to the BD–chameleon scalar field [Formula: see text] but decrease with time t for pure Chaplygin gas, modified Chaplygin gas, and VMCG models.

Does accelerating universe indicate Brans-Dicke theory?

The evolution of universe in Brans-Dicke (BD) theory is discussed in this paper. Considering a parameterized scenario for BD scalar field $\phi=\phi_{0}a^{\alpha}$ which plays the role of gravitational "constant" $G$, we apply the Markov Chain Monte Carlo method to investigate a global constraints on BD theory with a self-interacting potential according to the current observational data: Union2 dataset of type supernovae Ia (SNIa), high-redshift Gamma-Ray Bursts (GRBs) data, observational Hubble data (OHD), the cluster X-ray gas mass fraction, the baryon acoustic oscillation (BAO), and the cosmic microwave background (CMB) data. It is shown that an expanded universe from deceleration to acceleration is given in this theory, and the constraint results of dimensionless matter density $\Omega_{0m}$ and parameter $\alpha$ are, $\Omega_{0m}=0.286^{+0.037+0.050}_{-0.039-0.047}$ and $\alpha=0.0046^{+0.0149+0.0171}_{-0.0171-0.0206}$ which is consistent with the result of current experiment exploration, $\mid\alpha\mid \leq 0.132124$. In addition, we use the geometrical diagnostic method, jerk parameter $j$, to distinguish the BD theory and cosmological constant model in Einstein's theory of general relativity.

Brans-Dicke theory in anisotropic models with a viscous fluid

We consider anisotropic models of the Universe in the presence of a Brans-Dicke (BD) scalar field ϕ, a causal viscous fluid and a barotropic fluid. We show that, irrespective of the fluid, the causality theory provides a late-time acceleration of the Universe. If deceleration occurs in a radial direction and acceleration in the transverse direction, then the anisotropic Universe will accelerate for a particular condition in a power-law representation of the scale factors.

Coupling of Brans–Dicke scalar field with Horava–Lifshitz gravity

We look for a Brans–Dicke type generalization of Horava–Lifshitz gravity. It is shown that such a generalization is possible within the detailed balance condition. Classically, the resulting theory reduces in the low energy limit to the usual Brans–Dicke theory with a negative cosmological constant for certain values of parameters. We then consider homogeneous, isotropic cosmology and study the effects of the new terms appearing in the model.

Accelerating Universe in Brans-Dicke Theory in Presence of Chaplygin Gas

In this paper, we have considered a model for Brans-Dicke scalar field in presence of Chaplygin gas and interaction between them. We have shown that the BD parameter ω is constant or not, the Chaplygin gas provides early deceleration and late time acceleration of the universe. The graphical representation of statefinder parameters shows the total evolution of the universe starts from radiation era to phantom model.

Gravitational constraints of dS branes in an AdS Einstein–Brans–Dicke bulk

We derive the full projected Einstein–Brans–Dicke gravitational equations associated with an n-dimensional brane embedded in an (n + 1)-dimensional bulk. Using general conditions, as the positivity of the Brans–Dicke parameter and the effective Newton gravitational constant as well, we can constrain the brane cosmological constant in terms of the brane tension, the Brans–Dicke scalar field and the trace of the stress tensor on the brane, in order to achieve a dS brane. Applying these constraints to a specific five-dimensional model, a lower bound for the scalar field on the brane is elicited without solving the full equations. It is shown under which conditions the effective brane cosmological constant can be ignored in the brane projected gravitational field equations, suggesting a different fine tuning between the brane tension and the bulk cosmological constant.

The Brans-Dicke gravity as a theory of Dark Matter

The pure Brans-Dicke (BD) gravity with or without the cosmological constant Λ has been taken as a model theory for dark matter. Indeed, there has been a consensus that unless one modifies either the standard theory of gravity, namely, general relativity, or the standard model for particle physics, or both, one can never achieve a satisfying understanding of the phenomena associated with dark matter and dark energy. Along this line, our dark matter model in this work can be thought of as an attempt to modify the gravity side alone in the simplest fashion to achieve the goal. Among others, it is demonstrated that our model theory can successfully predict the emergence of a dark matter halo-like configuration in terms of a self-gravitating spacetime solution to the BD field equations and reproduce the flattened rotation curve in this dark halo-like object in terms of the non-trivial energy density of the BD scalar field, which was absent in the context of general relativity, where Newton’s constant is strictly a “constant” having no dynamics. Our model theory, however, is not entirely without flaw, such as the prediction of relativistic jets in all types of galaxies, which actually is not the case.

