Cosmic acceleration in Brans–Dicke cosmology

We consider Brans–Dicke theory with a self-interacting potential in Einstein conformal frame. We show that an accelerating expansion is possible in a spatially flat universe for large values of the Brans–Dicke parameter consistent with local gravity experiments.     

Equations of state in the Brans–Dicke cosmology

We investigate the Brans–Dicke (BD) theory with the potential as cosmological model to explain the present accelerating universe. In this work, we consider the BD field as a perfect fluid with the energy density and pressure in the Jordan frame. Introducing the power-law potential and the interaction with the cold dark matter, we obtain the phantom divide which is confirmed by the native and effective equation of state. Also we can describe the metric f(R) gravity with an appropriate potential, which shows a future crossing of the phantom divide in viable f(R) gravity models when employing the native and effective equations of state.     

Third quantization of Brans–Dicke cosmology

We study the third quantization of a Brans–Dicke toy model, we calculate the number density of the universes created from nothing and found that it has a Planckian form. Also, we calculated the uncertainty relation for this model by means of functional Schrödinger equation and we found that fluctuations of the third-quantized universe field tends to a finite limit in the course of cosmic expansion.     

Friedmann cosmology with decaying vacuum density in Brans–Dicke theory

The European Physical Journal C - We study numerical solutions corresponding to spherically symmetric gravitating electroweak monopole and magnetically charged black holes of the...     

Interacting entropy-corrected new agegraphic dark energy in Brans–Dicke cosmology

Motivated by a recent work of one of us (Sheykhi in Phys Rev D 81: 023525, 2010), we extend it by using quantum (or entropy) corrected new agegraphic dark energy in the Brans–Dicke cosmology. The correction terms are motivated from the loop quantum gravity which is one of the competitive theories of quantum gravity. Taking the non-flat background spacetime along with the conformal age of the universe as the length scale, we derive the dynamical equation of state of dark energy and the deceleration parameter. An important consequence of this study is the phantom divide scenario with entropy-corrected new agegraphic dark energy. Moreover, we assume a system of dark matter, radiation and dark energy, while the later interacts only with dark matter. We obtain some essential expressions related with dark energy dynamics. The cosmic coincidence problem is also resolved in our model.     

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.     

FRW cosmology from five dimensional vacuum Brans–Dicke theory

We follow the approach of induced-matter theory for a five-dimensional (5D) vacuum Brans–Dicke theory and introduce induced-matter and induced potential in four dimensional (4D) hypersurfaces, and then employ a generalized FRW type solution. We confine ourselves to the scalar field and scale factors be functions of the cosmic time. This makes the induced potential, by its definition, vanishes, but the model is capable to expose variety of states for the universe. In general situations, in which the scale factor of the fifth dimension and scalar field are not constants, the 5D equations, for any kind of geometry, admit a power–law relation between the scalar field and scale factor of the fifth dimension. Hence, the procedure exhibits that 5D vacuum FRW-like equations are equivalent, in general, to the corresponding 4D vacuum ones with the same spatial scale factor but a new scalar field and a new coupling constant, [(w)\tilde]{\tilde{\omega}} . We show that the 5D vacuum FRW-like equations, or its equivalent 4D vacuum ones, admit accelerated solutions. For a constant scalar field, the equations reduce to the usual FRW equations with a typical radiation dominated universe. For this situation, we obtain dynamics of scale factors of the ordinary and extra dimensions for any kind of geometry without any priori assumption among them. For non-constant scalar fields and spatially flat geometries, solutions are found to be in the form of power–law and exponential ones. We also employ the weak energy condition for the induced-matter, that gives two constraints with negative or positive pressures. All types of solutions fulfill the weak energy condition in different ranges. The power–law solutions with either negative or positive pressures admit both decelerating and accelerating ones. Some solutions accept a shrinking extra dimension. By considering non-ghost scalar fields and appealing the recent observational measurements, the solutions are more restricted. We illustrate that the accelerating power–law solutions, which satisfy the weak energy condition and have non-ghost scalar fields, are compatible with the recent observations in ranges −4/3 < ω ≤ −1.3151 for the coupling constant and 1.5208 ≤ n < 1.9583 for dependence of the fifth dimension scale factor with the usual scale factor. These ranges also fulfill the condition ${\tilde{\omega} > -3/2}${\tilde{\omega} > -3/2} which prevents ghost scalar fields in the equivalent 4D vacuum Brans–Dicke equations. The results are presented in a few tables and figures.     

