Role of chameleon field in accelerating Universe

In this work, we have considered a model of the flat Friedmann–Robertson–Walker (FRW) Universe filled with cold dark matter and a chameleon field where the scale function is taken as (i) intermediate expansion and (ii) logamediate expansion. In both cases we find the expressions of the chameleon field, chameleon potential, statefinder parameters, and slow-roll parameters. Also, it has been shown that the potential always decreases with the chameleon field in both scenarios. The nature of the slow-roll parameters has been shown diagrammatically.     

Magnetized chameleonic Brans-Dicke cosmology and phase space analysis

This paper deals with the study of dynamical or phase space analysis of Bianchi I universe in Brans-Dicke gravity with chameleon scalar field. For this purpose, the matter contents are taken to be perfect fluid with magnetic field effects described by the non-linear Maxwell Lagrangian density. By taking some ansatz for the field potential and the interaction function in chameleon cosmology, we discuss three cases: Bianchi I universe with perfect fluid, FRW universe with magnetized perfect fluid and Bianchi I universe with magnetized perfect fluid. In all cases, we calculate fixed or critical points and discuss stability of the respective configuration for radiation as well as matter dominated eras. We also evaluate some cosmological parameters in each case for matter dominated era only and investigate their cosmological implications.     

Thermodynamics and cosmic expansion in magnetized chameleonic Brans-Dicke universe

This study is emphasized to explore the validity of generalized second law of thermodynamics in the context of non-linear electrodynamics (magnetic effects only) with Brans-Dicke chameleon scalar field as dark energy candidate. For this purpose, we consider FRW universe model with perfect fluid matter contents. We evaluate matter energy density and magnetic field by taking interacting and non-interacting cases of magnetic field and matter as well as the power law ansatz for scalar field. The validity of this law is discussed by using the first law of thermodynamics for four different horizons: Hubble, apparent, particle and event horizons. We conclude that this law may hold for all four horizons with small positive red-shift when chameleon mechanism is taken into account in Brans-Dicke gravity. Finally, we investigate the statefinders in order to check the viability of the model.     

Observational constraints on tachyonic chameleon dark energy model

It has been recently shown that tachyonic chameleon model of dark energy in which tachyon scalar field non-minimally coupled to the matter admits stable scaling attractor solution that could give rise to the late-time accelerated expansion of the universe and hence alleviate the coincidence problem. In the present work, we use data from Type Ia supernova (SN Ia) and Baryon Acoustic oscillations to place constraints on the model parameters. In our analysis we consider in general exponential and non-exponential forms for the non-minimal coupling function and tachyonic potential and show that the scenario is compatible with observations.     

Equivalence principle (EP) and solar system constraints on $R(1\pm\epsilon\ln({R \over R_{c}}))$ model of gravity

Experiments on the violation of equivalence principle (EP) and solar system give a number of constraints in which any modified gravity model must satisfy them. We study these constraints on a kind of f(R) gravity as f(R) = R(1±eln([(R)/(R_c)]))f(R) = R(1\pm\epsilon\ln({R \over R_{c}})). For this investigation we use of chameleon mechanism and show that a spherically body has thin-shell in this model. So that we obtain an effective coupling of the fifth force which is suppressed through a chameleon mechanism. Also, we obtain γ _PPN =1±1.13×10⁻⁵ which is agreement with experiment results. At last, we show that for R _c ≈ρ _c this model is consistent with EP, thin shell condition and fifth force of chameleon mechanism for ε⋍10⁻¹⁴.     

Chameleon mechanism with a new potential

Chameleon scalar field is a new model, which introduced to provide a mechanism for exhibiting accelerating universe. Chameleon field has several interesting aspects, such as field dependence on the local matter density. For this model we introduce a new kind of potential which has run away form and satisfies chameleon constraints. The results are acceptable in comparison with the other potentials which studied up to now.     

Entropy and statefinder diagnosis in chameleon cosmology

In this paper, the generalized second law (GSL) of thermodynamics and entropy is revisited in the context of cosmological models with bouncing behavior such as chameleon cosmology where the boundary of the universe is assumed to be enclosed by the dynamical apparent horizon. From a thermodynamic point of view, to link between thermodynamic and geometric parameters in cosmological models, we introduce “entropy rate of change multiplied by the temperature” as a model independent thermodynamic state parameter together with the well known {r,s} statefinder to differentiate the dark energy models.     

Notes on the Chameleon Brans-Dicke gravity

We consider a generalized Brans-Dicke model in which the scalar field has a potential function and is also allowed to couple non-minimally with the matter sector. This anomalous gravitational coupling can in principle avoid the model to pass local gravity experiments. One then usually assumes that the scalar field has a chameleon behavior in the sense that it acquires a density-dependent effective mass. While it can take a small effective mass in cosmological (low-density environment) scale, it has a sufficiently heavy mass in Solar System (large-density environment) and then hides gravity tests. We will argue that such a chameleon behavior can not be generally realized and depends significantly on the forms attributed to the potential and the coupling functions.     

New Agegraphic Dark Energy model in chameleon Brans-Dicke cosmology for different forms of the scale factor

In this work, we study the New Agegraphic Dark Energy (NADE) model (which contains the conformal time η as infrared cut-off) in the framework of Brans-Dicke cosmology with chameleon scalar field which is non-minimally coupled to the matter field. Considering interacting Dark Energy and Dark Matter (DM), we calculate some relevant cosmological parameters, i.e. the equation of state (EoS) parameter, the deceleration parameter q and the evolution of the energy density parameter $\varOmega_{D}'$ for different forms of scale factors, i.e. the power-law, the emergent, the intermediate and the logamediate ones, which leads to different expressions of η.     

