Brans-Dicke Research Articles

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.

Quantum cosmology in coupled Brans–Dicke gravity: A Noether symmetry analysis

This work deals with a multi-field cosmological model in a spatially flat FLRW spacetime geometry. The usual Brans–Dicke (BD) field and another scalar field are minimally coupled to gravity while they interact with each other through the Kinetic terms. The main aim of this work is to examine whether the model is compatible with cosmic observations. So cosmological solutions are obtained using symmetry analysis only. By imposing Noether Symmetry to the Lagrangian of the system, the potential of the scalar field as well as the coupling function had been determined. The classical solutions are determined after simplifying the Lagrangian using cyclic variables. Finally, Wheeler–DeWitt (WD) equation in quantum cosmology has been formulated and conserved momenta corresponding to Noether symmetry will show the periodic part of the wave function and it helps to have the complete integral for the wave function.

Cosmological aspects of anisotropic chameleonic Brans–Dicke gravity

We study the cosmological implications of non-minimal coupling of a scalar field with matter sector in the Brans–Dicke gravity. In particular, in the context of locally rotationally symmetric Bianchi-I (LRS BI) metric, we derived the field equations and used these equations to find a relation between scale factor and scalar field. We use this relation to find two different solutions, namely an eternally accelerating universe and a transit universe. By the power-law solutions for scalar field and coupling function, we find the allowed range of exponents for an accelerating universe. We further explore the direction and magnitude of energy transfer between scalar field and matter sector. We show by using the second law of thermodynamics that the direction and magnitude of energy transfer are closely related to the behavior of coupling function. In the transit universe, we investigate the dependence of the effective equation of state parameter and the effective energy density on the anisotropy parameter. The model explains the late-time accelerating universe evolution.

Stationary black holes and stars in the Brans-Dicke theory with Λ>0 revisited

It was shown a few years back that for a stationary regular black hole or star solution in the Brans-Dicke theory with a positive cosmological constant $\mathrm{\ensuremath{\Lambda}}$, endowed with a de Sitter or cosmological event horizon in the asymptotic region, not only there exists no nontrivial field configurations, but also the inverse Brans-Dicke parameter ${\ensuremath{\omega}}^{\ensuremath{-}1}$ must be vanishing. This essentially reduces the theory to Einstein's general relativity. The assumption of the existence of the cosmological horizon was crucial for this proof. However, since the Brans-Dicke field $\ensuremath{\phi}$, couples directly to the $\mathrm{\ensuremath{\Lambda}}$-term in the energy-momentum tensor as well as $\mathrm{\ensuremath{\Lambda}}$ acts as a source in $\ensuremath{\phi}$'s equation of motion, it seems reasonable to ask: can $\ensuremath{\phi}$ become strong instead and screen the effect of $\mathrm{\ensuremath{\Lambda}}$, at very large scales, so that the asymptotic de Sitter structure is replaced by some alternative, yet still acceptable boundary condition? In this work we analytically argue that no such alternative exists, as long as the spacetime is assumed to be free of any naked curvature singularity. We further support this result by providing explicit numerical computations. Thus we conclude that in the presence of a positive $\mathrm{\ensuremath{\Lambda}}$, irrespective of whether the asymptotic de Sitter boundary condition is imposed or not, a regular stationary black hole or even a star solution in the Brans-Dicke theory always necessitates ${\ensuremath{\omega}}^{\ensuremath{-}1}=0$, and thereby reducing the theory to general relativity. The qualitative differences of this result with that of the standard no hair theorems are also pointed out.

Constraint on Brans-Dicke theory from intermediate/extreme mass ratio inspirals

Intermediate/Extreme mass ratio inspiral (I/EMRI) system provides a good toolto test the nature of gravity in strong field.Based on the method of osculating orbits,we compute the orbital evolutions of I/EMRIs on quasi-elliptic orbits inboth Einstein's general relativity and Brans-Dicke theory.The extra monopolar and dipolar channels in Brans-Dicke theory accelerate the orbital decay,so it is important to consider the effects of monopolar and dipolar emissions on the waveform.With the help of accurate orbital motion,we generate waveform templates which include both monopolar and dipolar contributions for I/EMRIs on eccentric orbits in Brans-Dicke theory.With a two-year observation of gravitational waves emitted from I/EMRIs by LISA,we get the most stringent constraint on the Brans-Dicke coupling parameter ω 0 > 106.

Decoupled quark stars relativistic models in the regime of self-interacting Brans–Dicke gravity

In this work, we generate two static anisotropic solutions for a sphere containing quark matter in the framework of self-interacting Brans–Dicke theory. For this purpose, we add an anisotropic source in the seed distribution and decouple the field equations through deformation in the radial metric function. As a result of this transformation, the field equations are disintegrated into two systems which separately include the effects of isotropic and anisotropic sources. The system related to the additional source is solved via the MIT bag model equation of state. We consider Tolman V spacetime and Karmarkar condition to formulate two solutions for the isotropic sector which are extended to the anisotropic domain via decoupling technique. The junction conditions at the boundary determine the unknown parameters in terms of mass and radius of the spherical object. We investigate the viability and stability of the constructed strange star models in the presence of massive scalar field corresponding to three strange star candidates: Her X-1, PSR J1614-2230 and 4U 1608-52. It is concluded that the anisotropic models are well-behaved as they fulfill the necessary requirements for lower as well as higher values of the decoupling parameter. • We generate two anisotropic spherical solutions with quark matter in self-interacting Brans–Dicke theory. • Viability and stability of the constructed strange star models are checked for three strange star candidates. • Anisotropic models are well-behaved for lower as well as higher values of the decoupling parameter.

