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Email: international@du.ac.ir

Damghan University

University Blvd, Damghan, IR

Shahab Shahidi

Assistant Professor of Physics

DOI: 10.1140/epjc/s10052-018-6318-7

The one-loop partition function of the f(R, Rμ νRμ ν) gravity theory is obtained around AdS 4 background. After a suitable choice of the gauge condition and computation of the ghost determinant, we obtain the one-loop partition function of the theory. The traced heat kernel over the thermal quotient of the AdS 4 space is also computed and the thermal partition function is obtained for this theory. We then consider quantum corrections to the thermodynamical quantities in some special cases. © 2018, The Author(s).

PUBLISHER: Springer New York LLC

Haghani, Z., Harko, T., Shahidi, S. The Einstein dark energy model (2018) 21, pp. 27-39.

DOI: 10.1016/j.dark.2018.05.006

In 1919 Einstein tried to solve the problem of the structure of matter by assuming that the elementary particles are held together solely by gravitational forces. In addition, Einstein also assumed the presence inside matter of electromagnetic interactions. Einstein showed that the cosmological constant can be interpreted as an integration constant, and that the energy content of the Universe should consist of 25% gravitational energy, and 75% electromagnetic energy. In the present paper we reinterpret Einstein's elementary particle theory as a vector type dark energy model, by assuming a gravitational action containing a linear combination of the Ricci scalar and the trace of the matter energy–momentum tensor, as well as a massive self-interacting vector type dark energy field, coupled with the matter current. Since in this model the matter energy–momentum tensor is not conserved, we interpret these equations from the point of view of the thermodynamics of open systems as describing matter creation from the gravitational field. In the vacuum case the model admits a de Sitter type solution. The cosmological parameters, including Hubble function, deceleration parameter, matter energy density are obtained as a function of the redshift by using analytical and numerical techniques, and for different values of the model parameters. For all considered cases the Universe experiences an accelerating expansion, ending with a de Sitter type evolution. © 2018 Elsevier B.V.

AUTHOR KEYWORDS: Cosmology; Modified gravity

PUBLISHER: Elsevier B.V.

DOI: 10.1142/S0218271818500499

The energy conditions of mimetic-f(R) gravity theory is analyzed. We will obtain the parameter space of the theory in some special forms of f(R) in which the self-acceleration is allowed. In this sense, the parameter space is obtained in a way that it violates the strong energy condition while satisfying the weak, null and dominant energy conditions. We will also consider the condition that the Dolgov-Kawasaki instability is avoided. This condition will be further imposed in the parameter space of the theory. We will show that the parameter space of the mimetic-f(R) gravity is larger than f(R) gravity theory. © 2018 World Scientific Publishing Company.

AUTHOR KEYWORDS: energy conditions; mimetic dark matter theory; Modified gravity

PUBLISHER: World Scientific Publishing Co. Pte Ltd

DOI: 10.1016/j.physletb.2017.06.040

We study the quantum corrections to the generalized Proca theory via matter loops. We consider two types of interactions, linear and nonlinear in the vector field. Calculating the one-loop correction to the vector field propagator, three- and four-point functions, we show that the non-linear interactions are harmless, although they renormalize the theory. The linear matter-vector field interactions introduce ghost degrees of freedom to the generalized Proca theory. Treating the theory as an effective theory, we calculate the energy scale up to which the theory remains healthy. © 2017 The Authors

PUBLISHER: Elsevier B.V.

PUBLISHER: Elsevier B.V.

DOI: 10.1140/epjc/s10052-017-5078-0

We consider the cosmological implications of a gravitational theory containing two vector fields coupled via a generalized Chern–Simons term. One of the vector fields is the usual Maxwell field, while the other is a constrained vector field with constant norm included in the action via a Lagrange multiplier. The theory admits a de Sitter type solution, with healthy cosmological perturbations. We also show that there are seven degrees of freedom that propagate on top of de Sitter space-time, consisting of two tensor polarizations, four degrees of freedom related to the two vector fields, and a scalar degree of freedom that makes one of the vector fields massive. We investigate the cosmological evolution of Bianchi type I space-time, by assuming that the matter content of the Universe can be described by the stiff and dust. The cosmological evolution of the Bianchi type I Universe strongly depends on the initial conditions of the physical quantities, as well as on the model parameters. The mean anisotropy parameter, and the deceleration parameter, are also studied, and we show that independently of the matter equation of state the cosmological evolution of the Bianchi type I Universe always ends in an isotropic de Sitter type phase. © 2017, The Author(s).

