Farhad Darabi

Farhad Darabi
Azarbaijan Shahid Madani University · Department of Physics

Professor

About

167
Publications
9,293
Reads
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1,416
Citations
Additional affiliations
January 2005 - December 2010
Azarbaijan University of Tarbiat Moallem
January 2003 - December 2013
Research Institute for Astronomy and Astrophysics
Position
  • Researcher
September 2000 - present
Azarbaijan Shahid Madani University
Position
  • Professor (Full)

Publications

Publications (167)
Preprint
Full-text available
In this paper, we study a hybrid combination of Einstein-Hilbert action with curvature scalar $R$, and a function $f(\mathcal{R})$ in Palatini gravity within the context of inflationary scenario, from the Swampland conjecture point of view. This hybrid model has been paid attention in recent cosmological studies, and its applications have been wide...
Article
In this paper, we study a hybrid combination of Einstein–Hilbert action with curvature scalar R, and a function f(R) in Palatini gravity within the context of inflationary scenario, from the Swampland conjecture point of view. This hybrid model has been paid attention in recent cosmological studies, and its applications have been widely studied in...
Article
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In this paper, we use the Hojman symmetry approach to find new [Formula: see text]-dimensional [Formula: see text] gravity solutions, in comparison to Noether symmetry approach. In the special case of Hojman symmetry vector [Formula: see text], we recover [Formula: see text]-dimensional Bañados–Teitelboim–Zanelli (BTZ) black hole and generalized [F...
Preprint
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We introduce a new kind of super warped product spaces $\bar{M}_{_{(I)}}=\textbf{I}^{1|0}\times_f M^{m|n}$, $\bar{M}_{_{(II)}}=\textbf{I}^{0|1}\times_{f} M^{m|n}$, and $\bar{M}_{_{(III)}}=\textbf{I}^{1|1}\times_{f} M^{m|n}$, where $M^{m|n}$ is a supermanifold of dimension $m|n$, $\textbf{I}^{\delta|\delta'}$ is standard superdomain with $\textbf{I}...
Preprint
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In this paper, we investigate the Einstein equations with cosmological constant for Randall-Sundrum (RS) and Dvali-Gabadadze-Porrati (DGP) models to determine the warp functions in the context of warp product spacetimes. In RS model, it is shown that Einstein's equation in the bulk is reduced into the brane as a vacuum equation, having vacuum solut...
Article
In this paper, we investigate the Einstein equations with cosmological constant for Randall–Sundrum (RS) and Dvali–Gabadadze–Porrati (DGP) models to determine the warp functions in the context of warp product spacetimes. In RS model, it is shown that Einstein’s equation in the bulk is reduced into the brane as a vacuum equation, having vacuum solut...
Article
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We study cosmological inflation in a Galileon inflationary model with the E-model potential to find possible [Formula: see text]-attractors. First, we calculate evolution of the perturbations in our setup. By adopting E-model potential, we show that values of the scalar spectral index and tensor-to-scalar ratio are universal. Also, we consider the...
Article
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We study the Vaidya black hole surrounded by the exotic quintessence-like, phantom-like and cosmological constant-like fields by means of entropic considerations. Explicitly, we show that for this thermodynamical system, the requirement of the identification of the D-bound and Bekenstein entropy bound can be considered as a thermodynamical criterio...
Preprint
In this paper, we use the Hojman symmetry approach in the context of $f(R)$ gravity to find new generalized (2+1)-dimensional BTZ black hole solutions as well as the associated symmetry vectors.
Article
In this paper, we find exact string cosmological solutions for FRW cosmology, by using the Hojman symmetry approach. The string cosmology under consideration includes a scalar field [Formula: see text] with the potential [Formula: see text], and a totally antisymmetric field strength [Formula: see text] which is specifically defined in terms of the...
Article
The cosmological candidate fields for dark energy as quintessence, phantom and cosmological constant are studied in terms of an entropic hypothesis imposed on the McVittie solution surrounded by dark energy. We certify this hypothesis as “D-bound-Bekenstein bound identification” for dilute systems and use it as a criterion to determine which candid...
Article
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We study [Formula: see text]-attractor models with both E-model and T-model potential in an extended Nonminimal Derivative (NMD) inflation where a canonical scalar field and its derivatives are nonminimally coupled to gravity. We calculate the evolution of perturbations during this regime. Then by adopting inflation potentials of the model we show...
