Searcharxiv⌕ Search

arXiv subjects

Davood Momeni

Publications and source records attributed to Davood Momeni.

89 records · Page 5Linked to original sources
arXiv2014

Observational constraints on non-minimally coupled Galileon model

As an extension of Dvali-Gabadadze-Porrati (DGP) model, the Galileon theory has been proposed to explain the "self-accelerating problem" and "ghost instability problem". In this Paper, we extend the Galileon theory by considering a non-minimally coupled Galileon scalar with gravity. The statefinder analysis, $Om(z)$ diagnostic and constraint the model parameters have been investigate , we find $Ω_\text{m0}=0.263_{-0.031}^{+0.031}$, $n=1.53_{-0.37}^{+0.21}$ (at the $95\%$ confidence level) with $χ^2_{\text{min}}=473.376$. Further we show that due to the SNe Ia + BAO data ,our model behaves like a phantom-like dark energy.

physics.gen-ph↗
arXiv2014

Pilgrim Dark Energy in $f(T, T_G)$ cosmology

We work on the reconstruction scenario of pilgrim dark energy" (PDE) in $f(T, T_G)$. In PDE model it is assumed that a repulsive force that is accelerating the Universe is phantom type with $(w_{DE}<-1)$ and it so strong that prevents formation of the black hole. We construct the $f(T, T_G)$ models and correspondingly evaluate equation of state parameter for various choices of scale factor. Also, we assume polynomial form of $f(T, T_G)$ in terms of cosmic time and reconstruct $H$ and $w_{DE}$ in this manner. Through discussion, it is concluded that PDE shows aggressive phantom-like behaviour for$s=-2$ in $f(T, T_G)$ gravity.

gr-qc↗
arXiv2014

Conformal Invariant Teleparallel Cosmology

We study teleparallel gravitational theories with are invariant under the conformal transformations. Wide family of the gravitational Lagrangians that are invariant under conformal transformations have investigated. Cosmological solutions inspired by the observational data for a flat Universe in the vacuum has been found. To add matter fields to the cosmological models by preserving the conformal symmetry there are significant limitations. Friedmann-Robertson-Walker (FRW) equations based on the effective terms of energy density and pressure have been rewriting and continuity equation for the effective quantities are derived. In the vacuum and in the absence of any matter field,the FRW equation has an exact solution for Hubble parameter which is consistent with the cosmological data,specially given analytical solution is in good agreement with $Λ$CDM model in the present time. Then the scalar field-Torsion models in the cosmological FRW background investigated. Massless scalar field equations are very complex with an exact analytical solution in special limits. We have shown that the Lagrangian scalar field with self interaction $V(ϕ)=\frac{1}{4!}μϕ^4$ can be written that the conformal symmetry is preserved.

gr-qc↗
arXiv2014

Cosmological reconstruction of $f(T,\mathcal{T})$ Gravity

Motivated by the newly proposal for gravity as the effect of the torsion scalar $T$ and trace of the energy momentum tensor $\mathcal{T}$,we investigate the cosmological reconstruction of different models of the Universe. Our aim here is to show that how this modified gravity model, $f(T,\mathcal{T})$ is able to reproduce different epoches of the cosmological history. We explicitly show that $f(T,\mathcal{T})$ can be reconstructed for $Λ$CDM as the most popular and consistent model. Also we study the mathematical reconstruction of $f(T,\mathcal{T})$ for a flat cosmological background filled by two fluids mixture. Such model describes phantom-non-phantom era as well as the purely phantom cosmology. We extend our investigation to more cosmological models like perfect fluid,Chaplygin gas and massless scalar field. In each case we obtain some specific forms of $f(T,\mathcal{T})$. These families of $f(T,\mathcal{T})$ contain arbitrary function of torsion and trace of the energy momentum.

