Yazar "Dogru, Melis Ulu" seçeneğine göre listele

Listeleniyor 1 - 12 / 12
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    Conformal and traversable wormholes with monopole and perfect fluid in f(R)-gravity
    (World Scientific Publ Co Pte Ltd, 2016) Taser, Dogukan; Dogru, Melis Ulu
    We investigate spherically symmetric spacetime filled with global monopole and perfect fluid in f(R)-gravity. We consider field equations of f(R)-gravity in order to understand the global monopole and the perfect fluid curve to the spacetime. It has taken advantages of conformal symmetry properties of the spacetime to solve these equations. The obtained solutions are improved in case of phantom energy. It is shown that obtained f(R) function is consistent with well-known models of the modified gravity. Also, it is examined whether the obtained solutions support a traversable wormhole geometry. Obtained results of the solutions have been concluded.
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    Conformal cylindrically symmetric spacetimes in modified gravity
    (World Scientific Publ Co Pte Ltd, 2015) Turkoglu, Murat Metehan; Dogru, Melis Ulu
    We investigate cylindrically symmetric spacetimes in the context of f(R) gravity. We firstly attain conformal symmetry of the cylindrically symmetric spacetime. We obtain solutions to use features of the conformal symmetry, field equations and their solutions for cylindrically symmetric spacetime filled with various cosmic matters such as vacuum state, perfect fluid, anisotropic fluid, massive scalar field and their combinations. With the vacuum state solutions, we show that source of the spacetime curvature is considered as Casimir effect. Casimir force for given spacetime is found using Wald's axiomatic analysis. We expose that the Casimir force for Boulware, Hartle-Hawking and Unruh vacuum states could have attractive, repulsive and ineffective features. In the perfect fluid state, we show that matter form of the perfect fluid in given spacetime must only be dark energy. Also, we offer that potential of massive and massless scalar field are developed as an exact solution from the modified field equations. All solutions of field equations for vacuum case, perfect fluid and scalar field give a special f(R) function convenient to Lambda-CDM model. In addition to these solutions, we introduce conformal cylindrical symmetric solutions in the cases of different f(R) models. Finally, geometrical and physical results of the solutions are discussed.
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    Conformal symmetric Bianchi type-I cosmologies in f(R) gravity
    (World Scientific Publ Co Pte Ltd, 2018) Taser, Dogukan; Dogru, Melis Ulu
    In this study, we investigate the Bianchi type-I cosmologies with string cloud attached to perfect fluid in f(R) gravity. The field equations and their exact solutions for Bianchi type-I cosmologies with string cloud attached to a perfect fluid are found by using the conformal symmetry properties. The obtained solutions under the varied selection of arbitrary constants indicate three cosmological models. Isotropy conditions for obtained cosmological models are investigated for large value of time. Whether or not the string cloud in conformal symmetric Bianchi type-I universe supports the isotropy condition for the large value of time has been investigated. Also, we examine the contracting and decelerating features of the obtained solutions by using Raychaudhuri equation. Finally, geometrical and physical results of the solutions are discussed.
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    Cylindrically Symmetric Domain Wall In Unimodular f(R) Gravity
    (Amer Inst Physics, 2019) Aydin, Huseyin; Dogru, Melis Ulu
    In this study, we examine cylindrically symmetric domain wall distributions in the context of unimodular f(R) gravity. We use unimodular and static cylindrically symmetric spacetime. Field equations and their solutions for the spacetime and the matter are considered. It is shown that unimodular f(R) gravity allows to static cylindrically symmetric spacetime geometry independent from domain wall tension. Other properties of the model are also discussed.
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    Cylindrically symmetric fluid in unimodular f (R) gravity
    (Springer, 2019) Aydin, Hueseyin; Dogru, Melis Ulu
    In this paper, we examine static cylindrical symmetric fluid in unimodular f (R) gravity. Unimodularity for cylindrical symmetric spacetime geometry is ensured with a coordinate transformation by using tetrad formalism. Field equation solutions for given matter and the spacetime are obtained. Resource of cosmic matter is discussed by using Equation of State (EoS) and energy conditions. We investigate weak and strong energy conditions in unimodular f (R) gravity context with the help of Raychaudhuri equations. Obtained results show that fluids which violate the energy conditions cause the proposed unimodular and cylindrical symmetric wormholes, but this kind of wormholes do not satisfy traversability conditions. Furthermore, the obtained solutions indicate that. coefficient in EoS must have the condition. = -1 for physically meaningful fluids. It is discussed whether the result is compatible with the unimodular gravitation theories.
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    Energy and Momentum of Higher Dimensional Black Holes
    (Springer/Plenum Publishers, 2012) Dogru, Melis Ulu; Varlikli, Neriman; Baykal, Derya; Kiy, Guliz; Taser, Dogukan; Caglar, Halife; Gunduz, Emine
    In this paper, we devote to investigate the energy-momentum problem of higher dimensional black holes in the general theory of relativity. The energy and momentum complex of Moller has been used for the calculations. Also, total energy and total momentum of some special cases for higher dimensional black holes such as Schwarzschild-like black holes, Reissner-Nordstrom-like charged black holes, AdS-like black holes, topological black holes, BTZ-like and charged BTZ-like black holes were obtained. It is invented that the momentum of black holes vanishes everywhere while the energy of black holes are not equal to zero in higher dimension. Also the results agree with Yang and Radinschi or Vagenas results in three and four dimensional black holes, respectively (Jang and Radinschi in AIP Conf. Proc. 895, 325, 2007; Vagenas in Mod. Phys. Lett. A 21, 1947, 2006).
