arXiv subjects

Shahab Shahidi

Publications and source records attributed to Shahab Shahidi.

50 records · Page 3Linked to original sources

arXiv2015

One-loop corrections to vector Galileon theory

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 non-renormalization 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.

hep-th↗

arXiv2015

Cosmology of a Lorentz violating Galileon theory

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.

gr-qc↗

arXiv2014

The scalar Einstein-aether theory

We consider an Einstein-aether type Lorentz-violating theory of gravity in which the aether vector field $V_{μ}$ is represented as the gradient of a scalar field $ϕ$, $V_{μ}=\nabla _{μ}ϕ$. A self interacting potential for the scalar aether field is considered, as well as the possibility of a coupling between the hydrodynamic matter flux and the aether field, with the imposition of the timelike nature of the aether vector. The gravitational field equations and the equation of motion of the scalar field are derived by varying the action with respect to the metric and $ϕ$. In the absence of matter flux and scalar field coupling the effective energy-momentum tensor of the scalar aether is conserved. The matter flux-aether coupling generates an extra force acting on massive test particles and consequently the motion becomes non-geodesic. The Newtonian limit of the theory is investigated and the generalized Poisson equation for weak gravitational fields is obtained. The cosmological implications of the theory is also considered and it is shown that in the framework of the Scalar Einstein-aether theory both decelerating and accelerating cosmological models can be constructed.

gr-qc↗

arXiv2014

Matter may matter

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.

gr-qc↗

arXiv2013

Further matters in space-time geometry: $f(R,T,R_{μν}T^{μν})$ gravity

We consider a gravitational model in which matter is non-minimally coupled to geometry, with the effective Lagrangian of the gravitational field being 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 field equations of the model are obtained in the metric formalism, and the equation of motion of a massive test particle is derived. In this type of models the matter energy-momentum tensor is generally not conserved, and this non-conservation determines the appearance of an extra-force acting on the particles in motion in the gravitational field. The Newtonian limit of the model is also considered, and an explicit expression for the extra-acceleration which depends on the matter density is obtained in the small velocity limit for dust particles. We also analyze in detail the so-called Dolgov-Kawasaki instability, and obtain the stability conditions of the model with respect to local perturbations. A particular class of gravitational field equations can be obtained by imposing the conservation of the energy-momentum tensor. We derive the corresponding field equations for the conservative case by using a Lagrange multiplier method, from a gravitational action that explicitly contains an independent parameter multiplying the divergence of the energy-momentum tensor. The cosmological implications of the model are investigated for both the conservative and non-conservative cases, and several classes of analytical solutions are obtained.

gr-qc↗

arXiv2013

Weyl-Cartan-Weitzenböck gravity through Lagrange multiplier

We consider an extension of the Weyl-Cartan-Weitzenböck (WCW) and teleparallel gravity, in which the Weitzenböck condition of the exact cancellation of curvature and torsion in a Weyl-Cartan geometry is inserted into the gravitational action via a Lagrange multiplier. In the standard metric formulation of the WCW model, the flatness of the space-time is removed by imposing the Weitzenböck condition in the Weyl-Cartan geometry, where the dynamical variables are the space-time metric, the Weyl vector and the torsion tensor, respectively. However, once the Weitzenböck condition is imposed on the Weyl-Cartan space-time, the metric is not dynamical, and the gravitational dynamics and evolution is completely determined by the torsion tensor. We show how to resolve this difficulty, and generalize the WCW model, by imposing the Weitzenböck condition on the action of the gravitational field through a Lagrange multiplier. The gravitational field equations are obtained from the variational principle, and they explicitly depend on the Lagrange multiplier. As a particular model we consider the case of the Riemann-Cartan space-times with zero non-metricity, which mimics the teleparallel theory of gravity. The Newtonian limit of the model is investigated, and a generalized Poisson equation is obtained, with the weak field gravitational potential explicitly depending on the Lagrange multiplier and on the Weyl vector. The cosmological implications of the theory are also studied, and three classes of exact cosmological models are considered.

gr-qc↗

arXiv2013

Curvature perturbations of quasidilaton non-linear massive gravity

We study the cosmological perturbations of the recently proposed extension of non-linear massive gravity with a scalar field. The added scalar field ensures a new symmetry on the field space of the theory. The theory has the property of having a flat dS solution, in contrast to the standard dRGT massive gravity. The tensor part is the same as that of the standard dRGT and shows gravitational waves with a time dependent mass which modifies the dispersion relation. We obtain the curvature perturbation of the model on superhorizon scales for a specific choice of $ω=0$ and find that the theory does not allow a constant curvature perturbation on the superhorizon scales and we have always a growing mode. The consistency of equations restricts the parameter space of the theory.

gr-qc↗

arXiv2012

Asymptotically Lifshitz Brane-World Black Holes

We study the gravity dual of a Lifshitz field theory in the context of a RSII brane-world scenario, taking into account the effects of the extra dimension through the contribution of the electric part of the Weyl tensor. We study the thermodynamical behavior of such asymptotically Lifshitz black holes. It is shown that the entropy imposes the critical exponent $z$ to be bounded from above. This maximum value of $z$ corresponds to a positive infinite entropy as long as the temperature is kept positive. The stability and phase transition for different spatial topologies are also discussed.

