arXiv subjects

Shahab Shahidi

Publications and source records attributed to Shahab Shahidi.

At least 37 records · Page 2Linked to original sources

arXiv2021

Cosmological implications of Rastall-$f(R)$ theory

Cosmological implications of a generalized Rastall theory where the non-conservation equation is promoted to accepts arbitrary functions of the Ricci scalar is considered. We have obtain the model parameters of the power-law ansatz $f\propto R^η$, using $H_0$ and $fσ_8$ data sets and show that the generalized Rastall theory could satisfy observational data. The dynamical analysis of the model shows that for $η\in(0,1)$, the dust dominated fixed point is lost and as a result the parameter range of $η$ should be restricted to $η\geq1$. We will also show that the genralized Rastall theory will predict more accelerating universe with larger radius, compared to the $Λ$CDM model.

gr-qc↗

arXiv2021

Non-minimal Energy-momentum squared gravity

We consider a gravitational theory with an additional non-minimal coupling between baryonic matter fields and geometry. The coupling is second order in the energy momentum tensor and can be seen as a generalization of the energy-momentum squared gravity model. We will add a constraint through a Lagrange multiplier to ensure the conservation of the energy-momentum tensor. Background cosmological implications together with its dynamical system analysis will be investigated in details. Also we will consider the growth of matter perturbation at first order, and estimate the model parameter from observations on $H$ and also $fσ_8$. We will show that the model parameter should be small and positive in 2$σ$ confidence interval. The theory is shown to be in a good agreement with observational data.

gr-qc↗

arXiv2021

Geodesic deviation, Raychaudhuri equation, Newtonian limit, and tidal forces in Weyl-type $f(Q,T)$ gravity

We consider the geodesic deviation equation, describing the relative accelerations of nearby particles, and the Raychaudhuri equation, giving the evolution of the kinematical quantities associated with deformations (expansion, shear and rotation) in the Weyl type $f(Q,T)$ gravity, in which the nonmetricity $Q$ is represented in the standard Weyl form, fully determined by the Weyl vector, while $T$ represents the trace of the matter energy-momentum tensor. The effects of the Weyl geometry and of the extra force induced by the nonmetricity-matter coupling are explicitly taken into account. The Newtonian limit of the theory is investigated, and the generalized Poisson equation, containing correction terms coming from the Weyl geometry, and from the geometry matter coupling, is derived. As a physical application of the geodesic deviation equation the modifications of the tidal forces, due to the nonmetricity-matter coupling, are obtained in the weak field approximation. The tidal motion of test particles is directly influenced by the gradients of the extra force, and of the Weyl vector. As a concrete astrophysical example we obtain the expression of the Roche limit (the orbital distance at which a satellite begins to be tidally torn apart by the body it orbits) in the Weyl type $f(Q,T)$ gravity.

gr-qc↗

arXiv2020

Black hole solutions in modified gravity induced by quantum metric fluctuations

The inclusion of the quantum fluctuations of the metric in the geometric action is a promising avenue for the understanding of the quantum properties of gravity. In this approach the metric is decomposed in the sum of a classical and of a fluctuating part, of quantum origin, which can be generally expressed in terms of an arbitrary second order tensor, constructed from the metric, and from the thermodynamic quantities describing matter fields. In the present paper we investigate the effects of the quantum fluctuations on the spherically symmetric static black hole solutions of the modified field equations, obtained from a variational principle, by assuming that the quantum correction tensor is given by the coupling of a scalar field to the metric tensor. After reformulating the field equations in a dimensionless form, and by introducing a suitable independent radial coordinate, we obtain their solutions numerically. We detect the formation of a black hole from the presence of a singularity in the metric tensor components. Several models, corresponding to different functional forms of the scalar field potential, are considered. The thermodynamic properties of the black hole solutions (horizon temperature, specific heat, entropy and evaporation time due to Hawking luminosity) are also investigated.

