H. R. Sepangi

Shahid Beheshti University, Teheran, Tehrān, Iran

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Publications (74)199.32 Total impact

  • A. Borzou, H. R. Sepangi, S. Shahidi, R. Yousefi
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    ABSTRACT: We consider a brane world scenario in which the bulk action is assumed to have the form of a generic function of the Ricci scalar f(R) and derive the resulting Einstein field equation on the brane. In a constant curvature bulk a conserved geometric quantity appears in the field equations which can be associated with matter. We present spherically symmetric solutions which account for galaxy rotation curves in a specific form. Then cosmological solutions by assuming a specific form for f(R) are derived which can explain an accelerated expanding universe.
    AIP Conference Proceedings. 10/2009; 1178(1):131-137.
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    N. Khosravi, H. R. Sepangi, B. Vakili
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    ABSTRACT: We employ the familiar canonical quantization procedure in a given cosmological setting to argue that it is equivalent to and results in the same physical picture if one considers the deformation of the phase-space instead. To show this we use a Probabilistic Evolutionary Process (PEP) to make the solutions of these different approaches comparable. Specific model theories are used to show that the independent solutions of the resulting Wheeler-DeWitt equation are equivalent to solutions of the deformation method with different signs for the deformation parameter. We also argued that since the Wheeler-DeWitt equation is a direct consequence of diffeomorphism invariance, this equivalence is only true provided that the deformation of phase-space does not break such an invariance. Comment: 19 pages, 31 figures; This paper is a complement to arXiv:0903.1914;
    General Relativity and Gravitation 09/2009; · 1.90 Impact Factor
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    H. R. Sepangi, S. Shahidi
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    ABSTRACT: We study the virial mass discrepancy in the context of a DPG brane-world scenario and show that such a framework can offer viable explanations to account for the mass discrepancy problem. This is done by defining a geometrical mass $\mathcal{N}$ that we prove to be proportional to the virial mass. Estimating $\mathcal{N}$ using observational data, we show that it behaves linearly with $r$ and has a value of the order of $M_{200}$, pointing to a possible resolution of the virial mass discrepancy. We also obtain the radial velocity dispersion of galaxy clusters and show that it is compatible with the radial velocity dispersion profile of such clusters. This velocity dispersion profile can be used to differentiate various models predicting the virial mass. Comment: 12 pages, 1 figure, to appear in CQG
    Classical and Quantum Gravity 08/2009; · 3.56 Impact Factor
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    A. S. Sefiedgar, K. Atazadeh, H. R. Sepangi
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    ABSTRACT: We use the collision-free Boltzmann equation in Palatini $f({\mathcal{R}})$ gravity to derive the virial theorem within the context of the Palatini approach. It is shown that the virial mass is proportional to certain geometrical terms appearing in the Einstein field equations which contribute to gravitational energy and that such geometric mass can be attributed to the virial mass discrepancy in cluster of galaxies. We then derive the velocity dispersion relation for clusters followed by the metric tensor components inside the cluster as well as the $f({\mathcal{R}})$ lagrangian in terms of the observational parameters. Since these quantities may also be obtained experimentally, the $f({\mathcal{R}})$ virial theorem is a convenient tool to test the viability of $f({\mathcal{R}})$ theories in different models. Finally, we discuss the limitations of our approach in the light of the cosmological averaging used and questions that have been raised in the literature against such averaging procedures in the context of the present work. Comment: 16 pages, to appear in PRD
    Physical review D: Particles and fields 08/2009;
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    S. Jalalzadeh, M. Mehrnia, H. R. Sepangi
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    ABSTRACT: The vacuum solutions in brane gravity differ from those in 4D by a number of additional terms and reduce to the familiar Schwarzschild metric at small distances. We study the possible roles that such terms may play in the precession of planetary orbits, bending of light, radar retardation and the anomaly in mean motion of test bodies. Using the available data from Solar System experiments, we determine the range of the free parameters associated with the linear term in the metric. The best results come from the anomalies in the mean motion of planets. Such studies should shed some light on the origin of dark energy via the solar system tests. Comment: 10 pages, no figures, to appear in CQG
    Classical and Quantum Gravity 06/2009; · 3.56 Impact Factor
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    K. Atazadeh, A. M. Ghezelbash, H. R. Sepangi
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    ABSTRACT: In the standard picture of cosmology it is predicted that a phase transition, associated with chiral symmetry breaking after the electroweak transition, has occurred at approximately 10 \mu seconds after the Big Bang to convert a plasma of free quarks and gluons into hadrons. We consider the quark-hadron phase transition in a DGP brane world scenario within an effective model of QCD. We study the evolution of the physical quantities useful for the study of the early universe, namely, the energy density, temperature and the scale factor before, during, and after the phase transition. Also, due to the high energy density in the early universe, we consider the quadratic energy density term that appears in the Friedmann equation. In DGP brane models such a term corresponds to the negative branch (\epsilon=-1) of the Friedmann equation when the Hubble radius is much smaller than the crossover length in 4D and 5D regimes. We show that for different values of the cosmological constant on a brane, \lambda, phase transition occurs and results in decreasing the effective temperature of the quark-gluon plasma and of the hadronic fluid. We then consider the quark-hadron transition in the smooth crossover regime at high and low temperatures and show that such a transition occurs along with decreasing the effective temperature of the quark-gluon plasma during the process of the phase transition.