Preprint
In this paper, we find exact string cosmological solutions for Bianchi type I cosmology, by using Hojman symmetry approach. The string cosmology under consideration includes a dilaton field $\psi$ with the potential $W(\psi)$, and a totally antisymmetric field strength $H_{\mu\nu\rho}$ which is specifically defined in terms of the scale factor $a(t...
Preprint
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The cosmological quintessence, phantom and vacuum fields, as candidates for dark energy, are studied by entropic considerations in a particular model of McVittie solution surrounded by dark energy. We show that the thermodynamical requirement of identifying D-bound and Bekenstein bound for any system can be considered as a thermodynamical criterion...
Preprint
We consider the possibility of the quantum vacuum states in f(R,T) gravity. Particularly, we study the Bogoliubov transformations associated to different vacuum states for some f(R,T) models. The method consists of fixing the f(R,T) free parameters by requiring the Bogoliubov coefficients to be minimized. In such a way, the particle production is r...
Article
We quantize a flat cosmological model in the context of the f(T) theory of modified gravity using the Dirac quantization approach for Hamiltonian constraint systems. In this regard, first we obtain the Wheeler-DeWitt equation as the operator equation of the Hamiltonian constraint and solve it for the typical cosmological models of f(T)=T−2Λ, f(T)=β...
Article
We consider the possibility of the quantum vacuum states in [Formula: see text] gravity. Particularly, we study the Bogoliubov transformations associated to different vacuum states for some [Formula: see text] models. The method consists of fixing the [Formula: see text] free parameters by requiring the Bogoliubov coefficients to be minimized. In s...
Preprint
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We study the entropic considerations on the Universe system and the Universe-Black hole system, filled by quintessence, phantom and cosmological constant having negative pressure, using their relevant entropic bounds. It turns out that for both systems these considerations single out the cosmological constant, among the negative pressure candidate...
Article
In this paper, in the framework of massive bigravity, we study all possible cosmic evolutions by using a method in which the modified Friedmann equation is written in a form in which the scale factor evolves like the motion of a particle under a “potential.” Massive bigravity provides this potential with the most general mass interaction term, whic...
Preprint
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The Bousso's D-bound approach is investigated for particular dynamical black holes, namely the surrounded Vaidya solution by cosmological fields. We derive the D-bound and the Bekenstein bound for these solutions and then compare them to obtain some interesting results. We show that the more cosmological fields are diluted, the more D-bound and Bek...
Preprint
We quantize a flat cosmological model in the context of $f(T)$ theory of modified gravity. First, we show that the correct study of f(T) gravity should be analyzed using the formalism of Dirac's Hamiltonian constraint systems. Then, we proceed to quantize this model using the Dirac's quantization approach for Hamiltonian constraint systems. In this...
Article
Full-text available
In the present work, we generalize our previous work (Heydarzade and Darabi in arXiv:1710.04485, 2018 on the surrounded Vaidya solution by cosmological fields to the case of Bonnor–Vaidya charged solution. In this regard, we construct a solution for the classical description of the evaporating-accreting charged Bonnor–Vaidya black holes in the gene...
Preprint
In this paper, in the framework of massive bigravity, we study all possible cosmic evolutions by using a method in which the modified Friedmann equation is written in a form where the scale factor evolves like the motion of a particle under a "potential". Massive bigravity provides this potential with the most general mass interaction term which ca...
Article
Full-text available
We study nonlinear cosmological perturbations and their possible non-Gaussian character in an extended nonminimal inflation where gravity is coupled nonminimally to both the scalar field and its derivatives. By expansion of the action up to the third order, we focus on the nonlinearity and non-Gaussianity of perturbations in comparison with recent...
Article
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We present non-critical Bianchi type $I$ string cosmology solutions in the presence of central charge deficit term $\Lambda$. The leading order string frame curvature appears to be in the high curvature limit $R\alpha'\gtrsim1$, which underlines the necessity of including higher order $\alpha'$-corrections. We give new solutions of two-loop (order...
Article
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In the present work, we study the general surrounded Vaidya solution by the various cosmological fields and its nature describing the possibility of the formation of naked singularities or black holes. Motivated by the fact that real astrophysical black holes as non-stationary and non-isolated objects are living in non-empty backgrounds, we focus o...
