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D. Momeni

Publications and source records attributed to D. Momeni.

65 records · Page 4Linked to original sources
arXiv2010

Cosmic strings in a model of non-relativistic gravity

Ho$\check{\textbf{r}}$ava proposed a non-relativistic renormalizable theory of gravitation, which is reduced to general relativity (GR) in large distances (infra-red regime (IR)). It is believed that this theory is an ultra-violet (UV) completion for the classical theory of gravitation. In this paper, after a brief review of some fundamental features of this theory, we investigate it for a static cylindrical symmetric solution which describes \emph{Cosmic string} as a special case. We have also investigated some possible solutions, and have seen that how the classical GR field equations are modified for generic potential $V (g)$. In one case there is an algebraic constraint on the values of three coupling constants. Finally as a pioneering work we deduce the most general \emph{cosmic string} in this theory. We explicitly show that how the \emph{coupling constants} distort the mass parameter of \emph{cosmic string}. We deduce an explicit function for mass per unit length of the space-time as a function of the \emph{coupling constants}. We compare this function with another which Aryal et al [58] have found in GR.Also we calculate the self-force on a massive particle near Ho$\check{\textbf{r}}$ava-Lifshitz straight string and we give a typical order for the \emph{coupling constants} $g_{9}$. This order of magnitude proposes a cosmological test for validity of this theory.

gr-qc↗
arXiv2010

Ferromagnetism of electron gas

In current work, we investigate the density and temperature dependence of polarization parameter; using the relativistic formalism for the electron-electron interaction within the Fermi liquid model. we calculate the spin dependent scattering matrix elements in relativistic region, and then obtain the non-relativistic behavior to study the magnetic properties of an electron gas. By varying the polarization parameter, we minimized the free energy and then obtain the polarization of the system as a function of density and temperature. At zero temperature the exact results for polarization and magnetic susceptibility have obtained. It has been shown that for a given temperature (density) there is critical density (temperature) that the ferromagnetic phase can appears in electron gas. Our results show that at nonzero temperatures and in very low and very high densities the ferromagnetism phase can not be exist.

cond-mat.stat-mech↗
arXiv2010

Plane symmetric solutions in Horava-Lifshitz theory

The purpose of this paper is to find and analyze plane symmetric, static(non static) solutions in Hořava- Lifshitz gravity. We discussed two versions of Horava gravity. First we showed that if the detailed balance principle have considered, there are both static and non-static solutions. We show that in static case there are two family of solvable models which either of them has a well defined EOS, in analogous to the perfect fluid solutions in GR. In non-static case we find a family of solutions. Some physical properties of these solutions was discussed. Secondly we investigated the plane symmetric solutions for a new modified version of Hořavaa gravity \cite{bla}, which has the new terms inserted action in it.

hep-th↗
arXiv2010

Some notes about the density of states for a negative pressure matter

The main goal of this paper is deriving Density of states $g(ε)$ (degeneracy function) per volume for an equation of state (EOS) $p=-ρ$ (we called it dark energy(DE)).We have concluded that thermodynamic quantities such as pressure and energy density are simple functions of temperature, fugacity, curvature and mass of Bosons. Our work has been expressed the origin of some claims about the negativity of the entropy for the scalar fields models of DE.

gr-qc↗
arXiv2010

A note on constant curvature solutions in cylindrically symmetric metric $f(R)$ Gravity

In the previous work we introduced a new static cylindrically symmetric vacuum solutions in Weyl coordinates in the context of the metric f(R) theories of gravity\cite{1}. Now we obtain a 2-parameter family of exact solutions which contains cosmological constant and a new parameter as $β$. This solution corresponds to a constant Ricci scalar. We proved that in $f(R)$ gravity, the constant curvature solution in cylindrically symmetric cases is only one member of the most generalized Tian family in GR. We show that our constant curvature exact solution is applicable to the exterior of a string. Sensibility of stability under initial conditions is discussed.

