Scalar field instability in multi-dimensional cosmology

The scalar field theory on the background of cosmological models with n(n ≥ 1) spaces of constant curvature is considered. We take the integrable case of Ricci flat internal spaces. The coupling between the scalar and the gravitational fields includes the minimal coupling as well as the conformal case. In the ground state of the scalar field we find the conditions for vacuum instability realized for most of the possible solutions to Einstein's equations if the coupling parameter takes appropriate values. For the excited states of the scalar field we show the induction of massive modes and discuss their properties.     

Inflationary solutions for anisotropic model in scalar-tensor theory

In this paper we have studied B-D theory and general scalar tensor theory of Gravitation for anisotropic cosmological model in the false vacuum state. The possibility of both exponential inflation and power function inflation are examined for constant or variable coupling parameter . Also asymptotic limit of the scalar field and are discussed.     

An Interacting Model of Dark Energy in Brans-Dicke Theory

In this paper it is shown that in non-minimally coupled Brans-Dicke theory containing a self-interacting potential, a suitable conformal transformation can automatically give rise to an interaction between the normal matter and the Brans-Dicke scalar field. Considering the scalar field in the Einstein frame as the quintessence matter, it has been shown that such a non-minimal coupling between the matter and the scalar field 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 ω. We have also studied the observational constraints on the model parameters considering the Hubble and Supernova data.     

Emergent universe with scalar (or tachyonic) field in higher derivative theory

We consider a spatially homogeneous and isotropic flat Robertson-Walker model filled with a scalar (or tachyonic) field minimally coupled to gravity in the framework of higher derivative theory. We discuss the possibility of the emergent universe with normal and phantom scalar fields (or normal and phantom tachynoic fields) in higher derivative theory. We find the exact solution of field equations in normal and phantom scalar fields and observe that the emergent universe is not possible in normal scalar field as the kinetic term is negative. However, the emergent universe exists in phantom scalar field in which the model has no time-like singularity at infinite past. The model evolves into an inflationary stage and finally admits an accelerating phase at late time. The equation of state parameter is found to be less than −1 in early time and tends to −1 in late time of the evolution. The scalar potential increases from zero at infinite past to a flat potential in late time. More precisely, we discuss the particular case for phantom field in detail. We also carry out a similar analysis in case of normal and phantom tachyonic field and observe that only phantom tachyonic field solution represents an emergent universe. We find that the coupling parameter of higher order correction affects the evolution of the emergent universe. The stability of solutions and their physical behaviors are discussed in detail.     

Kaluza-Klein dark energy cosmological model in scale Co-variant Theory of Gravitation

A five dimensional Kaluza-Klein dark energy model with variable EoS parameter is investigated in the scale co-variant theory of gravitation proposed by Canuto et al. (in Phys. Rev. 39:429, 1977) in a five dimensional Kaluza-Klein space-time in the presence of perfect fluid source. Using the special law of variation for Hubble’s parameter proposed by Berman (in Nuovo Cimento B 74:183, 1983), we have obtained a determinate solution which represents a dark energy cosmological model in the theory. We have also used the result that the scalar expansion is proportional to shear scalar of the space-time. It is observed that the EoS parameter, skewness parameter in the model turn out to be functions of cosmic time. Some physical and Kinematical properties of the model are also discussed.     

Cosmological models in generalized scalar-tensor theory with cosmological term and bulk viscosity

Exponential and power law solutions for homogeneous cosmological modelshave been discussed in a generalized scalar-tensor theory where both thecosmological term and coupling parameter are taken tobe functions of the scalar field , along with the bulk viscousstress of the cosmic fluid. Assuming simple relationships of the scalarfield with the cosmological term and the coupling parameter, all thesolutions are studied in Dicke's revised units. By use of a conformaltransformation, the solutions are also expressed in atomic units.     

Exact Kantowski–Sachs cosmological models in general theory of relativity

We have studied cosmological model generated by perfect fluid coupled with mass less scalar field for Kantowski–Sachs space–time in general theory of relativity. Two different physically viable models of the universe are obtained by using a special law of variation for Hubble’s parameter that yields a constant value of deceleration parameter. Some physical consequences of the models have been discussed in case of Zel’dovich fluid.     

Perturbation spectrum in the inflationary stage when a cosmic string is present

We examine the spectrum of perturbations for a scalar field with an arbitrary curvature coupling parameter in the de Sitter stage of cosmological expansion when a cosmic string is present. These perturbations are caused by vacuum fluctuations in the field and serve as seed perturbations for the formation of galaxies in the postinflationary stage. A cosmic string disrupts the homogeneity of a de Sitter space, so that the spectrum of the perturbations depends on the distance from the string. This dependence is oscillatory in character with a period on the order of the perturbation wavelength.     

