Aspherical accretion of cosmic strings

A simulation of accretion of a string loop has been done. It shows that aspherical accretion is essential for large-scale loops. All configurations found in the distribution of galaxies, such as filaments, pancakes, bubbles, and voids, can be formed by the accretion of cosmic strings.     

VLF goniometer observations at Halley Bay,Antarctica—I. The equipment and the measurement of signal bearing

A transistorized wide-band (0.5–11 kHz) VLF goniometer has been developed for the study of whistlers and ELF/VLF emissions. It consists of two crossed vertical loops from which a single loop aerial, rotating about a vertical axis at a frequency of 25 sec^?1, is synthesized electronically. During periods of high whistler activity, when the same propagation paths may be identified in successive whistler groups, it is possible to determine the bearing of the exit point of such a magnetospheric path with an error, typically, of ± 10–20°.     

A dynamical model of prominence loops

A dynamical model of prominence loops is constructed on the basis of the theory of hydromagnetic buoyancy force. A prominence loop is regarded as a flux rope immersed in the solar atmosphere above a bipolar region of the photospheric magnetic field. The motion of a loop is partitioned into a translational motion, which accounts for the displacement of the centroidal axis of the loop, and an expansional motion, which accounts for the displacement of the periphery of the loop relative to the axis. The translational motion is driven by the hydromagnetic buoyancy force exerted by the surrounding medium of the solar atmosphere and the gravitational force exerted by the Sun. The expansional motion is driven by the pressure gradient that sustains the pressure difference between internal and external gas pressures and the self-induced Lorentz force that results from interactions among internal currents. The main constituent of the hydromagnetic buoyancy force on a prominence loop is the diamagnetic force exerted on the internal currents by the external currents that sustain the pre-existing magnetic field. By spatial transformation between magnetic and mechanical stresses, the diamagnetic force is manifested through a mechanical force acting at various mass elements of the prominence. For a prominence loop in equilibrium, the gravitational force is balanced by the hydromagnetic buoyancy force and the Lorentz force of helical magnetic field is balanced by a gradient force of gas pressure.     

Magnetic Bianchi type II string cosmological model in loop quantum cosmology

The loop quantum cosmology of the Bianchi type II string cosmological model in the presence of a homogeneous magnetic field is studied. We present the effective equations which provide modifications to the classical equations of motion due to quantum effects. The numerical simulations confirm that the big bang singularity is resolved by quantum gravity effects.     

A magnetohydrodynamic theory of coronal loop transients

A magnetohydrodynamic theory is presented for coronal loop transients. It is shown that the heliocentrifugal motion of a transient loop, as exhibited by the translational displacement of the axis of the loop, is driven by the magnetohydrodynamic buoyancy force exerted by the ambient medium. Self-induced hydromagnetic force, which includes the magnetic force produced by the internally driven current and the thermal force produced by the pressure imbalance between the internal and external gas pressures, causes the peripheral expansion of the loop, as exhibited by the lateral broadening and longitudinal stretching. This contention is substantiated by an analysis based on a model structure for a coronal loop.Besides accounting for the acceleration and expansion of a transient loop, this magnetohydrodynamic theory also provides an explanation for the initial ejection of a coronal loop from stationary equilibrium. Magnetic unwinding in consequence of abrupt magnetic activities at the solar surface will cause the periphery of a stationary coronal loop to expand. The increase in volume will enhance the magnetohydrodynamic buyoyancy force to exceed the gravitational force. Once a coronal loop is ejected from the solar surface, it will be continually accelerated and undergo expansion. Eventually a transient loop will blend with the ambient solar wind. This is also indicated by the theory presented in this paper.     

Five dimensional Bianchi type-I string cosmological model in Brans-Dicke theory

A Bianchi type-I string cosmological model in Brans-Dicke theory in five dimension space-time has been investigated. For the determinate solution it has been assumed that the sum of energy density and tension density of the cosmic string dust source vanishes. Some physical and kinematical parameters are also discussed.     