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    ENERGY-MOMENTUM DISTRIBUTIONS OF FIVE-DIMENSIONAL HOMOGENEOUS-ANISOTROPIC UNIVERSES
    (World Scientific Publ Co Pte Ltd, 2012) Dogru, Melis Ulu; Baykal, Derya; Kiy, Guliz; Taser, Dogukan; Caglar, Halife; Varlikli, Neriman
    In this study, it has been investigated whether the energy and momentum can be localizable for five-dimensional homogeneous and anisotropic universes. In this connection, energy and momentum of five-dimensional Bianchi type-I, type-III and type-V spacetimes have been calculated in the framework of general relativity (GR) and teleparallel gravity (TG). Einstein, Bergmann-Thomson, Landau-Lifshitz, Papapetrou, Tolman and Moller energy-momentum complexes have been used to obtain these related quantities of given the spacetimes in GR, while Einstein, Bergmann-Thomson, Landau-Lifshitz and Moller prescriptions have been used to obtain these related quantities of the spacetimes in TG. It has been found that all of the energy and momentum distributions of five-dimensional Bianchi type-I spacetime are equal to zero in GR and TG. For five-dimensional Bianchi type-III and type-V spacetimes, Bergmann-Thomson, Einstein and Tolman energy and momentum components give the same results, however Moller, Landau-Lifshitz and Papapetrou energy-momentum distributions give different results in general relativity. Also, in TG, Bergmann-Thomson and Einstein energy and momentum components give the same results for the Bianchi type-III and type-V spacetimes, too. In this sense, it is seen that Einstein, Bergmann-Thomson and Landau-Lifshitz energy and momentum descriptions of these spacetimes have been given same results in both theories, GR and TG.
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    Expanding and nonexpanding conformal wormholes in scalar-tensor theory
    (World Scientific Publ Co Pte Ltd, 2015) Dogru, Melis Ulu; Yilmaz, Ihsan
    We study spherically symmetric spacetime with anisotropic fluid in the scalar-tensor theory of gravity based on Lyra geometry. We suggest two different solutions of field equations for the theory by using Casimir effect. Obtained static and nonstatic solutions are similar to nonexpanding Lorentzian wormhole and expanding FRW-type wormhole, respectively. Furthermore, we study singularities of obtained solutions. We emphasize whether the expanding and nonexpanding wormholes conform with Big Rip or Big Crunch scenarios. Also, physical and geometrical properties of the solutions have been discussed.
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    Global monopoles in f(R) gravity
    (World Scientific Publ Co Pte Ltd, 2015) Dogru, Melis Ulu; Taser, Dogukan
    In this study, we investigate whether global monopoles cause black holes or wormholes to form. Field equations for static spherically symmetric spacetimes with global monopoles are obtained in f(R) gravity. We found exact solutions for the field equations without using any perturbation or approximation methods. It is shown that the obtained f(R) function is in accordance with the Lambda-cold dark matter (Lambda-CDM) model. Also, it is shown that the static spherically symmetric spacetimes associated with global monopoles form black holes or wormhole structures under some restrictions. Finally, geometrical and physical results of the solutions are discussed.
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    Kantowski-Sachs model in non-minimally coupled scalar theory
    (World Scientific Publ Co Pte Ltd, 2022) Taser, Dogukan; Dogru, Melis Ulu; Aydin, Huseyin
    In this study, our main purpose is to study a model which is homogenous and anisotropic describing early stage of Universe in non-minimally coupled scalar theory. It is quite interesting to study a model describing early stage in modified theory which is related to scalar field. In this context, we introduced Kantowski-Sachs metric for perfect fluid in non-minimally coupled scalar theory. Exact solution of field equations is attained for f(R,phi) = (1 + xi eta(2)phi(2))R model. Matter distributions for radiation, cosmic string and cosmological constant are investigated for constructed model. For different selections of arbitrary constants, Kantowski-Sachs model shows three different behaviors related with Hubble parameter and deceleration parameter. Also, some observational parameters, such as luminosity distance, angular diameter distance and distance modulus, are obtained for Kantowski-Sachs Universe. Lastly, we get Statefinder parameters for the model. All properties of constructed model are discussed from a physical and geometric point of view.
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    Scalar fields in f (G) gravity
    (World Scientific Publ Co Pte Ltd, 2020) Taser, Dogukan; Dogru, Melis Ulu
    In this study, we investigated scalar field in f(G)-gravity by using LRS Bianchi type-I universe. Massless and massive scalar field models are separately constructed in f(G)-gravity. Massless scalar field models are examined in the cases of constant and exponential potential fields. For all models, solutions of field equations are obtained under the consideration of A = B-n. f(G) functions for each model are separately attained in theory. It is shown that constructed models in the presence of massless scalar field permit quintessence scalar field. Also, it is observed that each model indicates expanding universe with deceleration. Also, kinematical quantities are analyzed in the light of obtained solutions. All models are concluded with a geometric and physical perspective.
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    Spherically Symmetric Electromagnetic Field and Chaplygin Gas with Conformal Motion in f (R) Gravity
    (Amer Inst Physics, 2016) Taser, Dogukan; Dogru, Melis Ulu
    In this study, we investigate electromagnetic field and Chaplygin gas in f(R) - gravity. We use field equations and their solutions of spherically symmetric space-time in the presence of conformal symmetry. Exact solutions of conformal spherically symmetric Chaplygin gas and electromagnetic field are obtained to the accompaniment of important results. Obtained f (R) function is classified with viable f (R) models of the modified theory. Also, we have discussed geometrical and physical features of the obtained solutions.