Article
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We study the thermodynamical features and dynamical evolutions of various apparent horizons associated with the Vaidya evaporating black hole surrounded by the cosmological fields of dust, radiation, quintessence, cosmological constant-like and phantom. In this regard, we address in detail how do these surrounding fields contribute to the character...
Article
We study the Einstein static universe (ESU) in the framework of Generalized Uncertainty Principle (GUP) constructed by the Snyder non-commutative space. It is shown that the deformation parameter can induce an effective energy density subject to GUP which obeys the holographic principle (HP) and plays the role of a cosmological constant. Using the...
Article
We study the dynamics of interacting holographic dark energy model in Brans–Dicke cosmology for the future event horizon and the Hubble horizon cut-offs. We determine the system of first-order differential equations for the future event horizon and Hubble horizon cut-offs and obtain the corresponding fixed points, attractors, repellers and saddle p...
Article
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We investigate stability of the Einstein static solution against homogeneous scalar, vector and tensor perturbations in the context of Rastall theory of gravity. We show that this solution in the presence of perfect fluid and vacuum energy originating from conformally-invariant fields is stable. Using the fix point method and taking linear homogene...
Article
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We classify all warped product space-times in three categories as (i) generalized twisted product structures, (ii) base conformal warped product structures and (iii) generalized static space-times and then we obtain the Einstein equations with the corresponding cosmological constant by which we can determine uniquely the warp functions in these war...
Article
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In this work, following the approach of Kiselev for the static black holes, we seek a realistic solution for classical description of the evaporating-accreting Vaidya black hole in the generic dynamical backgrounds of dust, radiation, quintessence, cosmological constant and phantom field, and classify them according to their behaviors under imposin...
Article
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The Bousso's D-bound entropy for the various possible black hole solutions on a 4-dimensional brane is checked. It is found that the D-bound entropy here is apparently different from that of obtained for the 4-dimensional black hole solutions. This difference is interpreted as the extra loss of information, associated to the extra dimension, when a...
Article
In the Brans–Dicke cosmology framework, we study holographic dark energy models with interaction and with power-law and logarithmic entropy corrections for different cutoffs. We consider conditions on the Brans–Dicke parameter compared with the conditions for the acceleration and phantom phases to show which entropy-corrected models can have accele...
Article
We study the Generalized Uncertainty Principle (GUP) in the framework of Einstein static universe (ESU). It is shown that the deformation parameter corresponding to the Snyder non-commutative space can induce an energy density subject to GUP which obeys the holographic principle (HP) and plays the role of a cosmological constant. Using the holograp...
Article
Full-text available
A generalized version for the Rastall theory is proposed showing the agreement with the cosmic accelerating expansion. In this regard, a coupling between geometry and the pressureless matter fields is derived which may play the role of dark energy responsible for the current accelerating expansion phase. Moreover, our study also shows that the radi...
Article
Full-text available
We study the static cosmological solutions and their stability at background level in the framework of massive bigravity theory with Friedmann–Robertson–Walker (FRW) metrics. By the modification proposed in the cosmological equations subject to a perfect fluid we obtain new solutions interpreted as the Einstein static universe. It turns out that th...
Article
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In this work, we obtain uncharged$\setminus$charged Kiselev-like black holes as a new class of black hole solutions surrounded by perfect fluid in the context of Rastall theory. Then, we study the specific cases of the uncharged$\setminus$charged black holes surrounded by regular matter like dust and radiation, or exotic matter like quintessence, c...
Article
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Stability of the Einstein static universe versus the linear scalar, vector and tensor perturbations is investigated in the context of deformed Ho\v{r}ava-Lifshitz cosmology inspired by entropic force scenario. A general stability condition against the linear scalar perturbations is obtained. Using this general condition, it is shown that there is n...
Article
We consider the existence and stability of the Einstein static universe under the Generalized Uncertainty Principle (GUP) effects. We show that this solution in the presence of perfect fluid with a minimal length is cyclically stable around a center equilibrium point. By taking linear homogeneous perturbations, we find that the scale factor of Eins...
Article
We study the dynamics of interacting logarithmic entropy-corrected holographic dark energy model in Brans-Dicke cosmology for the future event horizon and the Hubble horizon cut-offs. Specifically, for the future event horizon cut-off, we determine the system of first-order differential equations and obtain the corresponding fixed points, attractor...
