String Fluid Research Articles

Bianchi—V string cosmological model and late time acceleration

We have searched for the existence of the late time acceleration of the universe with string fluid as the source of matter in Bianchi—V space-time. To derive a deterministic solution, we choose the scale factor to be an increasing function of time that yields a time dependent deceleration parameter, representing a model which generates a universe showing a transition from an early decelerating phase to a recent accelerating phase. The study reveals that strings dominate the early universe and eventually disappear from the universe for sufficiently large times, i.e. in the present epoch. This picture is consistent with current astronomical observations. The physical behavior of the universe is discussed in detail.

Analyzing Packer's Deformation of Tubular for Unsetting Process in HTHP Wells under Variable (T, P) Fields

In this paper, the axial stress and deformation of high temperature high pressure super-deep deviated gas wells are studied. A new model presents multiple nonlinear equation systems, which comprehensively consider the axial load of the tubular string, internal and external fluid pressure, normal pressure between the tube and well wall, friction and the viscous friction of fluid flowing under variable temperature and pressure fields, instead of the traditional methods. The initial axial load, the pressure effect, the friction, temperature effect and sucker-rod pumping effect are derived using a dimensionless iterative interpolation algorithm. Basic data from the X Well (high temperature-high pressure gas well), 6115 meters deep, are used for case history calculations. The results and some useful conclusions could provide technical reliability in oil and gas well testing design.

DYNAMICS OF MAGNETIZED BULK VISCOUS STRINGS IN BRANS–DICKE GRAVITY

We explore locally rotationally symmetric Bianchi I universe in Brans–Dicke gravity with self-interacting potential by using charged viscous cosmological string fluid. We use a relationship between the shear and expansion scalars and also take the power law for scalar field as well as self-interacting potential. It is found that the resulting universe model maintains its anisotropic nature at all times due to the proportionality relationship between expansion and shear scalars. The physical implications of this model are discussed by using different parameters and their graphs. We conclude that this model corresponds to an accelerated expanding universe for particular values of the parameters.

Radiating stars with generalised Vaidya atmospheres

We model the gravitational behaviour of a radiating star when the exterior geometry is the generalised Vaidya spacetime. The interior matter distribution is shear-free and undergoing radial heat flow. The exterior energy momentum tensor is a superposition of a null fluid and a string fluid. An analysis of the junction conditions at the stellar surface shows that the pressure at the boundary depends on the interior heat flux and the exterior string density. The results for a relativistic radiating star undergoing nonadiabatic collapse are obtained as a special case. For a particular model we demonstrate that the radiating fluid sphere collapses without the appearance of the horizon at the boundary.

Deformable particles with anisotropic interactions: unusual field-induced structural transitions in ultrasoft ionic microgel colloids

Ionic microgels are intriguing soft and deformable colloids with an effective pair potential that crosses over from Yukawa-like at large distances to a much softer repulsive interaction at short distances. Here we report the effect of adding an anisotropic dipolar contribution to colloids with such “ultra-soft” interactions. We use an alternating electric field to induce a tunable dipolar contribution, and study the resulting particle self-assembly and phase transitions in situ with confocal laser scanning microscopy. We find significant field-induced structural transitions at low as well as at very high effective volume fractions. At ϕeff = 0.1 we observe a transition from an isotropic to a string fluid. At ϕeff = 0.85, there is a reversible transition from an amorphous to a dipolar crystalline state, followed by the onset of a gas–(string) solid coexistence. At ϕeff = 1.6 and 2.0, i.e. far above close packing, evidence for a field-induced arrested phase separation is found.

Weakly anisotropic and string fluid phases in magnetorheological systems

Strongly coupled magnetorheological (MR) systems differ from regular simple or complex fluids since the effective interparticle interaction is controlled by external fields and thus provides a tunable anisotropic contribution. This, in turn, causes remarkable diversified phase diagrams of MR-systems. Our analysis of fluid–fluid transitions is based on a modified Ornstein–Zernike (OZ) equation which allows us to calculate structural properties of magnetorheological fluids with anisotropic (dipolar) interactions. The results are compared with MC simulations.

String-fluid transition in systems with aligned anisotropic interactions

Systems with aligned anisotropic interactions between particles exhibit numerous phase transitions. A remarkable example of the fluid phase transition occurring in such systems is the formation of particle strings--the so-called "string" or "chain" fluids. We employ an approach based on the Ornstein-Zernike (OZ) equation, which allows us to calculate structural properties of fluids with aligned anisotropic interactions. We show that the string-fluid transition can be associated with the bifurcation of the "isotropic" correlation length into two distinct scales which characterize the longitudinal and transverse order in string fluids and, hence, may be used as a fingerprint of this transition. The comparison of the proposed OZ theory with the Monte Carlo simulations reveals fairly good agreement.