Self-similar chameleon Jordan-Brans-Dicke cosmological models

In this paper we study the chameleon Jordan-Brans-Dicke (JBD) cosmological models under the hypothesis of self-similarity. Since there are several ways to define the matter Lagrangian for a perfect fluid: L _m =?ρ and L _m =γρ, we show that they bring us to obtain two completely different cosmological models. In the first approach, L _m =?ρ, there is ordinary matter conservation, while in the second approach, L _m =γρ, we get matter creation processes. We deduce for each approach the behaviour of each physical quantity, under the self-similar hypothesis, by employing the Lie group method. The results are quite general and valid for any homogeneous geometry (FRW, Bianchi types, etc.). As example, we calculate exact solutions for each approach by considering the case of a Bianchi II geometry. In this way we can determine the exact behaviour of each physical quantity and in particular of G _eff and U (the potential that mimics the cosmological constant).We compare the solutions with the obtained ones in the framework of the usual JBD models.     

Small-scale clumping in molecular clouds

Summary This review consists of five sections. In the introduction, we briefly review the development of the study of molecular clouds. In the second section, we review the theories of molecular cloud structure and compare these predictions with the statistical properties of the clouds. In Sect. 3 we give an overview of current approaches to determinations of mass and local density in clouds. In the fourth part, we discuss the observations of a selected sample of individual sources. The emphasis here is on high resolution studies of regions of star formation. The final section contains a discussion of instrumental limitations and mentions some future developments.     

Linear Stability of the Lagrangian Triangle Solutions for Quasihomogeneous Potentials

In this paper, we study the linear stability of the relative equilibria for homogeneous and quasihomogeneous potentials. First, in the case the potential is a homogeneous function of degree −a, we find that any relative equilibrium of the n-body problem with a>2 is spectrally unstable. We also find a similar condition in the quasihomogeneous case. Then we consider the case of three bodies and we study the stability of the equilateral triangle relative equilibria. In the case of homogeneous potentials we recover the classical result obtained by Routh in a simpler way. In the case of quasihomogeneous potentials we find a generalization of Routh inequality and we show that, for certain values of the masses, the stability of the relative equilibria depends on the size of the configuration.     

活动星系核的多波段能谱及其发射机制──Ⅱ．红外连续辐射能谱

本文讨论了１６个红外强ＰＧ类星体的红外辐射能谱。我们假设这些活动星系核的红外谱是由非热辐射机制和尘埃的热辐射共同产生的，通过对红外包的最佳拟合，我们发现大多数ＰＧ类星体的红外包位于７一２４μ的中远红外区，尘埃的热辐射机制能很好地产生观测到的红外包。通过模拟能定量地说明尘埃产生的热致辐射在这些天体的红外谱中的相对重要性，在模型与观测值之间的拟合中，我们得到了在这些天体中核加热的尘埃区的大小、尘埃的分布等模型参数。     

Models of late-type stellar photospheres

Summary In this review we discuss recent work and progress in the modelling of photospheres of stars of spectral types F and later. Special emphasis is laid on advances as regards the consideration of atomic and molecular blanketing, non-LTE and convection and other dynamic processes. In a special chapter we discuss the possibilities of semi-empirical modelling of late-type photospheres. In the conclusions we find that much important work remains in this field, but that a considerable part of this work may in fact be carried out in a near future.     

太阳光球磁场的螺度

本文首先给出了近两年来我们在太阳活动区光球电流螺度的观测与研究中所取得的一些最新结果。其次,我们也展望了太阳第２３ 周峰年期间有关磁螺度研究的若干课题     

Stability of viscous fluid in Bianchi type-VI model with cosmological constant

In this paper, we investigate Bianchi type-VI cosmological model for the universe filled with dark energy and viscous fluid in the presence of cosmological constant. Also, we show accelerating expansion of the universe by drawing volume scale, pressure and energy density versus cosmic time. In order to solve the Einstein’s field equations, we assume the expansion scalar is proportional to a component of the shear tensor. Therefore, we obtain the directional scale factors and show the EOS parameter crosses over phantom divided-line.     

Some Brans-Dicke fields generated from Einstein's vacuum fields,III

In the previous papers (I and II), we have obtained exact solutions of the Brans-Dicke scalar-tensor theory in plane and cylindrically-symmetric space-times. In the present work we have obtained a more general set of solutions from those given in Papers I and II, by Morganstern's unit transformation.     

Interacting holographic generalized cosmic Chaplygin gas model

In this paper we consider a correspondence between the holographic dark energy density and interacting generalized cosmic Chaplygin gas energy density in flat FRW universe. Then, we reconstruct the potential of the scalar field which describe the generalized cosmic Chaplygin cosmology. In the special case we obtain time-dependent energy density and study cosmological parameters. We find stability condition of this model which is depend on cosmic parameter.     

A Hamiltonian theory for an elastic earth: Elastic energy of deformation

In this paper we study only the perturbation due to the deformation of the elastic mantle by a tidal body force. In a previous publication (Getino and Ferrándiz, 1989a) we defined two canonical systems of variables - we gave them the names ofelastic variables of Euler and Andoyer respectively. Next, using them, we obtained the canonical expression of rotational kinetic energy. In the present paper, using the same variables, we build up the elastic energy which is produced by the deformation of the elastic mantle. We show that the three termsm = 0, 1, 2 corresponding to the second order of the development in spherical harmonics of the perturbing potential, a tidal potential, are of the same order of magnitude. In addition, the numerical integration for a particular Earth Model (Takeuchi's Model 2) is performed, with the aim of obtaining a numerical estimate of the coefficients which intervene in both this energy and the previously mentioned kinetic energy.