Yazar "Doğru, Melis Ulu" seçeneğine göre listele

Listeleniyor 1 - 8 / 8
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    Öğe
    Accelerately Expanding Cosmologies in f(R,?,X) Theory
    (2023) Eraslan, Erkan; Doğru, Melis Ulu
    In this study, beginning and today expansion of universe are viewed in f(R,?,X) gravity. Field equations and their solutions of Friedmann-Lemaître-Robertson-Walker cosmologies with perfect fluid are obtained by considering f(R,?,X)=f_0 R+f_1 X^q-V(?) model. Validity of both f(R,?,X) gravity and f(R,?,X)=f_0 R+f_1 X^q-V(?) model for non-static space-time geometries is discussed by making use of the obtained matter dynamics results such as pressure and energy density. It is seen that in all obtained solutions by taking into account early and late period expansion, f function is a constant. This indicates that f(R,?,X) function is a first-order dependent function of Ricci scalar. When f(R,?,X)=f_0 R+f_1 X^q-V(?) model is considered together, it is understood that the obtained solutions could be reduced to ?-CDM model for f(R) gravity in limits of ??0 and X?0. The fact that the obtained results agree with expected situations supports. So, f(R,?,X) theory is a consistent theory of gravity.
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    Öğe
    Black Holes with Anisotropic Fluid in Lyra Scalar-Tensor Theory
    (2018) Doğru, Melis Ulu; Demirtaş, Murat
    In this paper, we investigate distribution of anisotropic fluid which is aresource of black holes in regard to Lyra scalar-tensor theory. As part of thetheory, we obtain field equations of spherically symmetric space-time withanisotropic fluid. By using field equations, we suggest distribution of anisotropicfluid, responsible for space-time geometries such as Schwarzschild, Reissner-Nordström, Minkowski type, de Sitter type, Anti-de Sitter type, BTZ and chargedBTZ black holes. Finally, we discuss obtained pressures and density of the fluid fordifferent values of arbitrary constants, geometrically and physically.
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    Öğe
    Compact objects in f(R,Lm) theory
    (World Scientific Publ Co Pte Ltd, 2025) Taşer, Doğukan; Doğru, Melis Ulu; Kirezli, Pınar; Aydın, Hüseyin; Eraslan, Erkan
    In this study, it is aimed to investigate the Krori-Barua type compact stars within the scope of f(R, L-m) theory. In this context, first, the field equations are obtained by considering the f(R, L-m) = R/2 + alpha L-m(n) model. Interestingly, the model offers chance to attain all components of matter without any restrictions. In addition, data sets from LMC X-4, EXO 1785-248 and Cen X-3 are used to analyze physical stability of the solutions. It is obtained that all conditions such as causality and stability. which indicate consistency between compact objects and f(R, L-m) theory are valid for the constructed model. We obtained that small values of n in the f(R, L-m) function show significant effect on the consistency of the star models. Therefore, different n values are considered for each star data set when graphing the physical states. Finally, necessary quantities of the model are defined, compared with the considered stars and discussed in detail.
  • Küçük Resim
    Öğe
    Conformal wormholes in f(R,Lm) theory
    (Iop Publishing Ltd, 2024) Taşer, Doğukan; Doğru, Melis Ulu; Eraslan, Erkan; Aydın, Hüseyin
    In this study, we present a new static and conformally symmetric wormhole solution. We develop new wormhole model in f(R,L-m) gravity. Shape function of conformally symmetric f(R,L-m) wormholes, provided by homothetic vector field is obtained regarding to conformal factor. It is investigated whether the function provides a stable throat infrastructure that does not allow collapsing. Anisotropic fluid matter filled the obtained conformally symmetric traversable wormholes is discussed by examining the dynamical properties of the fluid. Constructed new wormhole model violates all energy conditions. Physical significance of the sources filled constructed wormholes is discussed using volume quantifier. It is revealed that f(R,L-m) theory allows traversable and conformally symmetric wormholes if supported by phantom field.
