Yazar "Doğru, Melis Ulu" seçeneğine göre listele

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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
    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.
  • [ X ]
    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.