Einstein and Brans–Dicke Frames in Multidimensional Cosmology

Inhomogeneous multidimensional cosmological models with a higher-dimensional space-time manifold _{0 i=1} ⁿ M_i (n 1) are in stigated under dimensional reduction to a D ₀-dimensional effective non-minimally coupled -model which generalizes the familiar Brans–Dicke model. The general form of the Einstein frame representation of multidimensional solutions known in the Brans–Dicke frame is given with respect to cosmic synchronous time. As an example, the transformation is demonstrated explicitly for the generalized Kasner solutions where it is shown that solutions in the Einstein frame show no inflation of the external space although they can undergo deflation after the cosmic synchronous time inversion.     

Brans–Dicke geometry

We reveal the non-metric geometry underlying ω→0$\omega \to 0$ Brans–Dicke theory by unifying the metric and scalar field into a single geometric structure. Taking this structure seriously as the geometry to which matter universally couples, we show that the theory is fully consistent with solar system tests. This is in striking contrast with the standard metric coupling, which grossly violates post-Newtonian experimental constraints.     

Instability analysis of a cylindrical stellar object in Brans–Dicke gravity

This paper investigates instability ranges of a cylindrically symmetric collapsing cosmic filamentary structure in the Brans–Dicke theory of gravity. For this purpose, we use a perturbating approach to the modified field equations as well as dynamic equations and construct a collapse equation. The collapse equation with an adiabatic index (Γ) is used to explore the instability ranges of both isotropic and anisotropic fluid in Newtonian and post-Newtonian approximations. It turns out that the instability ranges depend on the dynamic variables of collapsing filaments. We conclude that the system always remains unstable for 0 < Γ < 1, while Γ > 1 provides instability only in a special case.     

Holographic dark energy in Brans–Dicke theory

In this paper, the holographic dark-energy model is considered in Brans–Dicke theory, where the holographic dark-energy density ρ _Λ =3c ² M _pl² L ⁻² is replaced by ρ _h=3c ² Φ(t)L ⁻². Here is the time-variable Newton constant. With this replacement, it is found that no accelerated expansion for the universe will be achieved when the Hubble horizon is taken to play the role of an IR cut-off. When the event horizon is adopted as the IR cut-off, accelerated expansion for the universe is obtained. In this case, the equation of state of holographic dark energy, w _h, takes the modified form . In the limit α→0, the ‘standard’ holographic dark energy is recovered. In the holographic dark-energy dominated epoch, power-law and de Sitter time-space solutions are obtained.     

Anisotropic universe models in Brans–Dicke theory

This paper is devoted to study Bianchi type I cosmological model in Brans–Dicke theory with self-interacting potential by using perfect, anisotropic and magnetized anisotropic fluids. We assume that the expansion scalar is proportional to the shear scalar and also take a power law ansatz for the scalar field. The physical behavior of the resulting models are discussed through different parameters. We conclude that contrary to the universe model, the anisotropic fluid approaches isotropy at later times in all cases, which is consistent with observational data.     

Accelerated expansion of an anisotropic Brans–Dicke cosmology from nonlinear derivative interaction and Gauss–Bonnet invariant

The study of Brans–Dicke cosmology has attracted considerable attention in the recent years since it explains most of the important features of the progress of the universe. We discuss in this letter a homogeneous and anisotropic cosmological model in the framework of Brans–Dicke theory including together a non-linear derivative interaction which appears in theory with the Galilean shift symmetry, a Gauss–Bonnet invariant motivated from heterotic string theory which plays an important role in numerous alternatives cosmological frameworks, two scalar fields and their interactions to fit easier with universe history expansion. Several particular cases are studied and the properties related to scaling solutions and asymptotic behaviour are discussed in some details.     