    04/2009;
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    A. Borzou, H. R. Sepangi
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    ABSTRACT: Within the context of a $5D$ space-time, we construct a unified theory of gravity and electromagnetism from which the Einstein field equations and Maxwell equations emerge, with homogenous Maxwell equations appearing naturally. We also introduce a well-defined five dimensional energy-momentum tensor consistent with our unification scheme. A correction term appears in Maxwell equations which can be used to explain the recently discovered galactic magnetic fields. Comment: 10 pages
    04/2009;
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    N. Khosravi, H.R. Sepangi
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    ABSTRACT: We present a method, which we shall call the probabilistic evolutionary process, based on the probabilistic nature of quantum theory to offer a possible solution to the problem of time in quantum cosmology. It offers an alternative for perceiving an arrow of time which is compatible with the thermodynamical arrow of time and makes a new interpretation of the FRW universe in vacua which is consistent with creation of a de Sitter space–time from nothing. This is a completely quantum result with no correspondence in classical cosmology.
    Physics Letters B 03/2009; · 4.57 Impact Factor
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    H. R. Sepangi, B. Shakerin, B. Vakili
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    ABSTRACT: We study the effects of noncommutativity and deformed Heisenberg algebra on the evolution of a two dimensional minisuperspace cosmological model in classical and quantum regimes. The phase space variables turn out to correspond to the scale factor of a flat FRW model with a positive cosmological constant and a dilatonic field with which the action of the model is augmented. The exact classical and quantum solutions in commutative and noncommutative cases are presented. We also obtain some approximate analytical solutions for the corresponding classical and quantum cosmology in the presence of the deformed Heisenberg relations between the phase space variables, in the limit where the minisuperspace variables are small. These results are compared with the standard commutative and noncommutative cases and similarities and differences of these solutions are discussed.
    Classical and Quantum Gravity 02/2009; · 3.56 Impact Factor
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    ABSTRACT: Various properties of Nano-Scale cell bio-membranes or vesicle membranes in an external electric field and free salt environment are studied and reviewed in a theoretical and computational framework. This is done by calculating the radial forces acting on cells generated as the result of an applied electric field and the ensuing charge re-distribution. Our results could help to shed light on mechanisms responsible for the observed deformation and fusion of cells which can be designed by biotechnological devices.
    Physica A: Statistical Mechanics and its Applications 01/2009; 388:120-128. · 1.68 Impact Factor
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    A. Borzou, H. R. Sepangi, S. Shahidi, R. Yousefi
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    ABSTRACT: We consider a brane world scenario in which the bulk action is assumed to have the form of a generic function of the Ricci scalar f(\mathcal{R}) and derive the resulting Einstein field equations on the brane. In a constant curvature bulk a conserved geometric quantity appears in the field equations which can be associated with matter. We present cosmological and spherically symmetric solutions by assuming specific forms for f(\mathcal{R}) and show that the former can explain an accelerated expanding universe while the latter may account for galaxy rotation curves.
    EPL (Europhysics Letters) 01/2009; 88. · 2.26 Impact Factor
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    K. Atazadeh, H. R. Sepangi
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    ABSTRACT: We consider a five dimensional DGP-brane scenario endowed with a non-minimally coupled scalar field within the context of Brans-Dicke theory. This theory predicts that the mass appearing in the gravitational potential is modified by the addition of the mass of the effective intrinsic curvature on the brane. We also derive the effective four dimensional field equations on a 3+1 dimensional brane where the fifth dimension is assumed to have an orbifold symmetry. Finally, we discuss the cosmological implications of this setup, predicting an accelerated expanding universe with a value of the Brans-Dicke parameter $\omega$ consistent with values resulting from the solar system observations. Comment: 12 pages, 1 figure, to appear in JCAP
    Journal of Cosmology and Astroparticle Physics 11/2008; · 6.04 Impact Factor
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    ABSTRACT: We consider the Weyl–Dirac theory within the framework of the weak field approximation and show that the resulting gravitational potential differs from that of Newtonian by a repulsive correction term increasing with distance. The scale of the correction term appears to be determined by the time variation rate of the gravitational coupling. It is shown that if the time variation rate of gravitational coupling is adopted from observational bounds, the theory can explain the rotation curves of typical spiral galaxies without resorting to dark matter. To check the consistency of our theoretical model with observation we use likelihood analysis to find the best-fitting values for the free parameters. The mean value for the most important free parameter, β× 1014 (yr−1), using the top-hat and Gaussian priors are 6.38+2.44−3.46+6.18−6.71 and 5.72+1.22−1.18+2.90−2.69, respectively. Although the interval for which β is defined is wide, our results show that the goodness of the fit is, by and large, not sensitive to this quantity. The intergalactic effects and gravitational lensing of clusters of galaxies are estimated and seem to be consistent with observational data.