EXACT NONSPHERICAL RELATIVISTIC STAR

Einstein's equation with a negative cosmological constant admit solutions which are asymptotically anti-de Sitter. We obtain Vaidya-like solutions to include both a null fluid and a string fluid in nonspherical (plane symmetric and cylindrical symmetric) anti-de Sitter space–times. Assuming that string fluid diffuse, we find analytical solutions to Einstein's field equations. Thus we extend the approach proposed by Glass and Krisch to nonspherical space–times with a negative cosmological constant. Some other exacts solutions are also presented and discussed.

Electrorheological Complex Plasmas

Conventional electrorheological (ER) fluids consist of suspensions of microparticles in usually nonconducting fluids with a different dielectric constant. The interparticle interaction, and hence, the rheology of ER fluids, is determined by an external electric field, which polarizes grains and thus induces additional dipole-dipole coupling. The electric field plays a role of a new degree of freedom that allows us to ¿tune¿ the interaction between particles. This makes the phase diagram of ER fluids remarkably diversified. Here, we report on the experimental investigations of ¿ER plasmas,¿ where the control of the interparticle interaction by an externally applied electric field is due to distortion of the Debye spheres that surround microparticles in a plasma. Interactions in ER plasmas under weak ac fields are mathematically equivalent to those in conventional ER fluids. Microgravity experiments as well as molecular dynamics simulations show a phase transition from an isotropic to an anisotropic plasma states as the electric field is increased. The variational Gibbs-Bogoliubov approach allows us to recover the phase diagram of ER plasmas, which includes string fluids and anisotropic solid phases.

The generic model of General Relativity

We develop a generic spacetime model in General Relativity from which all existing model results are produced under specific assumptions, depending on the case. We classify each type of possible assumption, especially the role of observers and that of symmetries, and discuss their role in the development of a model. We apply the results in a step by step approach to the case of a Bianchi I spacetime and a string fluid.

Massive motion in area metric spacetimes

The motion of massive point matter in area metric spacetimes, which arise as refinements of the Lorentzian spacetime structure in various situations of physical interest, is shown to be governed by a Finslerian geometry. This finding is based on the study of interacting string fluids on area metric manifolds, and extends previous findings on the motion of light rays.

SCENARIOS OF COSMIC STRING WITH A VARIABLE COSMOLOGICAL CONSTANT

Exact solutions to the Einstein field equations with cosmic string and a space-varying cosmological constant, viz. Λ = Λ(r), in the energy–momentum tensors are presented. Three cases have been studied: the variable cosmological constant (1) has power law dependence, (2) is proportional to the string fluid density, and (3) is purely a constant. Some cases of interesting physical consequences have been found where (i) the variable cosmological constant can be represented by a power law of the type Λ = 3r-2, (ii) the variable cosmological constant and the cosmic string density are interdependent according to the relation Λ = -8πρs, and (iii) the cosmic string density can be scaled by a power law of the type ρs= r-2. It is also shown that several known solutions can be recovered from the general form of the solutions obtained here.

NONSPHERICAL COLLAPSE OF TWO-FLUID STAR

We obtain the analog of collapsing Vaidya-like solution to include both a null fluid and a string fluid, with a linear equation of state (p⊥ = kρ), in nonspherical (plane symmetric and cylindrically symmetric) anti-de Sitter space–times. It turns out that the nonspherical collapse of two fluid in anti-de Sitter space–times, in accordance with cosmic censorship, proceed to form black holes, i.e. on naked singularity ever forms, in accordance with cosmic censorship, violating hoop conjecture.

String fluid cosmological models in general relativity

LRS Bianchi type-I string cosmological models are studied in the frame work of general relativity when the source for the energy momentum tensor is a bulk viscous fluid containing one dimensional strings embedded in electromagnetic field. A barotropic equation of state for the pressure and density is assumed to get determinate solutions of the field equations. The bulk viscosity is assumed to be inversely proportional to the scalar expansion. The physical and kinematical properties of the models are discussed. The effect of viscosity and electromagnetic field on the physical and kinematical properties is also investigated.

