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Papers by ali kaya

This is a technical note on the iϵ prescription in cosmology where we consider a self-interacting... more This is a technical note on the iϵ prescription in cosmology where we consider a self-interacting scalar field in the Poincare patch of the de Sitter space whose Hamiltonian has explicit time dependence. We use both path integral and operator formalisms to work out the evolution of states from asymptotic past infinity with iϵ prescription, which becomes nontrivial even in the free theory, and explicitly show how arbitrary states are projected onto the vacuum. We establish that in perturbation theory the iϵ prescription can be implemented in Weinberg's commutator formula by just inserting ϵ dependent convergence factors that make the oscillating time integrals at infinity meaningful.

arXiv: General Relativity and Quantum Cosmology, 2019

Determining the initial state of the universe is a challenging problem in quantum cosmology and w... more Determining the initial state of the universe is a challenging problem in quantum cosmology and we argue that the issue is intractable if the basic postulates of quantum mechanics are not modified in a nontrivial way. Namely a "standard" quantum theory of gravity is expected to resolve the big-bang singularity either by yielding a regular past eternal evolution or by a smooth finite beginning; in both cases the initial state can in principle be totally arbitrary. We illustrate this point in a minisuperspace, gauge fixed, deparametrized toy model where there is a smooth beginning of the universe provided by the matter Hamiltonian degenerating to the zero operator. This arbitrariness is the source of several debates in the literature, especially in relation to inflation, which can only be solved by a new paradigm involving initial conditions that necessarily alters the usual quantum mechanical treatment, but we argue that this is highly improbable.

Mathematical and Computational Applications, 1998

Stress and strain distributions in the adhesive bonded joints subjected to distributed forces are... more Stress and strain distributions in the adhesive bonded joints subjected to distributed forces are investigated using finite element method. Two different cases are considered, the bonded materials are the same, and the bonded materials are different. The investigations are conducted on a three dimensional model. The finite element model of the joint is obtained using isoparametric three dimensional elements having eight nodes with three degrees of freedom each. The stress components and their distributions both on adhesive surface and on metallic elements are given in dimensionless form using three dimensional graphics. There is great technical interest in the analysis of adhesive joints. The main purpose is to assess the severity of the load carried by the joint. Adhesive joints have come to be widely used in the fields of aeronautics, space, automobile, semiconductor, and other industries with the recent development of tough structural adhesives and the substantial improvement in the strength of adhesive joints. Adhesive joints have come to be used in mechanical structures with the improvement in the properties of adhesive bond. They have not been used in principle load-bearing parts of structures. Early theoretical studies of the stresses in adhesive joints were directed towards the single-lap joint. The paper by Goland et al. [1] is regarded as a classical work in the area of static analysis of a simple lap joint. The peel and shear stresses were assumed to be constants across the adhesive thickness in the analy~is. Finite element method for the single lap joints is first used by Wooley and Carver [2]. They presented stress concentrations as functions of geometry and material parameters. The plane stress model has been used in that paper. In the paper by Erdogan and Ratwani [3], the stress distribution in plates bonded through stepped joints is analyzed. The problem is solved under the assumption of plane stress. In reference [4] the development of linear analytical procedure for design of flat bonded joints with tapered adherends is presented. Reducing

Mathematical and Computational Applications, 1999

AIP Conference Proceedings, 2007

Metallic blocks of Inconel 718 and Ti-6A1-4V alloys that contain artificially introduced defects ... more Metallic blocks of Inconel 718 and Ti-6A1-4V alloys that contain artificially introduced defects of known type, size, shape and location were prepared to serve as calibration standards in ultrasonic inspection. The synthetic defects employed to serve as reflectors were all pertinent to the specific alloy systems used, i.e. compositional defects termed as `dirty white' `white spot' and `freckle' for Inconel 718; `hard-alpha' for titanium alloy. Furthermore, as a defect type common to all three materials, spherical voids of various sizes were also incorporated into these calibration blocks. The aim of this study is to introduce defects of known type and size into metallic blocks made of superalloy Inconel 718 and titanium Ti-6A1-4V alloy. The scope of the study entailed determination of the correct parameters for manufacturing processes involved. Based on the results of the preceding phases of this study, it was decided that the method of Vacuum Hot Pressing (VHP) was to be used in this project to manufacture the metallic block containing artificial defects.