PUBLISHER: Springer New York LLC

DOI: 10.1140/epjc/s10052-017-4697-9

We consider a vector–tensor gravitational model with terms quadratic in the Maxwell tensor derivatives, called the Bopp–Podolsky term. The gravitational field equations of the model and the equations describing the evolution of the vector field are obtained and their Newtonian limit is investigated. The cosmological implications of a Bopp–Podolsky type dark energy term are investigated for a Bianchi type I homogeneous and anisotropic geometry for two models, corresponding to the absence and presence of the self-interacting potential of the field, respectively. The time evolutions of the Hubble function, of the matter energy density, of the shear scalar, of the mean anisotropy parameter, and of the deceleration parameter, respectively, as well as the field potentials are obtained for both cases by numerically integrating the cosmological evolution equations. In the presence of the vector type dark energy with quadratic terms in the Maxwell tensor derivatives, depending on the numerical values of the model parameters, the Bianchi type I Universe experiences a complex dynamical evolution, with the dust Universes ending in an isotropic phase. The presence of the self-interacting potential of the vector field significantly shortens the time interval necessary for the full isotropization of the Universe. © 2017, The Author(s).

PUBLISHER: Springer New York LLC

DOI: 10.1103/PhysRevD.93.124044

The effective action of the recently proposed vector Galileon theory is considered. Using the background field method, we obtain the one-loop correction to the propagator of the Proca field from vector Galileon self-interactions. Contrary to the so-called scalar Galileon interactions, the two-point function of the vector field gets renormalized at the one-loop level, indicating that there is no nonrenormalization theorem in the vector Galileon theory. Using dimensional regularization, we remove the divergences and obtain the counterterms of the theory. The finite term is analytically calculated, which modifies the propagator and the mass term and generates some new terms also. © 2016 American Physical Society.

PUBLISHER: American Physical Society

Haghani, Z., Khosravi, N., Shahidi, S. The Weyl-Cartan Gauss-Bonnet gravity (2015) 32 (21), art. no. 215016, .

DOI: 10.1088/0264-9381/32/21/215016

In this paper, we consider the generalized Gauss-Bonnet action in four-dimensional Weyl-Cartan spacetime. In this spacetime, the presence of a torsion tensor and Weyl vector implies that the generalized Gauss-Bonnet action will not be a total derivative in four-dimensional spacetime. It will be shown that the higher than two time derivatives can be removed from the action by choosing a suitable set of parameters. In the special case where only the trace part of the torsion remains, the model reduces to general relativity plus two vector fields, one of which is massless and the other is massive. We will then obtain the healthy region of the five-dimensional parameter space of the theory in some special cases. © 2015 IOP Publishing Ltd.

AUTHOR KEYWORDS: GaussBonnet action; Ostrogradski instability; Proca action; WeylCartan theory

PUBLISHER: Institute of Physics Publishing

DOI: 10.1088/1475-7516/2015/05/022

We modify the scalar Einstein-aether theory by breaking the Lorentz invariance of a gravitational theory coupled to a Galileon type scalar field. This is done by introducing a Lagrange multiplier term into the action, thus ensuring that the gradient of the scalar field is time-like, with unit norm. The theory can also be considered as an extension to the mimetic dark matter theory, by adding some derivative self interactions to the action, which keeps the equation of motion at most second order in time derivatives. The cosmological implications of the model are discussed in detail. In particular, for pressure-less baryonic matter, we show that the universe experiences a late time acceleration. The cosmological implications of a special coupling between the scalar field and the trace of the energy-momentum tensor are also explored. © 2015 IOP Publishing Ltd and Sissa Medialab srl .

AUTHOR KEYWORDS: gravity; modified gravity

PUBLISHER: Institute of Physics Publishing

DOI: 10.1142/S0218271814420164

We propose a gravitational theory in which the effective Lagrangian of the gravitational field is given by an arbitrary function of the Ricci scalar, the trace of the matter energy-momentum tensor, and the contraction of the Ricci tensor with the matter energy-momentum tensor. The matter energy-momentum tensor is generally not conserved, thus leading to the appearance of an extra-force, acting on massive particles in a gravitational field. The stability conditions of the theory with respect to local perturbations are also obtained. The cosmological implications of the theory are investigated, representing an exponential solution. Hence, a Ricci tensor-energy-momentum tensor coupling may explain the recent acceleration of the universe, without resorting to the mysterious dark energy. © 2014 World Scientific Publishing Company.

AUTHOR KEYWORDS: cosmology; extra force; Modified gravity

PUBLISHER: World Scientific Publishing Co. Pte Ltd