Positive tension 3-branes in an AdS 5 bulk

In this work, we review and extend the so-called consistency conditions for the existence of a braneworld scenario in arbitrary dimensions in the Brans-Dicke (BD) gravitational theory. After that, we consider the particular case of a five-dimensional scenario which seems to have phenomenological interesting implications. We show that, in the BD framework, it is possible to achieve necessary conditions pointing to the possibility of accommodating branes with positive tensions in an AdS bulk by the presence of the additional BD scalar field, avoiding in this way the necessity of including unstable objects in the compactification scheme. Furthermore, in the context of time variable brane tension, it is shown that the brane tension may change its sign, following the bulk cosmological constant sign.

Holographic dark energy in Brans–Dicke cosmology with chameleon scalar field

We study a cosmological implication of holographic dark energy in the Brans–Dicke gravity. We employ the holographic model of dark energy to obtain the equation of state for the holographic energy density in non-flat (closed) universe enclosed by the event horizon measured from the sphere of horizon named L. Our analysis shows that one can obtain the phantom crossing scenario if the model parameter α (of order unity) is tuned accordingly. Moreover, this behavior is achieved by treating the Brans–Dicke scalar field as a Chameleon scalar field and taking a non-minimal coupling of the scalar field with matter. Hence one can generate phantom-like equation of state from a holographic dark energy model in non-flat universe in the Brans–Dicke cosmology framework.

Dynamical behavior of the extended holographic dark energy with the Hubble horizon

The extended holographic dark energy model with the Hubble horizon as the infrared cutoff avoids the problem of the circular reasoning of the holographic dark energy model. Unfortunately, it is hit with the no-go theorem. In this paper, we consider the extended holographic dark energy model with a potential, $V(\ensuremath{\phi})$, for the Brans-Dicke scalar field. With the addition of a potential for the Brans-Dicke scalar field, the extended holographic dark energy model using the Hubble horizon as the infrared cutoff is a viable dark energy model, and the model has the dark energy dominated attractor solution.

Accelerated Brans-Dicke universe in higher-derivative induced gravity-like conformally coupling and dominated by Gauss-Bonnet dark energy

We investigate the late-time dynamics of a four-dimensional universe based on the effective action of a Brans-Dicke scalar field in the presence of the matter source term, conformal coupling of the scalar curvature to the scalar field, a dynamical cosmological constant and Gauss-Bonnet higher-order terms in the scalar curvature. Many new interesting features are revealed and discussed in some details.

Naked singularity formation in Brans–Dicke theory

Gravitational collapse of the Brans–Dicke scalar field with non-zero potential in the presence of matter fluid obeying the barotropic equation of state, p = wρ, is studied. Utilizing the concept of the expansion parameter, it is seen that the cosmic censorship conjecture may be violated for and which correspond to the cosmic string and domain wall, respectively. We have shown that physically, it is the rate of collapse that governs the formation of a black hole or a naked singularity as the final fate of dynamical evolution and only for these two cases can the singularity be naked as the collapse end state. Also the weak energy condition is satisfied by the collapsing configuration.

Fermions in Brans-Dicke cosmology

Using the Brans-Dicke theory of gravitation we put under investigation a hypothetical universe filled with a fermionic field (with a self interaction potential) and a matter constituent ruled by a barotropic equation of state. It is shown that the fermionic field (in combination with the Brans-Dicke scalar field) could be responsible for a final accelerated era, after an initial matter dominated period.

Quantum cosmology with effects of a preferred reference frame

Recently, we presented a gravity model by generalizing the Brans–Dicke theory which is suitable for studying the metric signature transition dynamics without using an imaginary time parameter. Adding a suitable scalar potential described in terms of the Brans–Dicke scalar field ‘’, this alternative theory is used to study the Wheeler–DeWitt approach of quantum cosmology. We assumed that the universe is defined in a flat Robertson–Walker metric with Lorentzian signature. In that case, the Wheeler–DeWitt wavefunctional is obtained as two-dimensional quantum harmonic oscillator convergent polynomials for both of the choices of positive and negative values of the Brans–Dicke parameter. Here we choose a preferred reference frame with a time coordinate of ‘γ’ which relates to time of cosmological free falling observer ‘t’ as ‘’.

