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Hossein Farajollahi

Publications and source records attributed to Hossein Farajollahi.

17 recordsLinked to original sources
arXiv2018

Polytropic Inspired Inflation on the Brane

The brane inflationary model inspired by a polytropic inflationary idea is studied. In the slow-roll approximation and high energy limit, for a chaotic potential, the model is developed, and its characteristics are discussed. We obtain explicit expressions for the scalar power spectrum, the tensor-scalar ratio, the scalar spectral index and its running in terms of the polytropic parameters. We find a new constraint on the energy scale of the inflation and the brane tension using the WMAP9 data.

gr-qc↗
arXiv2017

Normal DGP in varying speed of light cosmology

The varying speed of light (VSL) has been used in cosmological models in which the physical constants vary over time. On the other hand, the Dvali, Gabadadze and Porrati (DGP) brane world model, especially its normal branch has been extensively discussed to justify the current cosmic acceleration. In this article we show that the normal branch of DGP in VSL cosmology leads to a self-accelerating behavior and therefore can interpret cosmic acceleration. Applying statefinder diagnostics demonstrate that our result slightly deviates ΛCDM model.

gr-qc↗
arXiv2015

Phase plane analysis and statefinder diagnostic of agegraphic dark energy in 5D Brans-Dicke cosmology

We present an autonomous phase-plane describing the evolution of field equations containing an agegraphic dark energy in 5D Brans- Dicke cosmology. To observationally verify the numerical results, we simultaneously solve the equations by constraining the model parameters with SNe Ia data. We find conditions for the existence and stability of the critical points (states of the universe) and numerically examine the cosmological parameters. We also investigate the model by means of statefinder diagnostic.

gr-qc↗
arXiv2011

Dynamics of the self-interacting chameleon cosmology

In this article we study the properties of the flat FRW chameleon cosmology in which the cosmic expansion of the Universe is affected by the chameleon field and dark energy. In particular, we perform a detailed examination of the model in the light of numerical analysis. The results illustrate that the interacting chameleon filed plays an important role in late time universe acceleration and phantom crossing.

physics.gen-ph↗
arXiv2011

Constraints on scalar-tensor theories from observations

We study the dynamical description of scalar-tensor gravity by performing the best-fit analysis for two cases of exponential and power-law form of the potential and scalar field function coupled to the curvature. The models are then tested against observational data. The results show that in both scenarios the Universe undergoes an acceleration expansion period and the geometrical equivalent of dark energy is associated with a time-dependent equation of state.

gr-qc↗
arXiv2011

Stability analysis and observational constraints in scalar tensor theory

We study FRW cosmology for scalar tensor theory where two scalar functions nonminimally coupled to the geometry and matter Lagrangian. In a framework to study stability and attractor solutions of the model in the phase space, we simultaneously solve the dynamical system and best fit the stability and model parameters with the observational data. The approach imposes restrictions on the model constraints while providing information about the universe dynamics. The model predict current accelerating universe, with a phantom crossing in near future.

gr-qc↗
arXiv2011

Geometric and thermodynamic properties in Gauss-Bonnet gravity

In this paper, the generalized second law (GSL) of thermodynamics and entropy is revisited in the context of cosmological models in Gauss-Bonnet gravity with the boundary of the universe is assumed to be enclosed by the dynamical apparent horizon. The model is best fitted with the observational data for distance modulus. The best fitted geometric and thermodynamic parameters such as equation of state parameter, deceleration parameter and entropy are derived. To link between thermodynamic and geometric parameters, the "entropy rate of change multiplied by the temperature" as a model independent thermodynamic state parameter is also derived. The results show that the model is in good agreement with the observational analysis.

gr-qc↗
arXiv2011

Dynamics of the universe as a "test 3-brane" in a 5D bulk

In this paper we study the dynamics of a 5D bulk space-time with our universe as a "test 3-brane" located in the bulk, an idealized model of a topological object whose physical properties is negligible in comparison with that of the bulk. Our universe experiences acceleration and its equation of state parameter crosses the phantom divide line due to the geometry of the bulk space-time.