gr-qc↗
arXiv2010

Spherically symmetric massive scalar fields in GR

First we review some of the attempts made to find exact spherically symmetric solutions of Einstein field equations in the presence of scalar fields .Wyman solution in both static and non static scalar field is discussed briefly and it is show that why in the case of non static homogenous matter field, static metric can not be represented in terms of elementary functions. We mention here that if our spacetime be static, according to EFE there is two option for choose scalar field matter: static (time independent) and non static (time dependent). All these solutions are limited to the minimally coupled massless scalar fields and also in the absence of the cosmological constant . Then we show that if we are interesting to have a homogenous isotropic scalar field matter one can construct a series solution in terms of scalar field's mass and cosmological constant. This metric is static and posses a locally flat case as a special chooses of mass of scalar field and can be interpreted as an effective vacuum. Therefore mass of scalar field eliminates any locally gravitational effect as tidal forces. Finally we describe why this system is unstable in the language of dynamical systems.

gr-qc↗
arXiv2010

Spacing of the entropy spectrum for KS Black hole in Horava-Lifshitz gravity

In this paper we present the spectrum of entropy/area for Kehagias-Sfetsos (KS) black hole in Ho$\check{\textbf{r}}$ava-Lifshitz (HL)gravity via quasi-normal modes (QNM) approach. We show that in the massive case the mass parameter $μ$ disappears in the entropy spectrum and only the quasinormal modes modified by a term which is proportional to the mass square term. Our calculations show that the charge like parameter $\frac{1}{2ω}=Q^{2}$ appears in the surface gravity and our calculations can be applied to any spherically symmetric spacetime which has only one physically acceptable horizon. Our main difference between our calculations and what was done in \cite{1} is that the portion of charge and mass is included explicitly in the surface gravity and consequently in the QNM expression. Since the imaginary part of the QNM is related to the adiabatic invariance of the system and also to the entropy, surprisingly the mass parameter do not appear in the entropy spectrum. Our conclusion supported by some acclaims about that the scalar field parameters (charges) can not change the fundamental parameters in the 4-dimensional black holes.

hep-th↗
arXiv2009

Phase transition in Schwarzschild-de Sitter spacetime

Using a static massive spherically symmetric scalar field coupled to gravity in the Schwarzschild-de Sitter (SdS) background, first we consider some asymptotic solutions near horizon and their local equations of state(E.O.S) on them. We show that near cosmological and event horizons our scalar field behaves as a dust. At the next step near two pure de-Sitter or Schwarzschild horizons we obtain a coupling dependent pressure to energy density ratio. In the case of a minimally couplling this ratio is -1 which springs to the mind thermodynamical behavior of dark energy. If having a negative pressure behavior near these horizons we concluded that the coupling constant must be $ξ<{1/4}$ >. Therefore we derive a new constraint on the value of our coupling $ξ$ . These two different behaviors of unique matter in the distinct regions of spacetime at present era can be interpreted as a phase transition from dark matter to dark energy in the cosmic scales and construct a unified scenario.

gr-qc↗
arXiv2008

Cylindrical solutions in metric f(R) gravity

We study static cylindrically symmetric vacuum solutions in Weyl coordinates in the context of the metric f(R) theories of gravity. The set of the modified Einstein equations is reduced to a single equation and it is shown how one can construct exact solutions corresponding to different $f(R)$ models. In particular the family of solutions with constant Ricci scalar ($R=R_{0}$) is found explicitly which, as a special case (R=0), includes the exterior spacetime of a cosmic string. Another new solution with constant, non-zero Ricci scalar is obtained and its possible relation to the Linet-Tian solution in general relativity is discussed.

gr-qc↗
arXiv2005

Morgan-Morgan-NUT disk space via the Ehlers transformation

Using the Ehlers transformation along with the gravitoelectromagnetic approach to stationary spacetimes we start from the Morgan-Morgan disk spacetime (without radial pressure) as the seed metric and find its corresponding stationary spacetime. As expected from the Ehlers transformation the stationary spacetime obtained suffers from a NUT-type singularity and the new parameter introduced in the stationary case could be interpreted as the gravitomagnetic monopole charge (or the NUT factor). As a consequence of this singularity there are closed timelike curves (CTCs) in the singular region of the spacetime. Some of the properties of this spacetime including its particle velocity distribution, gravitational redshift, stability and energy conditions are discussed.

gr-qc↗