arXiv subjects

K. Karami

Publications and source records attributed to K. Karami.

At least 37 records · Page 2Linked to original sources

arXiv2012

Cosmological constraints on polytropic gas model

We study the polytropic gas scenario as the unification of dark matter and dark energy. We fit the model parameters by using the latest observational data including type Ia supernovae, baryon acoustic oscillation, cosmic microwave background, and Hubble parameter data. At 68.3% and 95.4% confidence levels, we find the best fit values of the model parameters as $\tilde{K}=0.742_{-0.024}^{+0.024}(1σ)_{-0.049}^{+0.048}(2σ)$ and $n=-1.05_{-0.08}^{+0.08}(1σ)_{-0.16}^{+0.15}(2σ)$. Using the best fit values of the model, we obtain the evolutionary behaviors of the equation of state parameters of the polytropic gas model and dark energy, the deceleration parameter of the universe as well as the dimensionless density parameters of dark matter and dark energy. We conclude that in this model, the universe starts from the matter dominated epoch and approaches a de Sitter phase at late times, as expected. Also the universe begins to accelerate at redshift $z_{\rm t}=0.74$. Furthermore in contrary to the $Λ$CDM model, the cosmic coincidence problem is solved naturally in the polytropic gas scenario.

astro-ph.CO↗

arXiv2012

The Effect of Twisted Magnetic Field on the Period Ratio P_1/P_2 of Nonaxisymmetric MHD Waves

The nonaxisymmetric magnetohydrodynamic (MHD) modes in a zero-beta cylindrical compressible thin magnetic flux tube modelled as a twisted core surrounded by a magnetically twisted annulus, both embedded in a straight ambient external field is considered. The dispersion relation is derived and solved analytically and numerically to obtain the frequencies of the nonaxisymmetric MHD waves. The main result is that the twisted magnetic annulus does affect the period ratio P_1/P_2 of the kink modes. For the kink modes, the magnetic twist in the annulus region can achieve deviations from P_1/P_2 = 2 of the same order of magnitude as in the observations. Furthermore, the effect of the internal twist on the fluting modes is investigated.

astro-ph.SR↗

arXiv2012

The generalized second law of gravitational thermodynamics on the apparent horizon in f(R)-gravity

We investigate the generalized second law (GSL) of thermodynamics in the framework of $f(R)$-gravity. We consider a FRW universe filled only with ordinary matter enclosed by the dynamical apparent horizon with the Hawking temperature. For a viable modified gravity model as $f(R)=R-α/R+βR^{2}$, we examine the validity of the GSL during the early inflation and late acceleration eras. Our results show that for the selected $f(R)$-gravity model minimally coupled with matter, the GSL in the early inflation epoch is satisfied only for the special range of the equation of state parameter of the matter. But in the late acceleration regime, the GSL is always respected.

physics.gen-ph↗

arXiv2012

Power Law Entropy Corrected New-Agegraphic Dark Energy in Hořava-Lifshitz Cosmology

We investigate the new agegraphic dark energy (NADE) model with power-law corrected entropy in the framework of Hořava-Lifshitz cosmology. For a non-flat universe containing the interacting power-law entropy-corrected NADE (PLECNADE) with dark matter, we obtain the differential equation of the evolution of density parameter as well as the deceleration parameter. To study parametric behavior, we used an interesting form of state parameter as function of redshift $ω_Λ(z)=ω_0+ω_1 z$. We found that phantom crossing occurs for the state parameter for a non-zero coupling parameter, thus supporting interacting dark energy model.

physics.gen-ph↗

arXiv2012

Generalized second law of thermodynamics in f(T) gravity

We investigate the validity of the generalized second law (GSL) of gravitational thermodynamics in the framework of f(T) modified teleparallel gravity. We consider a spatially flat FRW universe containing only the pressureless matter. The boundary of the universe is assumed to be enclosed by the Hubble horizon. For two viable f(T) models containing $f(T)=T+μ_1{(-T)}^n$ and $f(T)=T-μ_2 T(1-e^{β\frac{T_0}{T}})$, we first calculate the effective equation of state and deceleration parameters. Then, we investigate the null and strong energy conditions and conclude that a sudden future singularity appears in both models. Furthermore, using a cosmographic analysis we check the viability of two models. Finally, we examine the validity of the GSL and find that for both models it is satisfied from the early times to the present epoch. But in the future, the GSL is violated for the special ranges of the torsion scalar T.