Viscous generalized Chaplygin gas model in Brans–Dicke theory

The Kantowski–Sachs cosmological model for viscous generalized Chaplygin gas has been investigated in the Brans–Dicke theory of gravitation. To determine the solutions of field equations, we have considered the power-law relation for the average scale factor. We have computed some cosmological parameters and discussed their physical importance. Furthermore, we have discussed the nature of statefinder and [Formula: see text] diagnostics in our model. It is also worth noting that the conclusions of the cosmological parameter are consistent with modern observational data.

Self-gravitating spherically symmetric systems with Q h condition in chameleonic Brans–Dicke gravity

Abstract This paper is asserted to explore the self-gravitating spherically symmetric anisotropic fluids in Chameleonic Brans–Dicke theory as dark energy matter. The dissipative and non-dissipative cases for the evolution of the system are discussed evidently satisfying the quasi-homologous condition with vanishing complexity (Y TF ) factor, which is identified in the trace free part of the electric Riemann tensor in splitting of the curvature tensor. We formulate different equations through conformal tensor, mass function, shear stress tensor, scalar field to govern self-gravitating systems. A few models describe center filled fluid distribution whereas some of them have cavities surrounding the center by means of matching conditions on the boundary as well as on inner surfaces. The temperature of the respective models is also discussed here. Finally, we conclude the work by comparing it with GR.

Cosmological model in Brans–Dicke theory of gravitation

Cosmological model in Brans–Dicke theory of gravitation

Further considerations about the traversability of thin-shell wormholes

Traversability in relation with tides in thin-shell wormholes is revisited to investigate the possibility of improving recently noted restrictive conditions for a safe travel across a wormhole throat. We consider wormholes mathematically constructed starting from background geometries which are solutions of scalar–tensor theories as dilaton gravity and Brans–Dicke gravity. The advantages of working within such frameworks are studied by examining the dependence of the extrinsic curvature and tides at the throat with the parameters determining the departure from pure relativity; the associated behaviour of tides in the smooth regions of the geometries is also analyzed. Other related but different approaches within pure relativity are discussed in the appendices.

Some Bianchi Type Viscous Holographic Dark Energy Cosmological Models in the Brans–Dicke Theory

In this article, we analyze Bianchi type–II, VIII, and IX spatially homogeneous and anisotropic space-times in the background of the Brans–Dicke theory of gravity within the framework of viscous holographic dark energy. To solve the field equations, we have used the relation between the metric potentials as R = S n and the relation between the scalar field ϕ and the scale factor a as ϕ = a m . Also, we have discussed some of the dynamical parameters of the obtained models, such as the deceleration parameter q , the jerk parameter j , the EoS parameter ω v h d e , the density parameter Ω vhde , Om-diagnostic, squared speed of sound v s 2 , EoS plane ω v h d e − ω v h d e ′ , and statefinder plane r − s through graphical representation, which are significant in the discussion of cosmology. Furthermore, all the models obtained and graphically presented shown an expanding and accelerating Universe, which is in better agreement with the latest experimental data. The viscous holographic dark energy models are compatible with explaining the present cosmic accelerated expansion.

Thin-shell Solution of Chameleon Mechanism in Brans-Dicke Scalar-Tensor Model

We investigated the chameleon screening mechanism in Brans-Dicke scalar-tensor theory. We preserved the fundamental premise of the typical chameleon mechanism, which is that the field is huge in high-density environments but essentially free in low-density environments like the solar system. We discovered thin-shell solutions for a static and spherical symmetric body and demonstrated the model’s applicability to local data.

Validation of energy conditions in traversable wormhole for GBD modified theory

In this work, we explore the wormhole physics in a modified gravitational theory, called the generalized Brans–Dicke theory (GBD). In order to solve the equations in this theory, the parameterized forms for the shape function and the scalar-field function are taken. With the help of selecting a specific form of state equation, i.e. considering a relation between the transverse pressure (not the total pressure) and the energy density for matter that threads wormhole: [Formula: see text], we provide some special solutions of the traversable wormhole in GBD. Given that the violation of energy conditions (ECs) of matter often induces problems on the gravitational theory, it is important to inspect the ECs of matter in modified theories. In this GBD theory, we find that the weak energy condition, the null energy condition, the dominated energy condition and the strong energy condition could be satisfied for the matter near the throat of wormhole, depending on the parameterized models and the model-parameters values. Finally, using the classical reconstruction technique in the traversable wormhole physics, we derive a Lagrangian function of gravitational field for the GBD 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.

Modified-gravity theories with nondynamical background fields

We study the dynamics of a modified-gravity theory, which is supplemented by an extended Gibbons-Hawking-York boundary term and incorporates diffeomorphism violation through nondynamical background fields denoted as $u$ and $s^{\mu\nu}$ in the literature. An ADM decomposition allows us to project the modified Einstein equations into purely spacelike hypersurfaces, which implies the field equations for the induced dynamical three-metric. We also obtain the Hamilton-Jacobi equations of motion for the canonical variables of the theory based on its Hamiltonian, which was derived in a previous work. The computations show that the dynamical field equations obtained from the Lagrangian and Hamiltonian approaches are consistent with each other. Connections to Brans-Dicke theory and ghost-free massive gravity are established.