Bianchi type-I string cosmological model in the presence of a magnetic field: classical versus loop quantum cosmology approaches

A Bianchi type-I cosmological model in the presence of a magnetic flux along a cosmological string is considered. The first objective of this study is to investigate Einstein equations using a tractable assumption usually accepted in the literature. Quantum effects of the present cosmological model are examined in the framework of loop quantum cosmology. Finally we draw a parallel between the classical and quantum approaches.     

Dilaton stabilization in string cosmology. I

A new mechanism is proposed for stabilization of the scalar dilaton field within the framework of lowenergy string gravitation with loop corrections to the dilaton coupling functions. It is based on the assumption that the loop corrections generate a kinetic dilaton function, which is singular for some finite value of the dilaton field. For a nongravitational source of the barotropic type, the system of equations describing the evolution of homogeneous and isotropic cosmological models is represented in the form of a thirdorder, autonomous, dynamical system. The behavior of the general solution in the vicinity of singularities of the dilaton coupling function is investigated by methods of the qualitative theory of dynamical systems. It is shown that there is a class of solutions, different from solutions of the general theory of relativity, with a constant dilaton. The conditions under which these solutions are an attractor for a general solution with a variable dilaton are determined. Translated from Astrofizika, Vol. 43, No. 1, pp. 123-136, January–March, 2000.     

Absence of initial singularities in superstring cosmology

In a universe whose elementary constituents are point particles there does not seem to be any obvious mechanism for avoiding the initial singularities in physical quantities in the standard model of cosmology. In contrast in string theory these singularities can be absent even at the level where spacetime is treated classically. This is a consequence of the basic degrees of freedom of strings in compact spaces, which necessitate a reinterpretation of what one means by a very small universe. We discuss the basic degrees of freedom of a string at the classical and quantum level, the minimum size of strings (string uncertainty principle), the t-duality symmetry, and string thermodynamics at high energy densities, and then describe how these considerations suggest a resolution of the initial singularity problem. An effort has been made to keep this writeup self-contained and accessible to non-string theorists.     

Vacuumless topological defects in Lyra geometry

Few years ago, Cho and Vilenkin have proposed that topological defects can arise in symmetry breaking models without having degenerate vacua. These types of defects are known as vacuumless defects. In the present work, the gravitational field of a vacuumless global string and global monopole have been investigated in the context of Lyra geometry. We find the metric of the vacuumless global string and global monopole in the weak field approximations. It has been shown that the vacuumless global string can have repulsive whereas global monopole exerts attractive gravitational effects on a test particle. It is dissimilar to the case studied in general relativity.     

Dilaton stabilization in string cosmology. II

A mechanism for stabilization of the dilaton field within the framework of low-energy string gravitation with loop corrections to the dilaton coupling function was proposed in the first part of this paper. The mechanism is based on the assumption that loop corrections generate a singular dilaton kinetic function for a certain value of the dilaton field. For a nortgravitational source with a constant barotropic index, the system of cosmological equations reduces to an autonomous, third-order dynamical system. The behavior of the general solution in the vicinity of singularities of the dilaton coupling function is investigated by methods of the qualitative theory of dynamical systems for different values of the singularity index. The conditions under which solutions with a constant dilaton are attractors for a general solution with a variable dilaton are determined. The evolution of models is considered, the corresponding phase diagrams are constructed, and the question of the effectiveness of dilaton stabilization is investigated. Translated from Astrofizika, Vol. 43, No. 2, pp. 313–324, April–June, 2000.     

String cloud cosmologies for Bianchi type-III models with electromagnetic field

The Saez-Ballester field equations for spatially homogeneous and anisotropic Bianchi type-III cosmological models have been solved for pure geometric cosmic string cloud pervading the universe either in the absence or in presence of electromagnetic field. It has been established here that the model does not survive for geometric cosmic string cloud pervading the universe when there is no electromagnetic field. But in presence of electromagnetic field the model can have plausible solutions fostering the idea that strings forming the surface of the world sheet have to co-exist with electromagnetic field.     