gr-qc↗
arXiv2013

Noether gauge symmetry approach in quintom cosmology

In literature usual point like symmetries of the Lagrangian have been introduced to study the symmetries and the structure of the fields. This kind of Noether symmetry is a subclass of a more general family of symmetries, called Noether Gauge Symmetries (NGS). Motivated by this mathematical tool, in this article, we discuss the generalized Noether symmetry of Quintom model of dark energy, which is a two component fluid model of quintessence and phantom fields. Our model is a generalization of the Noether symmetries of a single and multiple components which have been investigated in detail before. We found the general form of the quintom potential in which the whole dynamical system has a point like symmetry. We investigated different possible solutions of the system for diverse family of gauge function. Specially, we discovered two family of potentials, one corresponds to a free quintessence (phantom) and the second is in the form of quadratic interaction between two components. These two families of potential functions are proposed from the symmetry point of view, but in the quintom models they are used as phenomenological models without clear mathematical justification. From integrability point of view, we found two forms of the scale factor: one is power law and second is de-Sitter. Some cosmological implications of the solutions have been investigated

astro-ph.CO↗
arXiv2013

Geometrothermodynamics of Myers-Perry black holes

We consider the thermodynamics and Geometrothermodynamics of the Myers-Perry black holes in five dimensions for three different cases, depending on the values of the angular momenta. We follow Davies approach to study the thermodynamics of black holes and find a non-trivial thermodynamic structure in all cases, which is fully reproduced by the analysis performed with the techniques of Geometrothermodynamics. Moreover, we observe that in the cases when only one angular momentum is present or the two angular momenta are fixed to be equal, i.e. when the thermodynamic system is two dimensional, there is a complete agreement between the divergences of the generalized susceptibilities and the singularities of the equilibrium manifold, whereas when the two angular momenta are fully independent, that is, when the thermodynamic system is three dimensional, additional singularities in the curvature appear. However, we prove that such singularities are due to the changing from a stable phase to an unstable one.

hep-th↗
arXiv2013

Reconstruction of f(T) and f(R) gravity according to (m,n)-type holographic dark energy

Motivated by earlier works on reconstruction of modified gravity models with dark energy components, we extend them by considering a newly proposed model of (m, n)- type of holographic dark energy for two models of modified gravity, f(R) and f(T) theories, where R and T represent Ricci scalar and torsion scalar respectively. Specifically we reconstruct the two later gravity models and discuss their viability and cosmography. The obtained gravity models are ghost free, compatible with local solar system tests and describe effective positive gravitational constant.

astro-ph.CO↗
arXiv2013

Geometrothermodynamics of higher dimensional black holes

We study the thermodynamics and geometrothermodynamics of different black hole configurations in more than four spacetime dimensions. We find the conditions under which second order phase transitions occur in higher-dimensional static Reissner-Nordström and stationary Kerr black holes. Our results indicate that the equilibrium manifold of all these black hole configurations is in general curved and that curvature singularities appear exactly at those places where second order phase transitions occur.

gr-qc↗
arXiv2013

Analytical Holographic Superconductor with Backreaction Using AdS3/CFT2

The holographic model for a two-dimensional superconductor has been investigated by considering the three-dimensional gravity in the bulk. To find the critical temperature, we used the Sturm-Liouville variational method. Where as the same method is applied for calculating the condensation of the dual operators on the boundary. We included the back reactions on the metric by a combination of the perturbation method of the fields with respect to the small parameter and then applying the variational integrals on the resulting equations of the motion. The critical temperature has been successfully obtained on the backreaction effects, and we showed that it dropped with a rise in the backreaction of the fields, and it makes the condensation harder. We can use our analytical results to support the numerical data which was reported previously.

physics.gen-ph↗
arXiv2013

More on Superconductors via Gauge/Gravity Duality with Nonlinear Maxwell Field

We have developed the recent investigations on the second-order phase transition in the holographic superconductor using the probe limit for a nonlinear Maxwell field strength coupled to a massless scalar field. By analytic methods, based on the variational Sturm- Liouville minimization technique, we study the effects of the space-time dimension and the non-linearity parameter on the critical temperature and the scalar condensation of the dual operators on the boundary. Further, as a motivated result, we analytically deduce the DC conductivity in the low and zero temperatures regime. Especially in the zero temperature limit and in two dimensional toy model, we thoroughly compute the conductivity analytically. Our work clarifies more features of the holographic superconductors both in different space dimensions and on the effect of the non-linearity in Maxwell's strength field.

