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K. Karami

Publications and source records attributed to K. Karami.

At least 55 records · Page 3Linked to original sources
arXiv2011

The generalized second law for the interacting generalized Chaplygin gas model

We investigate the validity of the generalized second law (GSL) of gravitational thermodynamics in a non-flat FRW universe containing the interacting generalized Chaplygin gas with the baryonic matter. The dynamical apparent horizon is assumed to be the boundary of the universe. We show that for the interacting generalized Chaplygin gas as a unified candidate for dark matter (DM) and dark energy (DE), 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 GSL is always satisfied throughout the history of the universe for any spatial curvature, independently of the equation of state of the interacting generalized Chaplygin gas model.

physics.gen-ph↗
arXiv2011

The generalized second law of gravitational thermodynamics on the apparent and event horizons in FRW cosmology

We investigate the validity of the generalized second law of gravitational thermodynamics on the apparent and event horizons in a non-flat FRW universe containing the interacting dark energy with dark matter. We show that for the dynamical apparent horizon, the generalized second law is always satisfied throughout the history of the universe for any spatial curvature and it is independent of the equation of state parameter of the interacting dark energy model. Whereas for the cosmological event horizon, the validity of the generalized second law depends on the equation of state parameter of the model.

gr-qc↗
arXiv2010

Reconstructing f(T)-gravity from the polytropic and different Chaplygin gas dark energy models

Motivated by a recent work of us [1], we reconstruct the different f(T)-gravity models corresponding to a set of dark energy scenarios containing the polytropic, the standard Chaplygin, the generalized Chaplygin and the modified Chaplygin gas models. We also derive the equation of state parameter of the selected f(T)-gravity models and obtain the necessary conditions for crossing the phantom-divide line.

physics.gen-ph↗
arXiv2010

The effect of density stratification on the resonant absorption of MHD waves in coronal loops

The standing quasi-modes of the ideal MHD in a zero-$β$ cylindrical magnetic flux tube that undergoes a longitudinal density stratification and radial density structuring is considered. The radial structuring is assumed to be a linearly varying density profile. Using the relevant connection formulae of the resonant absorption, the dispersion relation for the fast MHD body waves is derived and solved numerically to obtain both the frequencies and damping rates of the fundamental and first-overtone, $k=1,2$ modes of both the kink ($m=1$), and fluting ($m=2$) waves. Where $k$ and $m$ are the longitudinal and azimuthal mode numbers, respectively.

astro-ph.SR↗
arXiv2010

Interacting entropy-corrected new agegraphic dark energy in non-flat universe

Here we consider the entropy-corrected version of the new agegraphic dark energy model in the non-flat FRW universe. We derive the exact differential equation that determines the evolution of the entropy-corrected new agegraphic dark energy density parameter in the presence of interaction with dark matter. We also obtain the equation of state and deceleration parameters and present a necessary condition for the selected model to cross the phantom divide. Moreover, we reconstruct the potential and the dynamics of the phantom scalar field according to the evolutionary behavior of the interacting entropy-corrected new agegraphic model.

physics.gen-ph↗
arXiv2010

Holographic dark energy in a non-flat universe with Granda-Oliveros cut-off

Motivated by Granda and Oliveros (GO) model, we generalize their work to the non-flat case. We obtain the evolution of the dark energy density, the deceleration and the equation of state parameters for the holographic dark energy model in a non-flat universe with GO cut-off. In the limiting case of a flat universe, i.e. $k = 0$, all results given in GO model are obtained.

gr-qc↗
arXiv2010

Comment on "A holographic model of dark energy and the thermodynamics of a non-flat accelerated expanding universe", by M.R. Setare and S. Shafei (JCAP 09 (2006) 011, arXiv:gr-qc/0606103)

Authors of ref. [1], M.R. Setare and S. Shafei (JCAP 09 (2006) 011), studied the thermodynamics of a holographic dark energy model in a non-flat universe enclosed by the apparent horizon $R_A$ and the event horizon measured from the sphere of the horizon named $L$. In section 3 in ref. [1], Authors showed that for $R_A$ the generalized second law of thermodynamics is respected, while for $L$ it is satisfied for the special range of the deceleration parameter. Here we present that their calculations for $R_A$ should be revised. Also we show that their conclusion for $L$ is not true and the generalized second law is hold for the present time independently of the deceleration parameter. Also if we take into account the contribution of dark matter in the generalized second law which is absent in ref. [1], then the generalized second law for $L$ is violated for the present time.

