Presence Of Strings Research Articles

The role of density inhomogeneity and anisotropy in the final outcome of dissipative collapse

In this work, we employ the “horizon” function introduced by Ivanov (Int J Mod Phys D 25:1650049, 2016b) to study radiating stellar models with a generalized Vaidya exterior. Since the star is dissipating energy in the form of a radial heat flux, the radial pressure at the boundary is non-vanishing. We study the boundary condition which encodes the temporal behaviour of the model. Utilizing a scheme developed by Ivanov, we provide several solutions to the modified junction condition. We show that the presence of strings, allow for the collapse to a black hole or the complete burning of a star.

Insertion of intrauterine devices after cesarean section: a systematic review update.

BackgroundWomen who undergo a cesarean section (CS) are in a unique position to receive the intrauterine contraceptive device (IUD). They may also want to use the IUD as a long-acting reversible contraceptive method provided the IUD is safe and effective in the presence of a CS scar.Search strategyWe researched and reviewed the MEDLINE, POPLINE, Google Scholar, and ClinicalTrials.gov databases from January 1968 to June 2015.Selection criteriaEligible studies reported event rates or practical problems relating to IUD usage in post-placental or interval insertion (>90 days) after CS. Studies with ≥20 subjects were included.Data collection and analysisAnalysis of eligible data collected from the search followed the PRISMA guidelines.Main resultsTwelve eligible studies of post-placental IUD insertion after CS included four randomized controlled trials of post-placental versus delayed insertion. Women randomized to delayed insertion were less likely to receive a device. Six studies examined the problem of missing IUD threads at follow-up with only 30%–60% presence of strings observed.ConclusionThe IUD is a long-acting reversible contraceptive method that is suitable for use in all women undergoing CS. The problems of device expulsion, missing threads at follow-up, and the tendency of increased puerperal bleeding need to be solved. Solutions are proposed.

The Nature, Origin and Propagation of the Electric Field: A New Insight to Fundamental Physics

The nature, origin and propagation of the electric field are discussed for the first time on the basis of the presence of vibrating strings in the space and their self-excitation process. It is considered that the electron is formed from strings and it has specific vibrational frequency. This excites the strings which are close by with the self-excitation process. This procedure which is continuous in the space according to the symmetry and vibrational energy in the form of waves spreads near the electron (or the charge particle), which behaves and carries energy known as electric field. In fact, the electron does not continuously emit energy in any form but induces (or excites) and organizes energy in a self-sustain vibrational form and extends in three dimensional space. Only on the basis of the presence of strings (vibrational energy), several electromagnetic phenomena have been explained in a consistent way. The vibrational nature of the electric field is also examined with the help of Stark effect and X-ray diffraction approach to support the present view.

Locally rotationally symmetric (LRS) Bianchi type II string dust viscous fluid cosmological models in Lyra geometry

In the present study, a locally rotationally symmetric Bianchi type II string dust cosmological model with varying bulk viscosity in the framework of Lyra geometry is investigated. To get the deterministic solution, we have assumed that shear (σ) is proportional to the expansion (θ) and ζθ = constant, where ζ is the coefficient of bulk viscosity. The ansatz ζθ = constant, was found to coincide with the occurrence of Little Rip (LR) cosmology using Friedmann–Robertson–Walker metric (Breviket al. Phys. Rev. D, 84, 103508-1-6 (2011)). The model describes the pre-inflationary stage of the universe. The presence of viscosity leads to inflationary-like solutions. Anisotropy is maintained because of the presence of strings. The presence of bulk viscosity prevents the matter density from vanishing. In a special case, the model represents the isotropy of the universe. The universe passes through a decelerating stage to an accelerating phase, which is in agreement with Ries et al.’s (Astrophys. J. 607, 665 (2004). doi:10.1086/383612 ) type Ia supernovae astronomical observations. The displacement vector (β) of Lyra geometry is a decreasing function of time analogous to the cosmological constant (Λ) of general relativity. We have attempted to explore the possibility of constructing a string dust viscous fluid cosmological model in Bianchi type II space–time with displacement vector (β) in Lyra geometry using the geometric condition that shear is proportional to expansion.

Bianchi Type-I String Dust Models with Bulk Viscosity in Lyra Geometry

In the present study, we have investigated string dust models with bulk viscosity in Lyra geometry taking Bianchi type-I space–time as a source. To get the determinate model, we have assumed the conditions $$ \sigma_{1}^{1} \,\alpha \,\theta \,{\text{and}}\,\zeta \,\theta = \,{\text{constant}} $$ where $$ \sigma_{1}^{1} $$ the component of shear tensor $$ (\sigma_{\text{i}}^{\text{j}}) ,\,\theta$$ the expansion in the model, ζ the coefficient of bulk viscosity. We observed that string cosmological models are possible in the frame work of Lyra manifold with one of shear tensor component bears the constant ratio to expansion. The models satisfy reality condition in presence and absence of bulk viscosity. The anisotropy is maintained due to the presence of strings. The models in presence of bulk viscosity has point type singularity while in absence of bulk viscosity, the model represents cigar type singularity. The displacement vector of Lyra geometry is initially large but decreases with time in presence and absence of bulk viscosity. The other physical aspects are also discussed.