Anisotropic Bianchi type-I models in string cosmology

The present study deals with a spatially homogeneous and anisotropic Bianchi-I cosmological models representing massive strings. The energy-momentum tensor, as formulated by Letelier (1983), has been used to construct massive string cosmological models for which we assume the expansion scalar in the models is proportional to one of the components of shear tensor. The Einstein’s field equations have been solved by applying a variation law for generalized Hubble’s parameter in Bianchi-I space-time. We have analysed a comparative study of accelerating and decelerating models in the presence of string scenario. The study reveals that massive strings dominate in the decelerating universe whereas strings dominate in the accelerating universe. The strings eventually disappear from the universe for sufficiently large times, which is in agreement with current astronomical observations.     

Bianchi Type-V Bulk Viscous Fluid String Dust Cosmological Model In General Relativity

Bianchi Type-V bulk viscous fluid string dust cosmological model in General Relativity is investigated. It has been shown that if coefficient of bulk viscosity (ζ) is inversely proportional to the expansion (θ) in the model then string cosmological model for Bianchi Type-V space-time is possible. In absence of bulk viscosity (ζ), i.e. when ζ → 0, then there is no string cosmological model for Bianchi Type-V space-time. The physical and geometrical aspects of the model are also discussed.     

On the location of energy release and temperature profiles along coronal loops

Several mechanisms have been suggested to contribute to the heating of the solar corona, each of which deposits energy along coronal loops in a characteristic way. To compare the theoretical models with observations one has to derive observable quantities from the models. One such parameter is the temperature profile along a loop. Here numerical experiments of flux braiding are used to provide the spatial distribution of energy deposition along a loop. It is found that braiding produces a heat distribution along the loop which has slight peaks near the footpoints and summit and whose magnitude depends on the driving time. Using different examples of the heat deposition, the temperature profiles along the loop are determined assuming a steady state. Along with this, different methods for providing average temperature profiles from the time-series have been investigated. These give summit temperatures within approximately 10% of each other. The distribution of the heating has a significant impact on both the summit temperature and the temperature distribution along the loop. In each case the ratio between the heat deposited and radiation provides a scaling for the summit temperature.     

Cosmological perturbations of an expanding brane in an anti-de Sitter bulk : a short review

Since the Randall-Sundrum 1999 papers, braneworlds have been a favourite playground to test string inspired cosmological models. The subject has developped into two main directions: elaborating more complex models in order to strenghten the connection with string theories, and trying to confront them with observations, in particular the Cosmic Microwave Background anisotropies. We review here the latter and see that, even in the simple, `paradigmatic', case of a single expanding brane in a 5D anti-de Sitter bulk, there is still a missing link between the `view from the brane' and the `view from the bulk' which prevents definite predictions. This revised version was published online in July 2006 with corrections to the Cover Date.     

LRS Bianchi Type V bulk viscous fluid string dust cosmological model in general relativity

An LRS Bianchi Type V bulk viscous fluid dust distribution string cosmological model in General Relativity is investigated. It has been shown that if coefficient of bulk viscosity (ζ) is proportional to the expansion (θ) in the model then string cosmological model for Bianchi Type V space-time is possible. In absence of bulk viscosity(ζ) i.e. when ζ → 0 then there is no string cosmological model for Bianchi Type V space-time. The physical and geometrical aspects are also discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

On string cosmology with loop corrections

Homogeneous cosmological models are investigated within the framework of low- energy string gravitation with loop corrections. Various conformai representations of the effective action are considered. Without specifying the correction functions in the Lagrangian, cosmological solutions are found with an arbitrary curvature and with dilaton fields, moduli fields, and Kalb- Ramond fields corresponding to a source with an extremely stiff equation of state. They generalize previously known solutions of the tree approximation. The behavior of the solutions in different asymptotic domains is investigated. Translated from Astrofizika, Vol. 41, No. 2, pp. 277–295, April-June, 1998.     

Dilaton Stabilization in String Cosmology. III. Models with Curved Space

Within the framework of the mechanism of dilaton stabilization proposed in Part I of the present work, an analysis of homogeneous and isotropic cosmological models of low-energy string gravitation with loop corrections is continued. The behavior of models with curved space is investigated by methods of the qualitative theory of dynamical systems for different values of the singularity index of the dilaton kinetic function and the barotropic index of nongravitational matter. The conditions under which dilaton stabilization occurs as a result of cosmological expansion are determined, and the corresponding phase diagrams are constructed.