physics.gen-ph↗
arXiv2010

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

We investigate the validity of the generalized second law in irreversible thermodynamics in a non-flat FRW universe containing the interacting 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 the present time, the generalized second law in nonequilibrium thermodynamics is satisfied for the special range of the energy transfer constants.

gr-qc↗
arXiv2009

New holographic scalar field models of dark energy in non-flat universe

Motivated by the work of Granda and Oliveros [L.N. Granda, A. Oliveros, Phys. Lett. B {\bf 671}, 199 (2009)], we generalize their work to the non-flat case. We study the correspondence between the quintessence, tachyon, K-essence and dilaton scalar field models with the new holographic dark energy model in the non-flat FRW universe. We reconstruct the potentials and the dynamics for these scalar field models, which describe accelerated expansion of the universe. In the limiting case of a flat universe, i.e. $k = 0$, all results given in [L.N. Granda, A. Oliveros, Phys. Lett. B {\bf 671}, 199 (2009)] are obtained.

gr-qc↗
arXiv2009

Phase-Mixing and Dissipation of Standing Shear Alfven waves

We study the phase mixing and dissipation of a packet of standing shear Alfvén waves localized in a region with non-uniform Alfvén background velocity. We investigate the validity of the exponential damping law in time, $\exp(-At^3)$, presented by Heyvaerts & Priest (1983) for different ranges of Lundquist, $S$, and Reynolds, $R$, numbers. Our numerical results shows that it is valid for $(R,S)\geq 10^7$.

astro-ph.SR↗
arXiv2009

Comment on "Interacting holographic dark energy model and generalized second law of thermodynamics in a non-flat universe", by M.R. Setare (JCAP 01, 023, 2007)

Author of Ref. [1], M.R. Setare (JCAP 01, 023, 2007, arXiv:hep-th/0701242), by redefining the event horizon measured from the sphere of the horizon as the system's IR cut-off for an interacting holographic dark energy model in a non-flat universe, showed that the generalized second law of thermodynamics is satisfied for the special range of the deceleration parameter. His paper includes an erroneous calculation of the entropy of the cold dark matter. Also there are some missing terms and some misprints in the equations of his paper. Here we present that his conclusion is not true and the generalized second law is violated for the present time independently of the deceleration parameter.

gr-qc↗
arXiv2009

Interacting polytropic gas model of phantom dark energy in non-flat universe

By introducing the polytropic gas model of interacting dark energy, we obtain the equation of state for the polytropic gas energy density in a non-flat universe. We show that for even polytropic index by choosing $K>Ba^{\frac{3}{n}}$, one can obtain $ω^{\rm eff}_Λ<-1$, which corresponds to a universe dominated by phantom dark energy.

gr-qc↗
arXiv2009

Velocity Curve Analysis of the Spectroscopic Binary Stars V373 Cas, V2388 Oph, V401 Cyg, GM Dra, V523 Cas, AB And, and HD 141929 by Artificial Neural Networks

We used an Artificial Neural Network (ANN) to derive the orbital parameters of spectroscopic binary stars. Using measured radial velocity data of seven double-lined spectroscopic binary systems V373 Cas, V2388 Oph, V401 Cyg, GM Dra, V523 Cas, AB And, and HD 141929, we found corresponding orbital and spectroscopic elements. Our numerical results are in good agreement with those obtained by others using more traditional methods.

astro-ph.SR↗
arXiv2008

Third Order Effect of Rotation on Stellar Oscillations of a $β$-Cephei Star

Here the effect of rotation up to third order in the angular velocity of a star on the p, f and g modes is investigated. To do this, the third-order perturbation formalism presented by Soufi et al. (1998) and revised by Karami (2008), was used. I quantify by numerical calculations the effect of rotation on the oscillation frequencies of a uniformly rotating $β$-Cephei star with 12 $M_\odot$. For an equatorial velocity of 90 $\rm km s^{-1}$, it is found that the second- and third-order corrections for $(l,m)=(5,-4)$, for instance, are of order of 0.07% of the frequency for radial order $n=-3$ and reaches up to 0.6% for $n=-20$.

astro-ph↗
arXiv2008

The effects of twisted magnetic field on coronal loops oscillations and dissipation

The standing MHD modes in a zero-$β$ cylindrical magnetic flux tube modelled as a straight core surrounded by a magnetically twisted annulus, both embedded in a straight ambient external field is considered. The dispersion relation for the fast MHD waves is derived and solved numerically to obtain the frequencies of both the kink ($m=1$), and fluting ($m=2,3$) waves. Damping rates due to both viscous and resistive dissipations in presence of the twisted magnetic field is derived and solved numerically for both the kink and fluting waves.

astro-ph↗