Article
Full-text available
The Reissner-Nordstr\"om black hole solution in a generic cosmological constant background in the the context of Rastall gravity is obtained. It is shown that the cosmological constant arises naturally from the consistency of the non-vacuum field equations of the Rastall theory for a spherical symmetric spacetime, rather than its {\it ad-hoc} intro...
Article
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We study the inflation via logarithmic entropy-corrected holographic dark energy LECHDE model with future event horizon, particle horizon and Hubble horizon cut-offs, and compare the results with those of obtained in the study of inflation by holographic dark energy HDE model. In comparison, the spectrum of primordial scalar power spectrum in the L...
Article
It is shown that for every multidimensional metric in the multiply warped product form $\bar{M} = K\times_{f_1} M_1\times_{f_2}M_2$ with warp functions $f_1$, $f_2$, associated to the submanifolds $M_1$, $M_2$ of dimensions $n_1$, $n_2$ respectively, one can find the corresponding Einstein equations $\bar{G}_{AB}=-\bar{\Lambda}\bar{g}_{AB}$, with c...
Article
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The covariant entropy conjecture is invariant under time reversal and consequently its origin must be statistical rather than thermodynamical. This may impose a fundamental constraint on the number of degrees of freedom in nature. Indeed, the covariant entropy bound imposes an upper entropy bound for any physical system. Considering a cosmological...
Article
We study the structure formation by investigating the spherical collapse model in the context of new agegraphic dark energy model in flat FRW cosmology. We compute the perturbational quantities $g(a)$, $\delta_{c}(z_{c})$, $\lambda(z_{c})$, $\xi(z_{c})$, $\Delta_{vir}(z_{c})$, $\log[\nu f(\nu)]$ and $\log[n(k)]$ for the new agegraphic dark energy m...
Article
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We study the tachyon scalar field model in flat FRW cosmology with the particular potential and the scale factor behavior . We consider the spherical collapse model and investigate the effects of the tachyon scalar field on the structure formation in flat FRW universe. We calculate , , , , and for the tachyon scalar field model and compare the resu...
Article
Full-text available
We study the static cosmological solutions and their stability in the framework of massive bigravity theory with Friedmann-Robertson-Walker (FRW) metrics. By the modification proposed in the cosmological equations subject to a perfect fluid we obtain new solutions interpreted as the Einstein static universe. It turns out that the non-vanishing size...
Article
Full-text available
We study the cosmological models in which an extended Chaplygin gas universe is merged with the braneworld scenario. In particular, we examine the realization of Einstein static universe in these models and perform a detailed perturbation analysis. We extract the stability conditions and find their impacts on the geometric equation of state paramet...
Article
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Based on the Padmanabhan's proposal, the accelerated expansion of the universe can be driven by the difference between the surface and bulk degrees of freedom in a region of space, described by the relation $ dV/dt = N_{sur}-N_{bulk}$ where $N_{sur}$ and $N_{bulk}$ are the degrees of freedom assigned to the surface area and the matter-energy conten...
Article
We investigate the cosmological dynamics of interacting Logarithmic Entropy Corrected Holographic Dark Energy model with Cold Dark Matter. Fixed points are determined and their corresponding cosmological models are presented. Moreover, the dynamical properties of these fixed points are derived.
Article
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We consider a string model at one-loop related to a σ-model whose antisymmetric tensor field is constructed as complex structure on the background manifold, especially on a manifold R × N where N is a complex manifold. As an example, we consider a homogeneous anisotropic (1 + 4)-dimensional σ-model where space part of the background is a four-dimen...
Article
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We consider the FLRW universe in a loop quantum cosmological model filled with the radiation, baryonic matter (with negligible pressure), dark energy and dark matter. The dark matter sector is supposed to be of Bose-Einstein condensate type. The Bose-Einstein condensation process in a cosmological context by supposing it as an approximate first ord...
Article
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We investigate the Einstein static universe and the emergent universe scenario in the framework of Horava-Lifshitz F(R) gravity. We first perform a dynamical analysis in the phase space, and amongst others we show that a universe filled with usual matter satisfying the strong energy condition can stay in a static phase for very long times, and even...