Non-Existence Of <Emphasis Type="Italic">N</Emphasis>-Dimensional Static Plane Symmetric Solutions In Bimetric Relativity Theory

A problem of static plane symmetric metric in the perfect fluid, the mesonic massive scalar field and in their coupling is studied in Rosen’s (1973) bimetric theory of relativity. It was found that the matter field like either perfect fluid or mesonic massive scalar field or their coupling does not survive in bimetric theory of gravitation when the space–time is governed by n-dimensional static plane symmetric metric.     

On the holographic dark energy in chameleon scalar-tensor cosmology

We study the holographic dark energy (HDE) model in generalized Brans-Dicke scenario with a non-minimal coupling between the scalar field and matter Lagrangian namely Chameleon Brans Dicke (CBD) mechanism. In this study we consider the interacting and non-interacting cases for two different cutoffs. The physical quantities of the model such as, equation of state (EoS) parameter, deceleration parameter and the evolution equation of dimensionless parameter of dark energy are obtained. We shall show that this model can describe the dynamical evolution of fraction parameter of dark energy in all epochs. Also we find the EoS parameter can cross the phantom divide line by suitable choices of parameters without any mines kinetic energy term.     

Dark Matter from the inflaton field

We present a model where inflation and Dark Matter takes place via a single scalar field ?. Without introducing any new parameters we are able unify inflation and Dark Matter using a scalar field ? that accounts for inflation at an early epoch while it gives a Dark Matter WIMP particle at low energies. After inflation our universe must be reheated and we must have a long period of radiation dominated before the epoch of Dark Matter. Typically the inflaton decays while it oscillates around the minimum of its potential. If the inflaton decay is not complete or sufficient then the remaining energy density of the inflaton after reheating must be fine tuned to give the correct amount of Dark Matter. An essential feature here, is that Dark Matter-Inflaton particle is produced at low energies without fine tuning or new parameters. This process uses the same coupling g as for the inflaton decay. Once the field ? becomes non-relativistic it will decouple as any WIMP particle, since n_? is exponentially suppressed. The correct amount of Dark Matter determines the cross section and we have a constraint between the coupling g and the mass m_o of ?. The unification scheme we present here has four free parameters, two for the scalar potential V(?) given by the inflation parameter λ of the quartic term and the mass m_o. The other two parameters are the coupling g between the inflaton ? and a scalar filed φ and the coupling h between φ with standard model particles ψ or χ. These four parameters are already present in models of inflation and reheating process, without considering Dark Matter. Therefore, our unification scheme does not increase the number of parameters and it accomplishes the desired unification between the inflaton and Dark Matter for free.     

Conformal analogs of the Jordan-Brans-Dicke theory. II. (models including vacuum effects)

This paper is part of a series based on a modified Jordan tensor-scalar theory of gravitation. Given the current importance of research on vacuum phenomena in cosmic evolution, we examine several standard cosmological models with a scalar field and a physical vacuum, including models that have a dominant scalar field with the vacuum energy taken into account in various conformal representations of the Jordan theory, as well as models in which ordinary matter that obeys the conventional equations of state is present. Some noteworthy results are obtained which are, to a certain extent, consistent with currently available observational data.     

String theory, cosmology and varying constants

In string theory the coupling `constants' appearing in the low-energy effective Lagrangian are determined by the vacuum expectation values of some (a priori) mass less scalar fields (dilaton, moduli). This naturally leads one to expect a correlated variation of all the coupling constants, and an associated violation of the equivalence principle. We review some string-inspired theoretical models which incorporate such a space time variation of coupling constants while remaining naturally compatible both with phenomenological constraints coming from geochemical data (Oklo; Rhenium decay) and with present equivalence principle tests. Barring a very unnatural fine-tuning of parameters, a variation of the fine-structure constant as large as that recently `observed' by Webb et al. in quasar absorption spectra appears to be incompatible with these phenomenological constraints. Independently of any model, it is emphasized that the best experimental probe of varying constants are high-precision tests of the universality of free fall, such as MICROSCOPE and STEP. This revised version was published online in July 2006 with corrections to the Cover Date.     

Tachyonic teleparallel dark energy

Teleparallel gravity is an equivalent formulation of general relativity in which instead of the Ricci scalar R, one uses the torsion scalar T for the Lagrangian density. Recently teleparallel dark energy has been proposed by Geng et al. (in Phys. Lett. B 704, 384, 2011). They have added quintessence scalar field, allowing also a non-minimal coupling with gravity in the Lagrangian of teleparallel gravity and found that such a non-minimally coupled quintessence theory has a richer structure than the same one in the frame work of general relativity. In the present work we are interested in tachyonic teleparallel dark energy in which scalar field is responsible for dark energy in the frame work of torsion gravity. We find that such a non-minimally coupled tachyon gravity can realize the crossing of the phantom divide line for the effective equation of state. Using the numerical calculations we display such a behavior of the model explicitly.     

