Qualitative Properties of Scalar-Tensor Theories of Gravity

The qualitative properties of spatiallyhomogeneous stiff perfect fluid and minimally coupledmassless scalar field models within general relativityare discussed. Consequently, by exploiting the formal equivalence under conformal transformations and field redefinitions of certain classes of theories ofgravity, the asymptotic properties of spatiallyhomogeneous models in a class of scalar-tensor theories of gravity that includes the Brans-Dicke theory can be determined. For example, exact solutions are presented, which are analogues of the general relativistic Jacobs stiff perfect fluid solutions andvacuum plane wave solutions, which act as past andfuture attractors in the class of spatially homogeneousmodels in Brans-Dicke theory.     

Generation of LRS Bianchi type I universes filled with perfect fluids

A generating technique is presented which converts known LRS Bianchi type I models into new models of the same type. Starting from the general Kasner solutions new classes of models are obtained which add to the rare perfect-fluid solutions not satisfying the equation of state. The physical and kinematical properties of cosmological models are studied.     

Kaluza-Klein Cosmology: Friedmann Models with Phenomenological Matter

We reduce the FRIEDMANN models in generalized KALUZA -KLEIN cosmologies, in which the ordinary space-time is supplemented by internal factor spaces, to the motion of a tensorial mass particle in a scalar field. Some general properties of these models as well as exact solutions for the case of one internal space are discussed for different mixtures of phenomenological matter components.     

SOLUTIONS OF THE BOLTZMANN EQUATION FOR THE SPATIALLY HOMOGENEOUS MIXTURE OF MAXWELLIAN GASES

Using the moment method,the general solution of the nonlinear Boltzmann equation for the spatially uniform binary mixture of Maxwellian gases for isotropic initial conditions is obtained in Laguerre series.Its general properties and asymptotic behaviors are investjgated.For the mixture with arbitrary parameters,a class of similarity solutions is derived.Two BKW solutions for the isotropic scattering model first found by Krook and Wu,are extended to including nonisotropic scattering models.     

Thermodynamic analysis of black hole solutions in gravitating nonlinear electrodynamics

We perform a general study of the thermodynamic properties of static electrically charged black hole solutions of nonlinear electrodynamics minimally coupled to gravitation in three space dimensions. The Lagrangian densities governing the dynamics of these models in flat space are defined as arbitrary functions of the gauge field invariants, constrained by some requirements for physical admissibility. The exhaustive classification of these theories in flat space, in terms of the behaviour of the Lagrangian densities in vacuum and on the boundary of their domain of definition, defines twelve families of admissible models. When these models are coupled to gravity, the flat space classification leads to a complete characterization of the associated sets of gravitating electrostatic spherically symmetric solutions by their central and asymptotic behaviours. We focus on nine of these families, which support asymptotically Schwarzschild-like black hole configurations, for which the thermodynamic analysis is possible and pertinent. In this way, the thermodynamic laws are extended to the sets of black hole solutions of these families, for which the generic behaviours of the relevant state variables are classified and thoroughly analyzed in terms of the aforementioned boundary properties of the Lagrangians. Moreover, we find universal scaling laws (which hold and are the same for all the black hole solutions of models belonging to any of the nine families) running the thermodynamic variables with the electric charge and the horizon radius. These scale transformations form a one-parameter multiplicative group, leading to universal “renormalization group”-like first-order differential equations. The beams of characteristics of these equations generate the full set of black hole states associated to any of these gravitating nonlinear electrodynamics. Moreover the application of the scaling laws allows to find a universal finite relation between the thermodynamic variables, which is seen as a generalized Smarr law. Some particular well known (and also other new) models are analyzed as illustrative examples of these procedures.     

Generalized Chaplygin Gas Dominated Anisotropic Bianchi Type-I Cosmological Models

We present Bianchi type-I cosmological models in the presence of generalized Chaplygin gas and perfect fluid for early and late time epochs. Exact solutions of Einstein’s field equations for this model are obtained. The general solutions of gravitational field equations are expressed in an exact parametric form, with average scale factor as parameter. In the limiting cases of small and large values of the average scale factor, the solutions of the field equations are expressed in exact analytic forms. Moreover, this model predicts that the expansion of Universe is accelerating for the late times. The physical and geometrical properties of the corresponding cosmological models are discussed.     

Collapsing spheres satisfying an “Euclidean condition”

We study the general properties of fluid spheres satisfying the heuristic assumption that theirs areal and proper radius are equal (the Euclidean condition). Dissipative and non-dissipative models are considered. In the latter case, all models are necessarily geodesic and a subclass of the Lemaître–Tolman–Bondi solution is obtained. In the dissipative case solutions are non-geodesic and are characterized by the fact that all non-gravitational forces acting on any fluid element produces a radial three-acceleration independent on its inertial mass.     

Field-theoretic extended particles in two space dimensions

A family of Lagrangian models of two scalar fields in (2 + 1) dimensions is studied. All their localized static classical solutions are obtained, and interpreted as representing systems of extended particles. The models are generalized by taking into account general-relativistic gravirational coupling, for which case the general static solutions are also obtained explicitly.     

The extended symmetry approach for studying the general Korteweg-de Vries-type equation

The extended symmetry approach is used to study the general Korteweg-de Vries-type (KdV-type) equation. Several variable-coefficient equations are obtained. The solutions of these resulting equations can be constructed by the solutions of original models if their solutions are well known, such as the standard constant coefficient KdV equation and the standard compound KdV--Burgers equation, and so on. Then any one of these variable-coefficient equations can be considered as an original model to obtain new variable-coefficient equations whose solutions can also be known by means of transformation relations between solutions of the resulting new variable-coefficient equations and the original equation.     