Article
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In a Friedmann-Robertson-Walker (FRW) space-time background we study the classical cosmological models in the context of recently proposed theory of nonlinear minimal massive bigravity. We show that in the presence of perfect fluid the classical field equations acquire contribution from the massive graviton as a cosmological term which is positive...
Article
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We study the Interacting Logarithmic Entropy-Corrected Holographic Dark Energy model with different cut-offs in Brans-Dicke cosmology and obtain the equation of state and the squared of sound speed for each cut-off. The former is used to describe the accelerating or decelerating behaviour and the later is used to describe the classical stability or...
Article
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We study a scalar-tensor cosmological model where the Einstein tensor is non-minimally coupled to the free scalar field dynamics. Using FRW metric, we investigate the behavior of scale factor for vacuum, matter and dark energy dominated eras. Especially, we focus on the inflationary behavior at early universe. Moreover, we study the perturbation an...
Article
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According to Padmanbhan's proposal, the difference between the surface degrees of freedom and the bulk degrees of freedom in a region of space may result in the acceleration of Universe expansion through the relation $\Delta V/\Delta t = N_{\rm sur}-N_{\rm bulk}$. In this paper, we study the dynamical effect of the extrinsic geometrical embedding o...
Article
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We consider a $5D$ bulk spacetime together with a single $4D$ brane, on which the gravity is confined, and derive the effective $4D$ gravitational field equations. Then, we study the non-minimally kinetic coupled version of a braneworld gravity proposed by Dvali, Gabadadze, and Porrati, so called DGP model, where the kinetic term of the scalar fiel...
Article
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By presenting a relation between average energy of the ensemble of probe photons and energy density of the Universe, in the context of gravity’s rainbow or doubly general relativity scenario, we introduce a rainbow FRW Universe model. By analyzing the fixed points in flat FRW model modified by two well known rainbow functions, we find that the fini...
Article
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We generalize the scalar tensor bigravity models to the non-minimal kinetic coupling scalar tensor bigravity models with two scalar fields whose kinetic terms are non-minimally coupled to two Einstein tensors constructed by two metrics. We show that a broad class of expanding universes can be explained by some solutions of this model. Then, we stud...
Article
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We consider a string cosmological model related to a $\sigma$-model whose antisymmetric tensor field is constructed as a geometrical structure from complex structure on the background manifold, specially on a manifold $R\times N$, $N$ being a complex manifold. As an example, we consider an homogeneous anisotropic (1+4) string cosmological model whe...
Article
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By using the conformal symmetry between Brans-Dicke action with $\omega=-\frac{3}{2}$ and O'Hanlon action, we seek the O'Hanlon actions in Einstein frame respecting the Noether symmetry. Since the Noether symmetry is preserved under conformal transformations, the existence of Noether symmetry in the Brans-Dicke action asserts the Noether symmetry i...
Article
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In this work, we show that it is possible to study the GR equivalent notion of geodesic deviation in $f(T)$ gravity, in spite of the fact that in teleparallel gravity there is no notion of geodesics, and the torsion is responsible for the appearance of gravitational interaction. In this regard, we obtain the GR equivalent of $f(T)$ gravity whose eq...
Article
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Non-minimal kinetic coupled gravity is one the novel modification to the general relativity, which includes the non-minimal coupling of kinetic term of a scalar field $\phi$ with the curvature tensor by the coupling $\kappa$, and the minimal coupling with the metric tensor by the coupling $\varepsilon$. It has already been shown that such modified...
Article
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We show that the conservation of energy-momentum tensor of a gravitational model with Einstein-Hilbert like action on a nearly Kähler manifold with the scalar curvature of a curvature-like tensor, is consistent with the nearly Kähler properties. In this way, the nearly Kähler structure is automatically manifested in the action as a induced matter f...
Article
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We investigate stability of the Einstein static universe against the scalar, vector and tensor perturbations in the context of induced matter brane gravity. It is shown that in the framework of this model, the Einstein static universe has a positive spatial curvature. In contrast to the classical general relativity, it is found that a stable Einste...
Article
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The dynamics of large scale gravitational structures like galaxies, local groups and clusters is studied based on the so-called Liquid-Droplet model describing the saturation property of the nuclear force. Using the assumption that the gravitational force is also saturated over large scale structures, it is argued that the Newtonian gravitational p...