Brans–Dicke Theory and Primordial Black Holes in Early Matter-Dominated Era

We show that primordial black holes can be formed in the matter-dominated era with gravity described by the Brans–Dicke theory. Considering an early matter-dominated era between inflation and reheating, we found that the primordial black holes formed during that era evaporate at a quicker rate than those of early radiation-dominated era. Thus, in comparison with latter case, less number of primordial black holes could exist today. Again the constraints on primordial black hole formation tend towards the larger value than their radiation-dominated era counterparts indicating a significant enhancement in the formation of primordial black holes during the matter-dominaed era.     

Interacting holographic dark energy in Brans–Dicke theory

We study cosmological application of interacting holographic energy density in the framework of Brans–Dicke cosmology. We obtain the equation of state and the deceleration parameter of the holographic dark energy in a non-flat universe. As system's IR cutoff we choose the radius of the event horizon measured on the sphere of the horizon, defined as L=ar(t)$L=ar(t)$. We find that the combination of Brans–Dicke field and holographic dark energy can accommodate wD=−1$w_D=-1$ crossing for the equation of state of noninteracting   holographic dark energy. When an interaction between dark energy and dark matter is taken into account, the transition of wD$w_D$ to phantom regime can be more easily accounted for than when resort to the Einstein field equations is made.     

Non-singular Brans–Dicke collapse in deformed phase space

We study the collapse process of a homogeneous perfect fluid (in FLRW background) with a barotropic equation of state in Brans–Dicke (BD) theory in the presence of phase space deformation effects. Such a deformation is introduced as a particular type of non-commutativity between phase space coordinates. For the commutative case, it has been shown in the literature (Scheel, 1995), that the dust collapse in BD theory leads to the formation of a spacetime singularity which is covered by an event horizon. In comparison to general relativity (GR), the authors concluded that the final state of black holes in BD theory is identical to the GR case but differs from GR during the dynamical evolution of the collapse process. However, the presence of non-commutative effects influences the dynamics of the collapse scenario and consequently a non-singular evolution is developed in the sense that a bounce emerges at a minimum radius, after which an expanding phase begins. Such a behavior is observed for positive values of the BD coupling parameter. For large positive values of the BD coupling parameter, when non-commutative effects are present, the dynamics of collapse process differs from the GR case. Finally, we show that for negative values of the BD coupling parameter, the singularity is replaced by an oscillatory bounce occurring at a finite time, with the frequency of oscillation and amplitude being damped at late times.     

Cylindrically symmetric Brans–Dicke–Maxwell solutions

We investigate cylindrically symmetric vacuum solutions with both null and non-null electromagnetic fields in the framework of the Brans–Dicke theory and compare these solutions with some of the well-known solutions of general relativity for special values of the parameters of the resulting field functions. We see that, unlike general relativity where the gravitational force of an infinite and charged line mass acting on a test particle is always repulsive, it can be attractive or repulsive for Brans–Dicke theory depending on the values of the parameters as well as the radial distance from the symmetry axis.     

Phantom energy accretion and primordial black holes evolution in Brans–Dicke theory

In this work, we study the evolution of primordial black holes within the context of Brans–Dicke theory by considering the presence of a dark energy component with a super-negative equation of state, called phantom energy, as a background. Besides Hawking evaporation, here we consider two types of accretion—radiation accretion and phantom energy accretion. We found that radiation accretion increases the lifetime of primordial black holes whereas phantom accretion decreases the lifespan of primordial black holes. Investigating the competition between the radiation accretion and phantom accretion, we found that there is an instant during the matter-dominated era beyond which phantom accretion dominates radiation accretion. So the primordial black holes which are formed in the later part of radiation-dominated era and in matter-dominated era are evaporated at a quicker rate than by Hawking evaporation. But for presently evaporating primordial black holes, radiation accretion and Hawking evaporation terms are dominant over the phantom accretion term and hence presently evaporating primordial black holes are not much affected by phantom accretion.