Physics Letters B, 2003

We present a cosmological model in 1 + m + p dimensions, where in m-dimensional space there are u... more We present a cosmological model in 1 + m + p dimensions, where in m-dimensional space there are uniformly distributed p-branes wrapping over the extra p-dimensions. We find that during cosmological evolution m-dimensional space expands with the exact power-law corresponding to pressureless matter while the extra p-dimensions contract. Adding matter, we also obtain solutions having the same property. We show that this might explain in a natural way why the extra dimensions are small compared to the observed three spatial directions.

Physical Review D, 2009

We consider early time cosmic evolution in string gas cosmology dominated by open strings attache... more We consider early time cosmic evolution in string gas cosmology dominated by open strings attached to D-branes. After reviewing statistical properties of open strings in D-brane backgrounds, we use dilaton-gravity equations to determine the string frame fields. Although, there are distinctions in the Hagedorn regime thermodynamics and dilaton coupling as compared to closed strings, it seems difficult to avoid Jeans instability and assume thermal equilibrium simultaneously, which is already a known problem for closed strings. We also examine characteristics of a possible subsequent large radius regime in this setup.

Materials Letters, 2004

Adhesively bonded lap joints are used extensively in various industries. Some disadvantages like ... more Adhesively bonded lap joints are used extensively in various industries. Some disadvantages like holes, thermal effects occurring in the bolted, welded, riveted, and soldered joints are not in question in adhesively bonded joints. Strong adhesive materials used in bonding have been greatly developed in recent years, and then the properties of lightness, sealing, corrosion resistance, heat and sound isolation, damping, and quickly mounting facility have been highly improved. In this work, effects of various dynamic characteristics in the adhesively bonded joints subjected to dynamic forces are investigated using the finite element method. The investigation is conducted on a three-dimensional model. The finite element model of the joint is obtained using isoparametric three-dimensional elements having eight nodes with three degrees of freedom each. Mesh generation is accomplished automatically in a computer. The joint is modeled as a thin plate clamped from the left side. The in-plane vibration analysis is constructed. First, the natural frequencies and mode shapes are obtained, and then point and transfer receptances are extracted, employing structural damping. It is observed that the damping greatly decreased the resonance amplitudes.

International Journal of Infectious Diseases, 2008

Introduction: The rate of postoperative infections following spinal procedures varies widely depe... more Introduction: The rate of postoperative infections following spinal procedures varies widely depending most of the times in factors such as-the nature of the procedure, the patient' s diagnosis, the preoperative comorbidities and other risk factors. A retrospective case control analysis of 28 cases of postoperative infection following spinal procedures was performed. Material and Methods: A review of 200 spinal procedures revealed that a postoperative infection developed in 28 patients (14%). Preoperative risk factors reviewed included smoking and alcohol abuse, diabetes, previous spinal surgery, previous infections and body mass index. Intraoperative factors reviewed included the type of procedures, the operating time and the use of instrumentation. Results: The majority of infections (20 cases-71.4%) occurred during the early postoperative period (< 6 months). Factors such asage of >65 years, smoking and alcohol abuse, diabetes, previous surgical infection and increased body mass index were statistically significant preoperative risk factors. The most common organism cultured from the wounds was Staphylococcus aureus. All patients were treated with surgical irrigation and débridement, and appropriate antibiotics to treat the cultured organism. Conclusions: It may be appropriate to increase the duration of prophylactic antibiotics or implement other measures to decrease the incidence of infection for high risk patients. Whereas extended prophylactics should be considered in posterior instrumentation, especially when risk factors are apparent.

Öz: Bir deprem olayında can ve mal kaybını en aza indirmek veya önlemek için zeminin ve üzerine i... more Öz: Bir deprem olayında can ve mal kaybını en aza indirmek veya önlemek için zeminin ve üzerine inşa edilen yapının deprem anında karşılıklı etkileşiminin ne olacağının iyi bilinmesi gerekir. Bu da zeminin bir deprem anındaki davranışlarını ortaya koyabilen dinamik ve statik ...