gr-qc↗

arXiv2012

Holographic, new agegraphic and ghost dark energy models in fractal cosmology

We investigate the holographic, new agegraphic and ghost dark energy models in the framework of fractal cosmology. We consider a fractal FRW universe filled with the dark energy and dark matter. We obtain the equation of state parameters of the selected dark energy models in the ultraviolet regime and discuss on their implications.

physics.gen-ph↗

arXiv2011

Power-law entropy-corrected HDE and NADE in Brans-Dicke cosmology

Considering the power-law corrections to the black hole entropy, which appear in dealing with the entanglement of quantum fields inside and outside the horizon, the holographic energy density is modified accordingly. In this paper we study the power-law entropy-corrected holographic dark energy in the framework of Brans-Dicke theory. We investigate the cosmological implications of this model in detail. We also perform the study for the new agegraphic dark energy model and calculate some relevant cosmological parameters and their evolution. {As a result we find that this model can provide the present cosmic acceleration and even the equation of state parameter of this model can cross the phantom line $w_D=-1$ provided the model parameters are chosen suitably}.

astro-ph.CO↗

arXiv2011

Interacting viscous entropy-corrected holographic scalar field models of dark energy with time-varying G in modified FRW cosmology

We study the entropy-corrected version of the holographic dark energy (HDE) model in the framework of modified FRW cosmology. We consider a non-flat universe filled with an interacting viscous entropy-corrected HDE (ECHDE) with dark matter. We also include the case of variable gravitational constant G in our model. We obtain the equation of state and the deceleration parameters of the interacting viscous ECHDE. Moreover, we reconstruct the potential and the dynamics of the quintessence, tachyon, K-essence and dilaton scalar field models according to the evolutionary behavior of the interacting viscous ECHDE model with time-varying G.

gr-qc↗

arXiv2011

Power-law entropy corrected holographic and new agegraphic f(R)-gravity models

Motivated by the recent works of us [1], we establish a correspondence between f(R)-gravity with the power-law entropy corrected holographic dark energy (PLECHDE) and new agegraphic dark energy (PLECNADE) models. We reconstruct corresponding f(R)-gravities and obtain the equation of state parameters. We show that the selected f(R)-gravity models can accommodate the transition from the quintessence state to the phantom regime as indicated by the recent observations.

physics.gen-ph↗

arXiv2011

Thermodynamics of apparent horizon in modified FRW universe with power-law corrected entropy

We derive the modified Friedmann equation corresponding to the power-law corrected entropy-area relation $S_{\rm A}=\frac{A}{4}[1-K_α A^{1-\fracα{2}}]$ which is motivated by the entanglement of quantum fields in and out of the apparent horizon. We consider a non-flat modified FRW universe containing an interacting viscous dark energy with dark matter and radiation. For the selected model, we study the effect of the power-law correction term to the entropy on the dynamics of dark energy. Furthermore, we investigate the validity of the generalized second law (GSL) of gravitational thermodynamics on the apparent horizon and conclude that the GSL is satisfied for $α<2$.

physics.gen-ph↗

arXiv2011

f(T) modified teleparallel gravity models as an alternative for holographic and new agegraphic dark energy models

In the present work, we reconstruct different f(T)-gravity models corresponding to the original and entropy-corrected version of the holographic and new agegraphic dark energy models. We also obtain the equation of state parameters of the corresponding f(T)-gravity models. We conclude that the holographic and new agegraphic f(T)-gravity models behave like phantom or quintessence model. Whereas in the entropy-corrected models, the equation of state parameter can justify the transition from the quintessence state to the phantom regime as indicated by the recent observations.

gr-qc↗

arXiv2011

The effect of twisted magnetic field on the resonant absorption of MHD waves in coronal loops

