Researcher:
Akarsu, Özgür

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Özgür

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Akarsu

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Akarsu, Özgür

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Now showing 1 - 10 of 14
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    Publication
    Are dark energy models with variable EoS parameter w compatible with the late inhomogeneous Universe?
    (Institute of Physics (IOP) Publishing, 2015) Bouhmadi-Lopez, Mariam; Brilenkov, Maxim; Brilenkov, Ruslan; Eingorn, Maxim; Zhuk, Alexander; Department of Physics; Department of Physics; Akarsu, Özgür; Researcher; College of Sciences; N/A
    We study the late-time evolution of the Universe where dark energy (DE) is presented by a barotropic fluid on top of cold dark matter (CDM). We also take into account the radiation content of the Universe. Here by the late stage of the evolution we refer to the epoch where CDM is already clustered into inhomogeneously distributed discrete structures (galaxies, groups and clusters of galaxies). Under this condition the mechanical approach is an adequate tool to study the Universe deep inside the cell of uniformity. More precisely, we study scalar perturbations of the FLRW metric due to inhomogeneities of CDM as well as fluctuations of radiation and DE. For an arbitrary equation of state for DE we obtain a system of equations for the scalar perturbations within the mechanical approach. First, in the case of a constant DE equation of state parameter w, we demonstrate that our method singles out the cosmological constant as the only viable dark energy candidate. Then, we apply our approach to variable equation of state parameters in the form of three different linear parametrizations of w, e.g., the Chevallier-Polarski-Linder perfect fluid model. We conclude that all these models are incompatible with the theory of scalar perturbations in the late Universe.
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    Some anisotropic universes in the presence of imperfect fluid coupling with spatial curvature
    (Springer, 2011) Kılınç, Can Battal; Department of Physics; Department of Physics; Akarsu, Özgür; Researcher; College of Sciences; N/A
    We consider Bianchi VI spacetime, which also can be reduced to Bianchi types VI0-V-III-I. We initially consider the most general form of the energy-momentum tensor which yields anisotropic stress and heat flow. We then derive an energy-momentum tensor that couples with the spatial curvature in a way so as to cancel out the terms that arise due to the spatial curvature in the evolution equations of the Einstein field equations. We obtain exact solutions for the universes indefinitely expanding with constant mean deceleration parameter. The solutions are beriefly discussed for each Bianchi type. The dynamics of the models and fluid are examined briefly, and the models that can approach to isotropy are determined. We conclude that even if the observed universe is almost isotropic, this does not necessarily imply the isotropy of the fluid (e.g., dark energy) affecting the evolution of the universe within the context of general relativity.
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    Publication
    Accelerated expansion of the universe `a la the Stueckelberg mechanism
    (Iop Publishing Ltd, 2014) Arik, Metin; Katirci, Nihan; Kavuk, Mehmet; Department of Physics; Department of Physics; Akarsu, Özgür; Researcher; College of Sciences; N/A
    We investigate a cosmological model in which the Stueckelberg fields are nonminimally coupled to the scalar curvature in a gauge invariant manner. We present not only a solution that can be considered in the context of the late time acceleration of the universe but also a solution compatible with the inflationary cosmology. Distinct behaviors of the scalar and vector fields together with the real valued mass gained by the Stueckelberg mechanism lead the universe to go through the two different accelerated expansion phases with a decelerated expansion phase between them. On the other hand, in the solutions we present, if the mass is null then the universe is either static or exhibits a simple power law expansion due to the vector field potential.
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    Publication
    Cosmology with hybrid expansion law: scalar field reconstruction of cosmic history and observational constraints
    (Institute of Physics (IOP) Publishing, 2014) Kumar, Suresh; Myrzakulov, R.; Sami, M.; Xu, Lixin; Department of Physics; Department of Physics; Akarsu, Özgür; Researcher; College of Sciences; N/A
    In this paper, we consider a simple form of expansion history of Universe referred to as the hybrid expansion law − a product of power-law and exponential type of functions. The ansatz by construction mimics the power-law and de Sitter cosmologies as special cases but also provides an elegant description of the transition from deceleration to cosmic acceleration. We point out the Brans-Dicke realization of the cosmic history under consideration. We construct potentials for quintessence, phantom and tachyon fields, which can give rise to the hybrid expansion law in general relativity. We investigate observational constraints on the model with hybrid expansion law applied to late time acceleration as well as to early Universe a la nucleosynthesis.
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    PublicationOpen Access
    A divergence-free parametrization for dynamical dark energy
    (Institute of Physics (IOP) Publishing, 2015) Vazquez, J. Alberto; Department of Physics; Department of Physics; Dereli, Tekin; Akarsu, Özgür; Faculty Member; College of Sciences; 201358; N/A