Interactions of non-Abelian global strings

Non-Abelian global strings are expected to form during the chiral phase transition. They have orientational zero modes in the internal space, associated with the vector-like symmetry SU ( N ) L + R broken in the presence of strings. The interaction among two parallel non-Abelian global strings is derived for general relative orientational zero modes, giving a non-Abelian generalization of the Magnus force. It is shown that when the orientations of the strings are the same, the repulsive force reaches the maximum, whereas when the relative orientation becomes the maximum, no force exists between the strings. For the Abelian case we find a finite volume correction to the known result. The marginal instability of the previously known Abelian η ′ strings is discussed.

Vortex flow and cavitation in diesel injector nozzles

Flow visualization as well as three-dimensional cavitating flow simulations have been employed for characterizing the formation of cavitation inside transparent replicas of fuel injector valves used in low-speed two-stroke diesel engines. The designs tested have incorporated five-hole nozzles with cylindrical as well as tapered holes operating at different fixed needle lift positions. High-speed images have revealed the formation of an unsteady vapour structure upstream of the injection holes inside the nozzle volume, which is referred to as ‘string-cavitation’. Computation of the flow distribution and combination with three-dimensional reconstruction of the location of the strings inside the nozzle volume has revealed that strings are found at the core of recirculation zones; they originate either from pre-existing cavitation sites forming at sharp corners inside the nozzle where the pressure falls below the vapour pressure of the flowing liquid, or even from suction of outside air downstream of the hole exit. Processing of the acquired images has allowed estimation of the mean location and probability of appearance of the cavitating strings in the three-dimensional space as a function of needle lift, cavitation and Reynolds number. The frequency of appearance of the strings has been correlated with the Strouhal number of the vortices developing inside the sac volume; the latter has been found to be a function of needle lift and hole shape. The presence of strings has significantly affected the flow conditions at the nozzle exit, influencing the injected spray. The cavitation structures formed inside the injection holes are significantly altered by the presence of cavitation strings and are jointly responsible for up to 10% variation in the instantaneous fuel injection quantity. Extrapolation using model predictions for real-size injectors operating at realistic injection pressures indicates that cavitation strings are expected to appear within the time scales of typical injection events, implying significant hole-to-hole and cycle-to-cycle variations during the corresponding spray development.

Quantum birth of a hot universe

We consider the quantum birth of a hot FRW universe from a vacuum-dominated quantum fluctuation with admixture of radiation and strings which corresponds to quantum tunnelling from a discrete energy level with a non-zero temperature. The presence of strings with the equation of state p=−ε/3 mimics a positive curvature term which makes it possible, in the case of a negative deficit angle, the quantum birth of an open and a flat universe. In the pre-de-Sitter domain radiation energy levels are quantized. We calculate the temperature spectrum and estimate the range of the model parameters restricting temperature fluctuations by the observational constraint on the CMB anisotropy. For the GUT scale of initial de Sitter vacuum the lower limit on temperature at the start of classical evolution is close to the values as predicted by reheating theories, while the upper limit is far from the threshold for a monopole rest mass.

Analytic and simulation studies of dust grain interaction and structuring

For dust grains in stationary plasma, a quantitative assessment is made of the effect of centrifugal potential barriers on ion trajectories near a grain. It is shown that in most situations of interest the barriers are weak and only marginally affect the validity of the orbital-motion-limited (OML) theory. The OML theory is then used to show that the electrostatic interaction between grains is always repulsive. The ion-shadowing force is calculated, and it is shown that this force can lead to a weak net attraction between grains at long range, under certain conditions with large grains, dense plasma, and/or low gas pressure. For grains in streaming plasma at or near the sheath, it is shown that nonlinear effects are weak and the grains can be represented as dressed particles interacting via the dynamically shielded Coulomb interaction, which includes wakefields, Landau damping, and collisional damping. The Dynamically Shielded Dust (DSD) simulation code, which is based on this model, is described and a simulation is shown for strongly coupled grains in flowing plasma. The simulation shows ordering of the grains into rigid strings aligned with the ion flow, and looser glass-like organization of the strings in the transverse plane. The presence of strings with odd and even numbers of grains results in stratification of the grains into planes with an alternating structure.