The standing quasi modes in a cylindrical incompressible flux tube with magnetic twist that undergoes a radial density structuring is considered in ideal magnetohydrodynamics (MHD). The radial structuring is assumed to be a linearly varying density profile. Using the relevant connection formulae, the dispersion relation for the MHD waves is derived and solved numerically to obtain both the frequencies and damping rates of the fundamental and first-overtone modes of both the kink (m=1) and fluting (m=2,3) waves. It was found that a magnetic twist will increase the frequencies, damping rates and the ratio of the oscillation frequency to the damping rate of these modes. The period ratio P_1/P_2 of the fundamental and its first-overtone surface waves for kink (m=1) and fluting (m=2,3) modes is lower than 2 (the value for an untwisted loop) in the presence of twisted magnetic field. For the kink modes, particularly, the magnetic twists B_ϕ/B_z=0.0065 and 0.0255 can achieve deviations from 2 of the same order of magnitude as in the observations. Furthermore, for the fundamental kink body waves, the frequency bandwidth increases with increasing the magnetic twist.

astro-ph.SR↗

arXiv2011

Reconstructing f(R) modified gravity from ordinary and entropy-corrected versions of the holographic and new agegraphic dark energy models

Here, we peruse cosmological usage of the most promising candidates of dark energy in the framework of f(R) theory. We reconstruct the different f(R) modified gravity models in the spatially flat FRW universe according to the ordinary and entropy-corrected versions of the holographic and new agegraphic dark energy models, which describe accelerated expansion of the universe. We also obtain the equation of state parameter of the corresponding f(R)-gravity models. We conclude that the holographic and new agegraphic f(R)-gravity models can behave like phantom or quintessence models. Whereas the equation of state parameter of the entropy-corrected models can transit from quintessence state to phantom regime as indicated by recent observations.

gr-qc↗

arXiv2011

Torsional Alfven waves in stratified and expanding magnetic flux tubes

The effects of both density stratification and magnetic field expansion on torsional Alfven waves in magnetic flux tubes are studied. The frequencies, the period ratio P1/P2 of the fundamental and its first-overtone, and eigenfunctions of torsional Alfven modes are obtained. Our numerical results show that the density stratification and magnetic field expansion have opposite effects on the oscillating properties of torsional Alfven waves.

astro-ph.SR↗

arXiv2011

Radial velocity-acceleration curve analysis of the spectroscopic binary stars by the nonlinear regression

We will introduce a new method to derive the orbital elements of the spectroscopic binary stars. Fitting the adopted radial velocity curves on the measured experimental data, the corresponding radial acceleration data are obtained. The orbital parameters are estimated by the nonlinear least squares of the radial velocity-acceleration curves. This method is implemented for the three double line spectroscopic binary systems RZ CAS, CC Cas and V1130 Tau. Numerical calculations show that the result are in good agreement with the those obtained by others using the method of Lehmann-Filhés.

astro-ph↗

arXiv2011

The generalized second law for the interacting new agegraphic dark energy in a non-flat FRW universe enclosed by the apparent horizon

We investigate the validity of the generalized second law of gravitational thermodynamics in a non-flat FRW universe containing the interacting new agegraphic dark energy with cold dark matter. The boundary of the universe is assumed to be enclosed by the dynamical apparent horizon. We show that for this model, the equation of state parameter can cross the phantom divide. We also present that for the selected model under thermal equilibrium with the Hawking radiation, the generalized second law is always satisfied throughout the history of the universe. Whereas, the evolution of the entropy of the universe and apparent horizon, separately, depends on the equation of state parameter of the interacting new agegraphic dark energy model.

gr-qc↗

arXiv2011

Resonant absorption in dissipative flux tubes

We study the resonant absorption of MHD waves in magnetized flux tubes with a radial density inhomogeneity. Within the approximation that resistive and viscous processes are operative in thin layers surrounding the singularities of the MHD equations, we give the full spectrum of the eigenfrequencies and damping rates of the MHD quasi modes of the tube. Both surface and body modes are analyzed. \keywords{Sun -- corona: magnetohydrodynamics (MHD) -- Sun: magnetic fields -- Sun: oscillations

astro-ph↗

arXiv2011

Generalized second law of thermodynamics in modified FRW cosmology with corrected entropy-area relation

Using the corrected entropy-area relation motivated by the loop quantum gravity, we investigate the validity of the generalized second law of thermodynamics in the framework of modified FRW cosmology. We consider a non-flat universe filled with an interacting viscous dark energy with dark matter and radiation. The boundary of the universe is assumed to be the dynamical apparent horizon. We find out that the generalized second law is always satisfied throughout the history of the universe for any spatial curvature regardless of the dark energy model.

gr-qc↗