New Agegraphic Dark Energy in Brans–Dicke Theory

In this paper, we investigate the new agegraphic dark energy model in the framework of Brans–Dicke theory, which is a natural extension of the Einstein's general relativity. In this framework the form of the new agegraphic dark energy density takes as ρq = 3n2 Φ(t)η−2, where η is the conformal age of the universe and Φ(t) is the Brans–Dicke scalar field representing the inverse of the time-variable Newton's constant. We derive the equation of state of the new agegraphic dark energy and the deceleration parameter of the universe in the Brans–Dicke theory. It is very interesting to find that in the Brans–Dicke theory the agegraphic dark energy realizes quintom-like behavior, i.e., its equation of state crosses the phantom divide w = –1 during the evolution. We also compare the situation of the agegraphic dark energy model in the Brans–Dicke theory with that in the Einstein's theory. In addition, we discuss the new agegraphic dark energy model with interaction in the framework of the Brans–Dicke theory.

The cosmological constant and Pioneer anomaly from Weyl spacetimes and Mach's principle

It is shown how Weyl's geometry and Mach's holographic principle furnishes both the magnitude and sign (towards the sun) of the Pioneer anomalous acceleration aP∼−c2/RHubble firstly observed by Anderson et al. Weyl's geometry can account for both the origins and the value of the observed vacuum energy density (dark energy). The source of dark energy is just the dilaton-like Jordan–Brans–Dicke scalar field that is required to implement Weyl invariance of the most simple of all possible actions. A nonvanishing value of the vacuum energy density of the order of 10−123MPlanck4 is found consistent with observations. Weyl's geometry accounts also for the phantom scalar field in modern Cosmology in a very natural fashion.

CAN A HYPERBOLIC PHASE OF THE BRANS–DICKE FIELD ACCOUNT FOR DARK MATTER?

We show that the introduction of a hyperbolic phase of the Brans–Dicke (BD) field results in a flat vacuum cosmological solution of the Hubble parameter H and a fractional rate of change of the BD scalar field F, which asymptotically approach constant values. At later stages, the hyperbolic phase of the BD field behaves like dark matter.

Brans-Dicke scalar field as a chameleon

In this paper it is shown that in Brans-Dicke theory, if one considers a nonminimal coupling between the matter and the scalar field, it can give rise to a late time accelerated expansion for the Universe preceded by a decelerated expansion for very high values of the Brans-Dicke parameter $\ensuremath{\omega}$.

Evolution of primordial black holes in Jordan–Brans–Dicke cosmology

We consider the evolution of primordial black holes in a generalized Jordan-BransDicke cosmological model where both the Brans-Dicke scalar field and its coupling to gravity are dynamical functions determined from the evolution equations. The evaporation rate for the black holes is different from the case in standard cosmology. We show that the accretion of radiation can proceed effectively in the radiation dominated era. It follows that the black hole lifetime shortens for low initial mass, but increases considerably for larger initial mass, thus providing a mechanism for the survival of primordial black holes as candidates of dark matter. We derive a cut-off value for the initial black hole mass, below which primordial black holes evaporate out in the radiation dominated era, and above which they survive beyond the present era.

Can the Brans-Dicke Gravity with $lambda$ Possibly be a Theory of Dark Matter?

The pure Brans-Dicke (BD) gravity with or without the cosmological constant $\Lambda $ has been taken as a model theory for the dark matter. Indeed, there has been a consensus that unless one modifies either the standard theory of gravity, namely, general relativity, or the standard model for particle physics, or both, one can never achieve a satisfying understanding of the phenomena associated with dark matter and dark energy. Along this line, our dark matter model in this work can be thought of as an attempt to modify the gravity side alone in the simplest fashion to achieve the goal. Among others, it is demonstrated that our model theory can successfully predict the emergence of dark matter halo-like configuration in terms of a self-gravitating spacetime solution to the BD field equations and reproduce the flattened rotation curve in this dark halo-like object in terms of the non-trivial energy density of the BD scalar field, which was absent in the context of general relativity where Newton's constant is strictly a ``constant'' having no dynamics. Our model theory, however, is not entirely without flaw, such as the prediction of relativistic jets in all types of galaxies which actually is not the case.