gr-qc↗

arXiv2020

Weyl type $f(Q,T)$ gravity, and its cosmological implications

We consider an $f(Q,T)$ type gravity model in which the scalar non-metricity $Q_{αμν}$ of the space-time is expressed in its standard Weyl form, and it is fully determined by a vector field $w_μ$. The field equations of the theory are obtained under the assumption of the vanishing of the total scalar curvature, a condition which is added into the gravitational action via a Lagrange multiplier. The gravitational field equations are obtained from a variational principle, and they explicitly depend on the scalar nonmetricity and on the Lagrange multiplier. The covariant divergence of the matter energy-momentum tensor is also determined, and it follows that the nonmetricity-matter coupling leads to the nonconservation of the energy and momentum. The energy and momentum balance equations are explicitly calculated, and the expressions of the energy source term and of the extra force are found. We investigate the cosmological implications of the theory, and we obtain the cosmological evolution equations for a flat, homogeneous and isotropic geometry, which generalize the Friedmann equations of standard general relativity. We consider several cosmological models by imposing some simple functional forms of the function $f(Q,T)$, and we compare the predictions of the theory with the standard $Λ$CDM model.

gr-qc↗

arXiv2020

Distinguishing Brans-Dicke-Kerr type naked singularities and black holes with their thin disk electromagnetic radiation properties

The possible existence of naked singularities, hypothetical astrophysical objects, characterized by a gravitational singularity without an event horizon is still an open problem in present day astrophysics. From an observational point of view distinguishing between astrophysical black holes and naked singularities also represents a major challenge. One possible way of differentiating naked singularities from black holes is through the comparative study of thin accretion disks properties around these different types of compact objects. In the present paper we continue the comparative investigation of accretion disk properties around axially-symmetric rotating geometries in Brans-Dicke theory in the presence of a massless scalar field. The solution of the field equations contains the Kerr metric as a particular case, and, depending on the numerical values of the model parameter $γ$, has also solutions corresponding to non-trivial black holes and naked singularities, respectively. Due to the differences in the exterior geometries between black holes and Brans-Dicke-Kerr naked singularities, the thermodynamic and electromagnetic properties of the disks (energy flux, temperature distribution and equilibrium radiation spectrum) are different for these two classes of compact objects, consequently giving clear observational signatures that could discriminate between black holes and naked singularities.

gr-qc↗

arXiv2019

Cosmology in theories with derivative matter coupling

A new class of modified gravity theories with a healthy higher order derivative terms of a function of the matter Lagrangian $f(L_m)$ is considered. Generally the energy momentum tensor is not conserved, leading to the fifth force similar to $f(R,T)$ theories. We will however show that in the FRW background there exists two possibilities corresponding to conservative and non-conservative cases. Cosmological implications of both cases with different functions of the matter Lagrangian $f$ will be investigated in details and we will show that current observational data can be satisfied in this model. The non-conservative case however, predicts less matter sources at early times and more deceleration. Evolution of the matter density perturbation in the longitudinal gauge is also considered for dust matter sources. We will show that the late time behavior of the matter density perturbation is in agreement with current observational data at sub-horizon limit.

gr-qc↗

arXiv2019

Cosmology of a higher derivative scalar theory with non-minimal Maxwell coupling

Higher derivative scalar field theory in curved space-time belongs to the GLPV theory coupled non-minimally to the Maxwell field is considered. We will show that the theory admits two independent exact de Sitter solutions in the FRW background, one driven by the cosmological constant and the other by the GLPV scalar field. The dynamical system analysis of the theory shows that these two exact solutions are stable fixed points. Also, cosmological perturbations over these solutions shows that the cosmological constant based solution is healthy at linear level but the GLPV based solution suffers from a gradient instability in the scalar sector. This proves that the cosmological constant is needed in the GLPV-Maxwell system in order to have a healthy de Sitter solution.

gr-qc↗

arXiv2018

Cosmic acceleration via derivative matter couplings

A gravitational theory with derivative matter coupling is proposed which adopt de Sitter expansion at late times with ordinary baryonic matter. Matter components are conserved separately in the cosmological background and the Newtonian limit is well behaved. Also Conformal transformation of the theory to Jordan frame showing an effective cosmological constant explaining the nature of de Sitter acceleration.

gr-qc↗

arXiv2018

Reply to "Comment on 'Can accretion disk properties observationally distinguish black holes from naked singularities?'"