    Monthly Notices of the Royal Astronomical Society 03/2008; 385(2):986 - 994. · 5.52 Impact Factor
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    N. Khosravi, H. R. Sepangi
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    ABSTRACT: We study a de-Sitter model in the framework of a Deformed Special Relativity (DSR) inspired structure. The effects of this framework appear as the existence of a fundamental length which influences the behavior of the scale factor. We show that such a deformation can either be used to control the unbounded growth of the scale factor in the present accelerating phase or account for the inflationary era in the early evolution of the universe. Comment: 10 pages, 3 figures, to appear in JCAP
    Journal of Cosmology and Astroparticle Physics 03/2008; · 6.04 Impact Factor
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    K. Atazadeh, M. Farhoudi, H.R. Sepangi
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    ABSTRACT: We study a 5-dimensional f(R) brane gravity within the framework of scalar-tensor type theories. We show that such a model predicts, for a certain choice of f(R) and a spatially flat universe, an exponential potential, leading to an accelerated expanding universe driven solely by the curvature of the bulk space. This result is consistent with the observational data in the cosmological scale.
    Physics Letters B 02/2008; · 4.57 Impact Factor
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    N. Khosravi, H. R. Sepangi
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    ABSTRACT: We show that the existence of a fundamental length, introduced in Deformed Special Relativity (DSR) inspired minisuper (phase-) space, causes the behavior of the scale factor of the universe to change from that of a universe filled with dust to an accelerating universe driven by a cosmological constant. Comment: 8 pages, to appear in PLA
    Physics Letters A 02/2008; · 1.77 Impact Factor
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    M. Heydari-Fard, H. R. Sepangi
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    ABSTRACT: The generalized Chaplygin gas provides an interesting candidate for the present accelerated expansion of the universe. We explore a geometrical explanation for the generalized Chaplygin gas within the context of brane world theories where matter fields are confined to the brane by means of the action of a confining potential. We obtain the modified Friedmann equations, deceleration parameter and age of the universe in this scenario and show that they are consistent with the present observational data.
    Physical review D: Particles and fields 11/2007;
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    K. Atazadeh, H. R. Sepangi
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    ABSTRACT: We consider a DGP inspired brane scenario where the action on the brane is augmented by a function of the Ricci scalar, ${\cal L}(R)$. The cosmological implications that such a scenario entails are examined for $R^{n}$ and shown to be consistent with a universe expanding with power-law acceleration. It is shown that two classes of solutions exist for the usual FRW metric and small Hubble radii. When the Hubble radius becomes larger, we either have a transition to a fully 5D regime or to a self-inflationary solution which produces a late accelerated expansion such that the radius becomes a function of $n$.
    Journal of Cosmology and Astroparticle Physics 11/2007; · 6.04 Impact Factor
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    B. Vakili, H.R. Sepangi
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    ABSTRACT: The problem of time reparameterization is addressed at both the classical and quantum levels in a Bianchi-I universe in which the matter source is a massive Dirac spinor field. We take the scale factors of the metric as the intrinsic time and their conjugate momenta as the extrinsic time. A scalar character of the spinor field is identified as a representation of the extrinsic time. The construction of the field equations and quantization of the model is achieved by solving the Hamiltonian constraint after time identification has been dealt with. This procedure leads to a true Hamiltonian whose exact solutions for the above choices of time are presented.
    Annals of Physics 10/2007; · 3.32 Impact Factor
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    M. Heydari-Fard, H. Razmi, H. R. Sepangi
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    ABSTRACT: Using a confining potential, we consider spherically symmetric vacuum (static black hole) solutions in a brane-world scenario. Working with a constant curvature bulk, two interesting cases/solutions are studied. A Schwarzschild-de Sitter black hole solution similar to the standard solution in the presence of a cosmological constant is obtained which confirms the idea that an extra term in the field equations on the brane can play the role of a positive cosmological constant and may be used to account for the accelerated expansion of the universe. The other solution is one in which we can have a proper potential to explain the galaxy rotation curves without assuming the existence of dark matter and without working with new modified theories (modified Newtonian dynamics).
    Physical review D: Particles and fields 08/2007;