gr-qc↗
arXiv2011

Time and Observables in Unimodular General Relativity

A cosmological time variable is emerged from the hamiltonian formulation of unimodular theory of gravity to measure the evolution of dynamical observables in the theory. A set of constants of motion has been identified for the theory on the null hypersurfaces that its evolution is with respect to the volume clock introduced by the cosmological time variable.

gr-qc↗
arXiv2011

On Dynamics of Brans--Dicke Theory of Gravitation

We study longstanding problem of cosmological clock in the context of Brans-Dicke theory of gravitation. We present the Hamiltonian formulation of the theory for a class of spatially homogenous cosmological models. Then, we show that formulation of the Brans-Dicke theory in the Einstein frame allows how an identification of an appropriate cosmological time variable, as a function of the scalar field in the theory, can be emerged in quantum cosmology. The classical and quantum results are applied to the Friedmann-Robertson-Walker cosmological models.

gr-qc↗
arXiv2011

Attractors, Statefinders and Observational Measurement for Chameleonic Brans--Dicke Cosmology

We investigate chameleonic Brans--Dicke model applied to the FRW universes. A framework to study stability and attractor solutions in the phase space is developed for the model. We show that depending on the matter field and stability conditions, it is possible to realize phantom-like behavior without introducing phantom filed in the model while the stability is fulfilled and phantom crossing occurs. The statefinder parameters to the model for different kinds of matter interacting with the chameleon scalar field are studied. We also compare our model with present day observations.

gr-qc↗
arXiv2010

On the Dynamics of Bianchi IX cosmological models

A cosmological description of the universe is proposed in the context of Hamiltonian formulation of a Bianchi IX cosmology minimally coupled to a massless scalar field. The classical and quantum results are studied with special attention to the case of closed Friedmann-Robertson-Walker model.

gr-qc↗
arXiv2008

Stochastic quantisation of locally supersymmetric models

Stochastic quantisation normally involves the introduction of a fictitious extra time parameter, which is taken to infinity so that the system evolves to an equilibrium state.In the case of a locally supersymmetric theory, an interesting new possibility arises due to the existence of a Nicolai map. In this case it turns out that no additional time parameter is required, as the existence of the Nicolai map ensures that the same job can be done by the existing time parameter after Euclideanisation. This provides the quantum theory with a natural probabilistic interpretation, without any reference to the concept of an inner product or a Hilbert space structure.

gr-qc↗
arXiv2008

Time and Dirac Observables in Friedmann Cosmologies

A cosmological time variable is emerged from the Hamiltonian formulation of Friedmann model to measure the evolution of dynamical observables in the theory. A set of observables has been identified for the theory on the null hypersurfaces that its evolution is with respect to the volume clock introduced by the cosmological time variable.

gr-qc↗
arXiv2005

World-line observables and clocks in General Relativity

A proposal for the issue of time and observables in any parameterized theory such as general relativity is addressed. Introduction of a gauge potential 3-form A in the theory of relativity enables us to define a gauge-invariant quantity which can be used by observers as a clock to measure the passage of time. This dynamical variable increases monotonically and continuously along a world line. Then we define world line observables to be any covariantly defined quantity obtained from the field configurations on any such causal past with dynamical time T.

gr-qc↗
arXiv2001

Dirac observables and the phase space of general relativity

In the canonical approach to general relativity it is customary to parametrize the phase space by initial data on spacelike hypersurfaces. However, if one seeks a theory dealing with observations that can be made by a single localized observer, it is natural to use a different description of the phase space. This results in a different set of Dirac observables from that appearing in the conventional formulation. It also suggests a possible solution to the problem of time, which has been one of the obstacles to the development of a satisfactory quantum theory of gravity.

gr-qc↗