In the Comment on "Can accretion disk properties observationally distinguish black holes from naked singularities?", by Bertrand Chauvineau, Phys. Rev. D {\bf 98}, 088501 (2018), the author did show that the metric used in Z. Kovács and T. Harko, Phys. Rev. D {\bf 82}, 124047 (2010), and initially introduced in K. D. Krori and D. R. Bhattacharjee, J. Math. Phys. \textbf{23}, 637 (1982) and K. K. Nandi, P. M. Alsing, J. C. Evans, and T. B. Nayak, Phys. Rev. D \textbf{63}, 084027 (2001), does not satisfy the Einstein gravitational field equations with a minimally coupled scalar field. In our reply we would like to point out that this result is actually not new, but it was already published in the literature. Moreover, a rotating solution that generalizes the Kerr metric for a nonminimally coupled scalar field does exist. We briefly discuss the nature of the singularities for the generalized metric, and point out that it can be used as a testing ground to differentiate black holes from naked singularities. We also mention the existence of some other typing or technical errors existing in the literature.

gr-qc↗

arXiv2018

Axionic extension of the Proca action

In the context of Cartan theory, we will show that the Proca action can be obtained from the Gauss-Bonnet action for a special choice of the torsion tensor. This in fact equivalent to the special case of the 4th vector Galileon Lagrangian. The theory will then be promoted to contain an axion field. It will be proved that the model admits de Sitter expanding phase with healthy tensor and vector fluctuations. The scalar sector has 4 degrees of freedom, but only one of them remains dynamical in the limit $k\rightarrow\infty$. We will analyze the scalar fluctuations in the small scales limit and obtain the parameter space of the theory in which all the perturbations remain healthy.

gr-qc↗

arXiv2018

Modified gravity one-loop partition function

The one-loop partition function of the $f(R,R_{μν}R^{μν})$ gravity theory is obtained around AdS$_4$ background. After 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 AdS$_4$ space is also computed and the thermal partition function is obtained for this theory. We have then consider quantum corrections to the thermodynamical quantities in some special cases.

gr-qc↗

arXiv2018

The Einstein dark energy model

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.

gr-qc↗

arXiv2017

Energy conditions in mimetic-$f(R)$ gravity

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.

gr-qc↗

arXiv2017

The Maxwell-Chern-Simons gravity and its cosmological implications

We consider the cosmological implications of a gravitational theory containing two vector fields coupled minimally to gravity as well as a generalized Chern-Simons term that couples the two vector fields. 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 will show that there is 6 degrees of freedom propagate on top of de Sitter space-time, two tensor polarizations and four degrees of freedom related to two massless vector fields interacting with each other via Chern-Simons interaction term. We also investigate in detail the behavior of the geometric and physical parameters of a homogeneous and anisotropic Bianchi type I Universe, by using both analytical and numerical methods, by assuming that the matter content of the Universe can be described by the stiff causal and pressureless dust fluid equations of state. The time evolution of the Bianchi type I Universe strongly depends on the initial conditions of the physical and geometrical quantities, as well as on the numerical values of the model parameters. Two important observational parameters, the mean anisotropy parameter, and the deceleration parameter, are also studied in detail, 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 and exponentially accelerating, de Sitter type, phase.

gr-qc↗

arXiv2017

Vector dark energy models with quadratic terms in the Maxwell tensor derivatives

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.

gr-qc↗

arXiv2016

Quantum corrections to the generalized Proca theory via a matter field

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.

hep-th↗

arXiv2016

The Weyl-Cartan Gauss-Bonnet gravity

In this paper, we consider the generalized Gauss-Bonnet action in $4$-dimensional Weyl-Cartan space-time. In this space-time, the presence of torsion tensor and Weyl vector implies that the generalized Gauss-Bonnet action will not be a total derivative in four dimension space-time. It will be shown that the higher than two time derivatives can be removed from the action by choosing suitable set of parameters. In the special case where only the trace part of the torsion remains, the model reduces to GR plus two vector fields. One of which is massless and the other is massive. We will then obtain the healthy region of the 5-dimensional parameter space of the theory in some special cases.

gr-qc↗