Nonlinear noise waves in soft biological tissues

The study of intense waves in soft biological tissues is necessary both for diagnostics and therapeutic aims. Tissue represents an inherited medium with frequency-dependent dissipative properties, in which waves are described by nonlinear integro-differential equations. The equations for such waves are well known. Their group analysis has been performed, and a number of exact solutions have been found. However, statistical problems for nonlinear waves in tissues have hardly been studied. As well, for medical applications, both intense noise waves and waves with fluctuating parameters can be used. In addition, statistical solutions are simpler in structure than regular solutions; they are useful for understanding the physics of processes. Below a general approach is described for solving nonlinear statistical problems applied to the considered mathematical models of biological tissues. We have calculated the dependences of the intensities of the narrowband noise harmonics on distance. For wideband noise, we have calculated the dependence of the spectral integral intensity on distance. In all cases, wave attenuation is determined both by the specific dissipative properties of the tissue and the nonlinearity of the medium.     

Higher harmonic generation in nonlinear waveguides of arbitrary cross-section

This article concerns the generation and properties of double harmonics in nonlinear isotropic waveguides of complex cross-section. Analytical solutions of nonlinear Rayleigh-Lamb waves and rod waves have been known for some time. These solutions explain the phenomenon of cumulative double harmonic generation of guided waves. These solutions, however, are only applicable to simple geometries. This paper combines the general approach of the analytical solutions with semi-analytical finite element models to generalize the method to more complex geometries, specifically waveguides with arbitrary cross-sections. Supporting comparisons with analytical solutions are presented for simple cases. This is followed by the study of the case of a rail track. One reason for studying nonlinear guided waves in rails is the potential measurement of thermal stresses in welded rail.     

String Cloud and Domain Walls with Quark Matter in <Emphasis Type="Italic">n</Emphasis>-Dimensional Kaluza-Klein Cosmological Model

We have obtained Kaluza-Klein cosmological solutions in n-dimensions for quark matter coupled with the string cloud and domain walls in general relativity. Some properties of the models, thus obtained, are also studied.     

Long time tails in stationary random media II: Applications

In a previous paper we developed a mode-coupling theory to describe the long time properties of diffusion in stationary, statistically homogeneous, random media. Here the general theory is applied to deterministic and stochastic Lorentz models and several hopping models. The mode-coupling theory predicts that the amplitudes of the long time tails for these systems are determined by spatial fluctuations in a coarse-grained diffusion coefficient and a coarse-grained free volume. For one-dimensional models these amplitudes can be evaluated, and the mode-coupling theory is shown to agree with exact solutions obtained for these models. For higher-dimensional Lorentz models the formal theory yields expressions which are difficult to evaluate. For these models we develop an approximation scheme based upon projecting fluctuations in the diffusion coefficient and free volume onto fluctuations in the density of scatterers.Work supported by grant No. CHE 77-16308 from the National Science Foundation and by a Nato Travel Grant.     

On stability of solutions of theU(N) chiral model in two dimensions

We discuss the stability properties of classical solutions of theU(N) sigma models in two Euclidean dimensions. We show that all nontrivial solutions are unstable. For a general case we exhibit one mode of instability; in some special cases (corresponding to a grassmannian solution and an instantonic grassannian embedding) we exhibit two such independent modes.     

Lattice gauge-higgs theories with local x global symmetry groups including exact solutions

We discuss a class of lattice gauge-Higgs models with local x global symmetry groups. These may also be viewed as a new class of disordered spin models. We give general properties of these theories and present exact solutions for certain (infinite) classes of discrete 2D models. Given the strong gauge coupling limit involved, the latter constitute the first nontrivial exactly solved gauge-Higgs theories. Our results provide the first existence proof of theories which satisfy a necessary condition of realistic gauge-Higgs models, namely that the mass gaps for the Higgs and gauge sectors must both vanish and their ratio must approach a finite constant, in the continuum limit.     

Dielectric properties of aqueous solutions of α- -glucose at 915 MHz

Using a cavity perturbation technique, dielectric properties of aqueous solutions of α- -glucose at 915 MHz were investigated at concentrations varying from 10 to 70% (w/w) and temperatures ranging from 25–85°C. The dielectric constant increased with temperature but decreased with concentration, whereas the loss factor did the inverse. Dielectric properties for higher concentration glucose solutions showed the greatest variation at higher temperatures. Predictive models of the dielectric properties as functions of concentration and temperature were developed by stepwise regression. Such models are useful in estimating the volumetric heating of these solutions by microwave energy, studying the dielectric behavior of the glucose solutions, and chemical reactions involving glucose in aqueous solutions in a microwave field.     

Transient motions of bi-lattices

We develop formal solutions for the propagation of transient pulses on a variety of bi-lattice models. The lattices are composed of a finite homogeneous chain connected in series with a different semi-infinite homogeneous chain at a common location occupied by a single mass which is different from the masses of both chains. Exact analytic solutions of this general case are not possible. Some analytic solutions are, however, possible for a variety of special cases. The general solutions are illustrated by numerically inverting the Laplace transform functions. The exact solutions are found to correlate very well with the numerical inversion scheme. Such correlations give confidence in the numerical scheme's predictions of the solutions of the more complicated chains.     

Comments on curve veering in eigenvalue problems

The dependence of eigenvalues on a system parameter is frequently illustrated by a family of loci. When two loci approach each other, they often cross or abruptly diverge. The latter case, called “curve veering”, has been observed in approximate solutions associated with discretized models. The influence of discretization in producing curve veering has raised doubt on the validity of many approximate solutions. The existence of curve veering in continuous models is illustrated by presenting the exact solution of an elementary eigenvalue problem. Veering is then examined in a general eigenvalue problem. Criteria are established to distinguish veerings from crossings in both continuous and discretized models. The application of the criteria is illustrated by examples.