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    Cylindrically symmetric unimodular f(R) black holes
    (World Scientific Publishing, 2021) Aydın, Hüseyin; Doğru, Melis Ulu
    In this paper, we examine massless scalar field by using unimodular f(R) theory. It is taken into account unimodular and cylindrically symmetric spacetime which provides convenience in researching black hole. The field equations in unimodular f(R) theory for given spacetime with massless scalar field and additional Bianchi identities are solved. Cylindrically symmetric anti-de Sitter (AdS)-Schwarzschild-like and AdS-Reissner-Nordstrom-like black hole spacetimes are achieved. Equations of motion are derived by using Hamiltonian. Orbits of massless test particles are depicted. Obtained line element asymptotically converges to dS/AdS spacetime. Weak and strong energy conditions of the massless scalar field are obtained with Raychaudhuri equations in unimodular f(R) theory. Also, stiff fluid interpretation of scalar field is reviewed.
  • Küçük Resim
    Öğe
    Scalar field in unimodular f(R, T) theory
    (Springer, 2024) Aydın, Hüseyin; Doğru, Melis Ulu
    In this paper, we are fundamentally interested in minimally coupled scalar field as source of unimodular Friedmann–Robertson–Walker (FRW) spacetime. We obtain dynamical quantities related with scalar field in FRW cosmologies by using field equations of unimodular f(R, T) theory. Equation of motion for a test particle and kinematical quantities are also obtained. We achieve Newtonian limit of unimodular f(R, T) theory by using weak field approximation. Energy conditions are presented for scalar field in unimodular f(R, T) theory. Energy conditions are examined by using graphics for massive and massless scalar field. Obtained results are discussed by comparing with viable cosmological models and previous works. Validity of unimodular f(R, T) theory is concluded.
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    Unimodular f(G, T) gravity
    (World Scientific Publ Co Pte Ltd, 2023) Doğru, Melis Ulu; Aydın, Hüseyin; Taşer, Doğukan
    In this study, unimodular 𝑓⁡(G,𝑇) gravity is proposed as an alternative theory of gravity. Field equations of unimodular 𝑓⁡(G,𝑇) gravity are suggested. The field equation and their solutions are obtained for exponential expansion and power-law expansion models. Energy conditions of unimodular 𝑓⁡(G,𝑇) gravity are defined by using Raychaudhuri equations. Energy conditions for exponential expansion and power-law expansion models are investigated. Hubble parameter and deceleration parameter are calculated by considering power-law expansion model and obtained solutions. It has been shown that both deceleration parameter and Hubble parameter are compatible with the current observational data.
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    Öğe
    Unimodüler f(R) Teoride Skaler Alanlı (1+1)-Boyutlu Karadelikler
    (2018) Aydın, Hüseyin; Doğru, Melis Ulu
    Bu çalışmada, unimodüler f(R) gravitasyon teorisi çerçevesinde (1+1)-boyutlu Schwarzschild karadelikleri skaler alan varlığında araştırılmıştır. Unimodüler f(R) gravitasyon teorisinin (3+1)-boyutlu uzay-zaman geometrileri için tanımlanmış olan alan denklemleri, (1+1)-boyut için elde edilmiştir. Unimodüler f(R) gravitasyon teorisine ait enerji-momentum korunum denklemi, etkin enerji-momentum tensörü kullanılarak (3+1) ve (1+1)-boyutlu uzay-zamanlar için tanımlanmıştır. Tanımlanan korunum denklemi ve Klein-Gordon denklemlerinden faydalanılarak alan denklemi çözümleri elde edilmiştir. Keyfi sabitlerin farklı seçimleri için f(R) ve skaler alan fonksiyonlarının grafikleri gösterilmiştir. Elde edilen sonuçlar fiziksel ve geometrik açıdan tartışılmıştır.