Exact solutions for cosmic strings in closed universes

Vacuum spacetimes with two-parameter symmetry groups have been investigated with the presence of strings along the two symmetry axes. The simplest spacetime metrics of this type are given explicitly. These spacetimes model closed (three-sphere space topology) gravitational-wave-dominated universes with long straight strings whose mass per unit length can have any value. Because the strings sit at the symmetry axes, they have no local effects on the evolution of the spacetime or the motion of test particles in it. However, the strings do have global effects---they cause a constant rescaling of time which affects the expansion rate in these models. Contrary to the usual arguments, these long straight strings do not seem to evolve so as to dominate these radiation-dominated universes.

Investigation of Structure and Dynamics in the Fibre-Type Nematic Phase of nBABA

Abstract Neutron diffraction measurements have been carried out with chain deuterated (D-) and non-deuterated (H-) compounds, p-n-AlkoxyBenzylidene-p-Amino Benzoic Acids (nBABA, n = 1, 3, 4 and 7) in the fibre-type nematic (N F) phase. For n = 3 and 4, our results indicate the presence of strings of one dimensional (1-d) correlated molecules parallel to ň, the nematic director. The number of strings as well as the correlation length, ξ1 ⊥, perpendicular to ň, decrease rapidly with increasing temperature. However, ξ1 ‖, the correlation length parallel to ň, remains almost constant for T PN ≤ T < T PN + 30°C (T PN = crystal to nematic transition temperature), but decreases for larger T. Neutron quasi-elastic scattering experiments carried out with 4BABA (both D- and H-) in the NF phase have shown that, (a) the rotational diffusion times, τ r 1 and τ f 1, for the rigid and flexible parts of the molecule, are not very different, and are ∼10−11s for T ≈ TPN , and (b) these motions are not influenced greatly by...

Anomalies and fermion zero modes on strings and domain walls

the parity anomaly in 2n + 1 dimensions, and the Dirac index density in 2n + 2 dimensions can be understood in terms of the physics of fermion zero modes on strings and domain walls. We show that the Dirac equation possesses chiral zero modes in the presence of strings in 2n +2 dimensions (such as occur in axion theories) or domain walls in 2n + 1 dimensions. We show that the anomalies due to the chiral zero modes are exactly cancelled by anomalies due to the coupling of axion-like fields to the Dirac index density or by anomalies due to the induced topological mass term. Anomalies arise when the quantum-mechanical vacuum functional of a field theory fails to have all the symmetries of the classical lagrangian from which it is derived. Ifthe symmetry in question is a gauge symmetry associated with a dynamical gauge field, the anomaly is intolerable and must be eliminated. The usual method is direct cancellation by the addition of extra fermion species with appropriate quantum numbers. In this paper, we will show that in certain contexts involving defects (i.e. strings and domain walls), anomalies arise which may be compensated by quantum number flows in the higher-dimensional space in which the defect is embedded, rather than cancelled. This phenomenon gives a particularly simple physical interpretation of the recently discovered mathematical connection between chiral anomalies in 2n +2 dimensions, parity anomalies in 2n + l dimensions and gravitational and non-abelian anomalies in 2n dimensions [1-5]. It also suggests some interesting phenomenological possibilities for the axion strings which may have played a role in the development of density perturbations in the early universe

Renormalization group studies of phase diagrams of Ising and XY-model films of variable thickness

We compare the critical behavior of spin- 1 2 Ising model (ordinary phase transition in two dimensions) and classical XY-model (topological phase transition in two dimensions) films, with two flat surfaces and nearest neighbor couplings K S between surface spins and K B between all others, as a function of film thickness. We carry out a real space Migdal-style renormalization in two stages. In the bulk stage the film is first renormalized towards a double layer, with the renormalized parameters as inter- and intra-layer couplings. Then the double layer is renormalized with those couplings as initial parameters. From the RG-equations for the bulk stage we find a tricritical point, not only for the Ising model (in which case it is well known) but also for the XY-model. It signals the existence of distinct surface and bulk transitions for sufficiently large values of K S K B . For the Ising model the complete program can be carried out and the phase diagram for films of arbitrary thickness is constructed. For the double layer XY-model a sufficiently complex Migdal-style renormalization appears to be unfeasible, presumably due to the possible presence of strings between the layers. Therefore, in an alternative approach, two representations for the partition function of the double layer XY-model are given. The system can be described in terms of its topological excitations, i.e., vortices on each layer and strings, either closed or terminating in vortices, between them. The system is also written as three coupled Coulomb gases. Based on this representation a renormalization group is found, and used, together with a Griffiths inequality for the correlation functions, to obtain information on the phase diagram. If there is a single phase transition, there is one phase where the correlations exhibit power law behavior and another where they fall off exponentially. The transition temperature increases monotonously with interlayer coupling to twice its value in two dimensions, but the nature of the phase transition for any finite inter-layer coupling appears to be different from that at zero coupling. We suggest that this is associated with the behavior of the internal strings. These results should be relevant for the renormalization of the film with isotropic bulk couplings as well as for layered systems of finite thickness, with different inter- and intra-layer couplings.