physics.gen-ph↗
arXiv2012

Wormholes in a viable f(T) gravity

In this paper, we derive some new exact solutions of static wormholes in f(T) gravity. We discuss independent cases of the pressure components including isotropic and anisotropic pressure. Lastly we consider radial pressure satisfying a barotropic equation of state. We also check the behavior of null energy condition (NEC) for each case and observe that it is violated for the anisotropic case, while it is satisfied for isotropic and barotropic cases.

gr-qc↗
arXiv2012

Noether Gauge Symmetry of Modified Teleparallel Gravity Minimally Coupled with a Canonical Scalar Field

This paper is devoted to the study of Noether gauge symmetries of $f(T)$ gravity minimally coupled with a canonical scalar field. We explicitly determine the unknown functions of the theory $f(T),V(ϕ), W(ϕ)$. We have shown that there are two invariants for this model, one of which defines the Hamiltonian $H$ under time invariance (energy conservation) and the other is related to scaling invariance. We show that the equation of state parameter in the present model can cross the cosmological constant boundary. The behavior of Hubble parameter in our model closely matches to that of $Λ$CDM model, thus our model is an alternative to the later.

astro-ph.CO↗
arXiv2012

Statefinder Analysis of f(T) Cosmology

In this paper, we intend to evaluate and analyze the statefinder parameters in $f(T)$ cosmology. Friedmann equation in $f(T)$ model is taken, and the statefinder parameters ${r,s}$ are calculated. We consider a model of $f(T)$ which contains a constant, linear and non-linear form of torsion. We plot $r$ and $s$ in order to characterize this model in the ${r,s}$ plane. We found that our model $f(T)=2C_1 \sqrt{-T} +αT+C_2,$ predicts the decay of dark energy in the far future while its special case namely teleparallel gravity predicts that dark energy will overcome over all the energy content of the Universe.

physics.gen-ph↗
arXiv2012

Phase space analysis of interacting dark energy in f(T) cosmology

In this paper, we examine the interacting dark energy model in $f(T)$ cosmology. We assume dark energy as a perfect fluid and choose a specific cosmologically viable form $f(T)= β\sqrt{T} $. We show that there is one attractor solution to the dynamical equation of $f(T)$ Friedmann equations. Further we investigate the stability in phase space for a general $f(T)$ model with two interacting fluids. By studying the local stability near the critical points, we show that the critical points lie on the sheet $u^*=(c-1)v^*$ in the phase space, spanned by coordinates $(u,v,Ω,T)$. From this critical sheet, we conclude that the coupling between the dark energy and matter $c\in (-2,0)$.

gr-qc↗
arXiv2011

Holographic superconductors in a model of non-relativistic gravity

We have studied holographic superconductors with spherical symmetry in the Ho$\check{\textbf{r}}$ava-Lifshitz gravity by using a semi analytical method, and also we have calculated the critical temperature and shown when the condensation will appear in a similar pattern as in the Einstein-Gauss- Bonnet gravity. We have computed the dependency of the conductivity as a function of frequency in this new non-relativistic model of quantum gravity.

hep-th↗
arXiv2011

Investigation of particle and molecular extinction effects in remote sensing by ultraviolet DIAL in the lower atmosphere

This study presents theoretical investigation of the effects of particle and molecular extinction in horizontal remote sensing near the ground for several visibilities at UV wavelengths by neglecting the spatial inhomogeneity of aerosol in the atmosphere and taking into account the dependence of refracting on air temperature and pressure. Due to weak attenuation of oxygen and other gaseous atmospheric constituents in this region, we have only considered the effect of ozone in calculation. The results are important to estimate systematic errors in measuring gas concentration introduced by large wavelength separation in UV-DIAL. The total attenuation (km-1) at wavelengths is listed in the form of a table from 200 to 400 nm for several values of visibilities. It is found the aerosol attenuation in UV region varies quite smoothly with wavelength and therefore systematic error caused by aerosol scattering is negligible in remote sensing by UV-DIAL even with large wavelength separation. Moreover, it has been found that only aerosol extinction is dominant in lidar remote sensing in the lower atmosphere in UV region. In large altitude that aerosol concentration is lower; the molecular scattering is important especially for wavelengths larger than 310 nm.

physics.ao-ph↗