Kaluza-Klein FRW dark energy models in Saez-Ballester theory of gravitation

This article is concerned with the investigation of dynamical behaviour of Kaluza-Klein(KK) FRW type dark energy cosmological models in the framework of a scalar-tensor theory of gravitation formulated by Saez and Ballester (Phys. Lett. A113,467:1986). Three cosmological models, in this theory, are presented by solving the field equations using (i) hybrid expansion law given by Pradhan et al. (Ind.J.Phys.89,5032015), (ii)varying deceleration parameter proposed by Mishra et al.(Int. J. Theor. Phy.52, 2546: 2013) and(iii)linearly varying deceleration parameter defined by Akarsu and Dereli (Int. J. Theor. Phys. 51, 612: 2012). We have evaluated the dynamical parameters for each of the models, namely, the equation of state (EoS) parameter, the deceleration parameter, statefinder parameter and total energy density parameter of dark energy. We have also found the scalar field in the models. We have discussed the dynamical behavior of the parameters through graphical representation with special reference to Planck Collaboration data. It is observed that our models describe accelerated expansion of the universe and our theoretical results are, reasonably, in good agreement with the observational data.     

Torsion,massive electrodynamics and gravitation induced by scalar fields

The equivalence of Lagrangian containing gravitational, electromagnetic, scalar, and torsion fields is discussed. It is shown that the equation for the variation of the scalar field leads to a torsion wave equation generated by electromagnetic field leads to a torsion wave equation generated by electromagnetic fields. The system is proved to be equivalent to a Proca field coupling torsion non-minimally to a massive photon and having the scalar Higgs field as a strength of this photon-torsion coupling. The generalized Maxwell equations containing the scalar fields are obtained. The torsion potential around the Sun or a more massive collapsing star in the weak field limit is estimated.     

A Cosmological Model with Negative Constant Deceleration Parameter in a Scalar-Tensor Theory

With the help of a special law of variation for Hubble's parameter presented by Bermann [Nuovo Cimento B (1983), 74, 182], a cosmological model with negative constant deceleration parameter is obtained in the framework of Saez-Ballester [Phys. Lett (1985), Al 13, 467] scalar -- tensor theory of gravitation. Some physical and kinematical properties of the model are, also, discussed.     

Interacting holographic dark energy with variable deceleration parameter and tachyon scalar field dark energy model in LRS Bianchi type-II universe

In this work, we have considered the spatially homogeneous and anisotropic Bianchi type-II universe filled with two interacting fluids; dark matter and holographic dark energy components. Assuming the proportionality relation between one of the components of shear scalar and expansion scalar which yields time dependent deceleration parameter, an exact solution to Einstein’s field equations in Bianchi type-II line element is obtained. We have investigated geometric and kinematics properties of the model and the behaviour of the holographic dark energy. It is observed that the mean anisotropic parameter is uniform through the whole evolution of the universe and the coincidence parameter increases with increasing time. The solutions are also found to be in good agreement with the results of recent observations. We have applied the statefinder diagnostics method to study the behaviour of different stages of the universe and to differentiate the proposed dark energy model from the ΛCDM model. We have also established a correspondence between the holographic dark energy model and the tachyon scalar field dark energy model. We have reconstructed the potential and the dynamics of the tachyon scalar field, which describes accelerated expansion of the universe.     

Modified equations in the theory of induced gravity. Solution to the cosmological constant problem

This research is an extension of the author’s works, in which conformally invariant generalization of string theory was suggested to higher-dimensional objects. Special cases of the proposed theory are Einstein’s theory of gravity and string theory. This work is devoted to the formation of self-consistent equations of the theory of induced gravity in the presence of matter in the form of a perfect fluid that interacts with scalar fields. The study is done to solve these equations for the case of the cosmological model. In this model time-evolving gravitational and cosmological “constants” take place which are determined by the square of scalar fields. The values of which can be matched with the observational data. The equations that describe the theory have solutions that can both match with the solutions of the standard theory of gravity as well as it can differ from it. This is due to the fact that the fundamental “constants” of the theory, such as gravitational and cosmological, can evolve over time and also depend of the coordinates. Thus, in a rather general case the theory describes the two systems (stages): Einstein and “evolving”. This process is similar to the phenomenon of phase transition, where the different phases (Einstein gravity system, but with different constants) transit into each other.     

Relativistic Cosmology with Zero Deceleration Parameter

The possibility that the deceleration parameter q might be a null constant is discussed; such possibilitiy is interesting because solves the horizon problem and the flatness problem with no need of inflation. A simple way to get q = const = 0 is explored: the way assumes Einstein's field equations without cosmological term and introduces a massless scalar field V with negative energy density. Both in the early and in the present universe one finds the Whitrow-Randall relation Gut² = const 1 G gravitational coupling, u mass-energy density, t cosmic time). The interaction between the V-field and the ordinary matter is briefly discussed; as possible consequence of this interaction the true value of the Hubble parameter might be one half the observed value. This revised version was published online in July 2006 with corrections to the Cover Date.