hep-th↗

arXiv2012

Virial mass in DGP cosmology

In this proposal we study the problem of the virial mass discrepancy in the context of DGP brane gravity. DGP model is a kind of brane-world model such that the corrections to the usual gravity occurred in the large distance limit relative to the distance scale of the model defined as a ratio of the brane Planck scale to the bulk Planck scale. The extra dimension of this model is noncompact. This model is composed with an Einstein-Hilbert action in 5 dimensions plus an induced 4D action guarantees the existence of gravity on the brane. The importance of this model is that it can explain the self-acceleration of our universe without use of any type of dark energy. The virial mass discrepancy is an important problem in cosmology and it can be explained by dark matter. This is due to our various ways in measurement of the mass of the galaxy clusters. One of the ways we can measure the mass of a cluster of galaxies is to measure the galaxy masses and then add them up to obtain the cluster mass. Another way is to use the virial theorem. Almost in all clusters the virial mass is bigger than 20-30 times from the observed mass. In this proposal we show that the virial mass discrepancy can be addressed by the DGP model.

gr-qc↗

arXiv2012

Weyl-Cartan-Weitzenböck gravity as a generalization of teleparallel gravity

We consider a gravitational model in a Weyl-Cartan space-time, in which the Weitzenböck condition of the vanishing of the sum of the curvature and torsion scalar is also imposed. Moreover, a kinetic term for the torsion is also included in the gravitational action. The field equations of the model are obtained from a Hilbert-Einstein type variational principle, and they lead to a complete description of the gravitational field in terms of two fields, the Weyl vector and the torsion, respectively, defined in a curved background. The cosmological applications of the model are investigated for a particular choice of the free parameters in which the torsion vector is proportional to the Weyl vector. Depending on the numerical values of the parameters of the cosmological model, a large variety of dynamic evolutions can be obtained, ranging from inflationary/accelerated expansions to non-inflationary behaviors. In particular we show that a de Sitter type late time evolution can be naturally obtained from the field equations of the model. Therefore the present model leads to the possibility of a purely geometrical description of the dark energy, in which the late time acceleration of the Universe is determined by the intrinsic geometry of the space-time.

gr-qc↗

arXiv2012

Massive cosmological scalar perturbations

We study the cosmological perturbations of the new bi-metric gravity proposed by Hassan and Rosen \cite{hasan} as a representation of massive gravity. The mass term in the model, in addition of ensuring ghost freedom for both metrics, causes the two scale factors to mix at the cosmological level and this affects the cosmological perturbation of the model. We find two combinations corresponding to the entropy and adiabatic perturbations of the theory. In this sense we show that the adiabatic perturbations could be a source for the entropy perturbations. So in addition to the adiabatic perturbations, entropy perturbations can also be present in this theory. We also show that the adiabatic perturbations are not constant at the super horizon scales, implying that the theory could not be used to describe the inflationary epoch, even if it can impose some corrections to the standard inflationary scenarios.

gr-qc↗

arXiv2012

Cosmological dynamics of brane f(R) gravity

The cosmological dynamics of a brane world scenario where the bulk action is taken as a generic function of the Ricci scalar is considered in a framework where the use of the $\mathbb{Z}_2$ symmetry and Israel junction conditions are relaxed. The corresponding cosmological solutions for some specific forms of $f(\mc{R})$ are obtained and shown to be in the form of exponential as well as power law for a vacuum brane space-time. It is shown that the existence of matter dominated epoch for a bulk action in the form of a power law for $\cal R$ can only be obtained in the presence of ordinary matter. Using phase space analysis, we show that the universe must start from an unstable matter dominated epoch and eventually falls into a stable accelerated expanding phase.

gr-qc↗

arXiv2012

Multi-Metric Gravity via Massive Gravity

A generalization to the theory of massive gravity is presented which includes three dynamical metrics. It is shown that at the linear level, the theory predicts a massless spin-2 field which is decoupled from the other two gravitons which are massive and interacting. In this regime the matter naturally should be coupled to massless graviton which introduces a preferred metric that is the average of the primary metrics. The cosmological solution of the theory shows the de-Sitter behavior with a function of mass as its cosmological constant. Surprisingly, it lacks any non-trivial solution when one of the metrics is taken to be Minkowskian and seems to enhance the predictions which suggests that there is no homogeneous, isotropic and flat solution for the standard massive cosmology.

hep-th↗

arXiv2010

Brane worlds and dark matter

Two problems related to dark matter is considered in the context of a brane world model in which the confinement of gauge fields on the brane is achieved by invoking a confining potential. First, we show that the virial mass discrepancy can be addressed if the conserved geometrical term appearing in this model is considered as an energy momentum tensor of an unknown type of matter, the so-called X-matter whose equation of state is also obtained. Second, the galaxy rotation curves are explained by assuming an anisotropic energy momentum tensor for the X-matter.

gr-qc↗