    We introduce a new parametrization for the dark energy, led by the same idea to the linear expansion of the equation of state in scale factor a and in redshift z, which diverges neither in the past nor future and contains the same number of degrees of freedom with the former two. We present constraints of the cosmological parameters using the most updated baryon acoustic oscillation (BAO) measurements along with cosmic microwave background (CMB) data and a recent reanalysis of Type Ia supernova (SN) data. This new parametrization allowed us to carry out successive observational analyses by decreasing its degrees of freedom systematically until ending up with a dynamical dark energy model that has the same number of parameters with ACDM. We found that the dark energy source with a dynamical equation of state parameter equal 2/3 at the early universe and -1 today fits the data slightly better than A.
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    PublicationOpen Access
    Probing kinematics and fate of the universe with linearly time-varying deceleration parameter
    (Springer, 2014) Kumar, Suresh; Xu, Lixin; Department of Physics; Department of Physics; Dereli, Tekin; Akarsu, Özgür; Faculty Member; College of Sciences; 201358; N/A
    The parametrizations q = q 0+q 1 z and q = q 0+q 1(1 - a/a 0) (Chevallier-Polarski-Linder parametrization) of the deceleration parameter, which are linear in cosmic redshift z and scale factor a, have been frequently utilized in the literature to study the kinematics of the Universe. In this paper, we follow a strategy that leads to these two well-known parametrizations of the deceleration parameter as well as an additional new parametrization, q = q 0+q 1(1 - t/t 0), which is linear in cosmic time t. We study the features of this linearly time-varying deceleration parameter in contrast with the other two linear parametrizations. We investigate in detail the kinematics of the Universe by confronting the three models with the latest observational data. We further study the dynamics of the Universe by considering the linearly time-varying deceleration parameter model in comparison with the standard ?CDM model. We also discuss the future of the Universe in the context of the models under consideration.
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    PublicationOpen Access
    Accelerating anisotropic cosmologies in Brans-Dicke gravity coupled to a mass-varying vector field
    (Institute of Physics (IOP) Publishing, 2014) Department of Physics; Department of Physics; Akarsu, Özgür; Dereli, Tekin; Oflaz, Neslihan; PhD Student; College of Sciences; N/A; 201358; N/A
    The field equations of Brans-Dicke gravity coupled to a mass-varying vector field are derived. Anisotropic cosmological solutions with a locally rotationally symmetric Bianchi type I metric and time-dependent scalar and electric vector fields are studied. A particular class of exact solutions for which all the variable parameters have a power-law time dependence is given. The universe expands with a constant expansion anisotropy within this class of solutions. We show that the accelerating expansion is driven by the scalar field and the electric vector field can be interpreted as an anisotropic dark matter source.
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    PublicationOpen Access
    A comparison of the LVDP and {\Lambda} CDM cosmological models
    (Springer, 2012) Department of Physics; Department of Physics; Akarsu, Özgür; Dereli, Tekin; Faculty Member; College of Sciences; N/A; 201358
    We compare the cosmological kinematics obtained via our law of linearly varying deceleration parameter (LVDP) with the kinematics obtained in the Lambda CDM model. We show that the LVDP model is almost indistinguishable from the Lambda CDM model up to the near future of our universe as far as the current observations are concerned, though their predictions differ tremendously into the far future.
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    PublicationOpen Access
    Late time acceleration of the 3-space in a higher dimensional steady state universe in dilaton gravity
    (Institute of Physics (IOP) Publishing, 2013) Department of Physics; Department of Physics; Akarsu, Özgür; Dereli, Tekin; Faculty Member; College of Sciences; N/A; 201358
    We present cosmological solutions for (1+3+n)-dimensional steady state universe in dilaton gravity with an arbitrary dilaton coupling constant w and exponential dilaton self-interaction potentials in the string frame. We focus particularly on the class in which the 3-space expands with a time varying deceleration parameter. We discuss the number of the internal dimensions and the value of the dilaton coupling constant to determine the cases that are consistent with the observed universe and the primordial nucleosynthesis. The 3-space starts with a decelerated expansion rate and evolves into accelerated expansion phase subject to the values of w and n, but ends with a Big Rip in all cases. We discuss the cosmological evolution in further detail for the cases w = 1 and w = 1/2 that permit exact solutions. We also comment on how the universe would be conceived by an observer in four dimensions who is unaware of the internal dimensions and thinks that the conventional general relativity is valid at cosmological scales.
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    PublicationOpen Access
    The dynamical evolution of 3-space in a higher dimensional steady state universe
    (Springer, 2013) Department of Physics; Department of Physics; Dereli, Tekin; Akarsu, Özgür; Faculty Member; College of Sciences; 201358; N/A
    We investigate a class of cosmological solutions of Einstein's field equations in higher dimensions with a cosmological constant and an ideal fluid matter distribution as a source. We discuss the dynamical evolution of the universe subject to two constraints that (i) the total volume scale factor of the universe is constant and (ii) the effective energy density is constant. We obtain various interesting new dynamics for the external space that yield a time varying deceleration parameter including oscillating cases when the flat/curved external and curved/flat internal spaces are considered. We also comment on how the universe would be conceived by an observer in four dimensions who is unaware of the extra dimensions.