Cylindrically Symmetric Inhomogeneous Universes with a Cloud of Strings

Cylindrically symmetric inhomogeneous string cosmological models are investigated in presence of string fluid as a source of matter. To get the three types of exact solutions of Einstein’s field equations we assume A=f(x)k(t), B=g(x)ℓ(t) and C=h(x)ℓ(t). Some physical and geometric aspects of the models are discussed.

Exact BPS Kink in BCFT, DBI EFT, NCFT, and BSFT

We discuss single topological BPS kinks with non-zero thickness, which are exact solutions in various contexts including Dirac-Born-Infeld type effective field theory, noncommutative field theory, boundary conformal field theory, and boundary string field theory. The tachyon field is monotonically increasing (or decreasing) and connects the isolated vacua at positive and negative infinity irrespective of the theories of our interest. It is expressed by a simple linear function, τ (X) ∝ X, in the case of boundary conformal field theory. The obtained tachyon kink in the presence of critical electromagnetic field is interpreted as a single thick BPS D(p− 1)-brane (or D(p− 1)F1-composite) inside an unstable Dp-brane with background fluid of fundamental strings.

TIMELIKE AND SPACELIKE MATTER INHERITANCE VECTORS IN SPECIFIC FORMS OF ENERGY–MOMENTUM TENSOR

This paper is devoted to the investigation of the consequences of timelike and spacelike matter inheritance vectors in specific forms of energy–momentum tensor, i.e. for string cosmology (string cloud and string fluid) and perfect fluid. Necessary and sufficient conditions are developed for a space–time with string cosmology and perfect fluid to admit a timelike matter inheritance vector, parallel to ua and spacelike matter inheritance vector, parallel to xa. We compare the outcome with the conditions of conformal Killing vectors. This comparison provides us the conditions for the existence of matter inheritance vector when it is also a conformal Killing vector. Finally, we discuss these results for the existence of matter inheritance vector in the special cases of the above mentioned space–times.

D-string fluid in conifold, I: Topological gauge model

Motivated by similarities between quantum Hall systems à la Susskind and aspects of topological string theory on conifold as well as results obtained in [E.H. Saidi, Topological SL ( 2 ) gauge theory on conifold and noncommutative geometry, hep-th/0601020], we study the dynamics of D-string fluids running in deformed conifold in presence of a strong and constant RR background B-field. We first introduce the basis of D-string system in fluid approximation and then derive the holomorphic noncommutative gauge invariant field action describing its dynamics in conifold. This study may be also viewed as embedding Susskind description for Laughlin liquid in type IIB string theory. FQH systems on real manifolds R × S 2 and S 3 are shown to be recovered by restricting conifold to its Lagrangian sub-manifolds. Aspects of quantum behaviour of the string fluid are discussed.

General symmetries of a string fluid space-time

It is shown that a string fluid is the simplest anisotropic fluid with vanishing heat flux. Furthermore it has the property that the Ricci tensor is obtained from the energy momentum tensor, and vice versa, if one interchanges the fluid variables. We use previous works on the collineations of anisotropic fluids, which include the string fluid as a particular case, to compute the kinematic and the dynamic effects of certain collineations of a string fluid. It is found that the possible spacetimes, which can carry a string fluid, are severely restricted and the possible string fluids in spacetimes, which can admit them are more or less fixed. We recover previous results on the effect of symmetries in string fluid spacetimes and get many new ones, for example the matter inheritance collineations. The study and the results are presented in a systematic manner, which allows the comprehension and the comparison of the restrictions imposed by each collineation. Finally one can use the same method of work for a systematic study of similar problems.

A FLUID OF STRINGS AS A VIABLE CANDIDATE FOR THE DARK SIDE OF THE UNIVERSE

We investigate the possibility that part of the dark matter is not made out of the usual cold dark matter (CDM) dust-like particles, but is in the form of a fluid of strings with barotropic factor ws= -1/3 of cosmic origin. To this aim, we split the dark matter density parameter into two terms and investigate the dynamics of a spatially flat universe filled with baryons, CDM, a fluid of strings and dark energy, modeling the latter as a cosmological constant or a negative pressure fluid with a constant equation of state w < 0. To test the viability of the models and to constrain their parameters, we use the Type Ia supernovae Hubble diagram and data on the gas mass fraction in galaxy clusters. We also discuss the weak field limit of a model comprising a significant fraction of dark matter in the form of a fluid of strings and show that this mechanism makes it possible to reduce the need for the elusive and up to now undetected CDM. We finally find that a model comprising both a cosmological constant and a fluid of strings fits the data very well and eliminates the need for phantom dark energy, thus representing a viable candidate for alleviating some of the problems plaguing the dark side of the universe.