Article
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We have studied the Hawking radiation from {\it generalized} rotating and static $(2+1)$-dimensional BTZ black holes. In this regard, we have benefited the quantum tunneling approach with WKB approximation and obtained the tunneling rate of outgoing scalar and spinor particles across the horizons. We have also obtained the Hawking temperature at th...
Article
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The existence and stability conditions of Einstein static universe against homogeneous scalar perturbations in the context of Lyra geometry is investigated. The stability condition is obtained in terms of the constant equation of state parameter $\omega=p/\rho$ depending on energy density $\rho_0$ and scale factor $a_0$ of the initial Einstein stat...
Article
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Spherically symmetric solutions for f(T) gravity models are derived by the so called Noether Symmetry Approach. First, we present a full set of Noether symmetries for some minisuperspace models. Then, we compute analytical solutions and find that spherically symmetric solutions in f(T) gravity can be recast in terms of Schwarzschild-like solutions...
Article
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We study two different possibilities of constructing the energy-momentum tensors for non-commutative Abelian Proca field, by using (i) general Noether theorem and (ii) coupling to a weak external gravitational field. Both energy-momentum tensors are not traceless due to the violation of Lorentz invariance in non-commutative spaces. In particular, w...
Article
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We have studied the Hawking radiation from generalized rotating and static (2+1)-dimensional BTZ black holes. In this regard, we have benefited the quantum tunneling approach with WKB approximation and obtained the tunneling rate of outgoing scalar and spinor particles across the horizons. We have also obtained the Hawking temperature at the horizo...
Article
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In the framework of f(R) scalar-tensor cosmology, we use the Noether symmetry approach to find the cosmological models consistent with the Noether symmetry. We obtain the functions f(R) and H(a), or the corresponding differential equations, according to specific choices for the scalar field potential V (Φ), the Brans-Dicke function ω(Φ), some cosmo...
Article
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Modified gravity is one of the most favorable candidates for explaining the current accelerating expansion of the Universe. In this regard, we study the viability of an alternative gravitational theory, namely f(R,G), by imposing energy conditions. We consider two forms of f(R,G), commonly discussed in the literature, which account for the stabilit...
Article
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The stability of Einstein static universe against homogeneous scalar perturbations in the context of braneworld scenario is investigated. The stability regions are obtained in terms of the constant geometric linear equation of state parameter $\omega_{extr}=p_{extr}/\rho_{extr}$ and are studied for each evolutionary era of the universe. The results...
Article
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Vector inflation is a newly established model where inflation is driven by non-minimally coupled massive vector fields with a potential term. This model is similar to the model of chaotic inflation with scalar fields, except that for vector fields the isotropy of expansion is achieved either by considering a triplet of orthogonal vector fields or $...
Chapter
Generalized noncommutativity between the coordinates and momenta of typical phase spaces is suggested by string theory and theories of quantum gravity. Generalized Uncertainty Principle (GUP) in typical phase spaces is suggested by the possible existence of a minimal observable Planckian length. In this chapter, we briefly review the canonical and...
Article
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We study a purely kinetic coupled scalar-tensor gravity. We use FRW metric and obtain the modified Friedmann equations subject to an effective perfect fluid including energy density $\rho_{\phi}=\rho_{g}+\rho_{_G}$ and pressure $p_{\phi}=p_{g}+p_{_G}$, where $(\rho_{g}, p_{g})$ and $(\rho_{_G}, p_{_G})$ define the perfect fluids corresponding to th...
Article
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We study the chaotic inflation in the context of a gravity theory where the Friedman equation is modified, inspired by the polytropic gas equation of state. It is seen that in the $n=1$ case for the polytropic index the inflaton field at the end of inflation $\phi_e$, depends on the Planck mass, while for $n\neq1$ it generally depends on the polytr...
Article
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We study an special law for the deceleration parameter, recently proposed by Akarsu and Dereli, in the context of f(R), f(T) and $f(\mathcal{G})$ theories of modified gravity. This law covers the law of Berman for obtaining exact cosmological models to account for the current acceleration of the universe, and also gives the opportunity to general...
Article
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We introduce a scenario in which the breakdown of conformal symmetry is responsible for the acceleration of universe in the matter dominant era. In this regard, we consider a self interacting scalar field non-minimally coupled to the Ricci scalar and the trace of energy-momentum tensor. For a traceless energy-momentum tensor in radiation dominant e...