arXiv subjects

Kayoomars Karami

Publications and source records attributed to Kayoomars Karami.

At least 37 records · Page 2Linked to original sources

arXiv2022

Primordial black holes in nonminimal derivative coupling inflation with quartic potential and reheating consideration

We investigate the generation of Primordial Black Holes (PBHs) with the aid of gravitationally increased friction mechanism originated from the NonMinimal field Derivative Coupling (NMDC) to gravity framework, with the quartic potential. Applying the coupling parameter as a two-parted function of inflaton field and fine-tuning of five parameter assortments we can acquire ultra slow-roll phase to slow down the inflaton field due to high friction. This enables us to achieve enough enhancement in the amplitude of curvature perturbations power spectra to generate PBHs with different masses. The reheating stage is considered to obtain criteria for PBHs generation during radiation dominated era. We demonstrate that three cases of asteroid mass PBHs ($10^{-12}M_{\odot}$, $10^{-13}M_{\odot}$, and $10^{-15}M_{\odot})$ can be very interesting candidates for comprising $100\%$ , $98.3\%$ and $99.1\%$ of the total Dark Matter (DM) content of the universe. Moreover, we analyse the production of induced Gravitational Waves (GWs), and illustrate that their spectra of current density parameter $(Ω_{\rm GW_0})$ for all parameter Cases foretold by our model have climaxes which cut the sensitivity curves of GWs detectors, ergo the veracity of our outcomes can be tested in light of these detectors. At last, our numerical results exhibit that the spectra of $Ω_{\rm GW_0}$ behave as a power-law function with respect to frequency, $Ω_{\rm GW_0} (f) \sim (f/f_c)^{n} $, in the vicinity of climaxes. Also, in the infrared regime $f\ll f_{c}$, the power index satisfies the relation $n=3-2/\ln(f_c/f)$.

gr-qc↗

arXiv2021

Mechanism of primordial black holes production and secondary gravitational waves in $α$-attractor Galileon inflationary scenario

We study the process of the Primordial Black Holes (PBHs) production in the novel framework, namely $α$-attractor Galileon inflation (G-inflation) model. In our framework, we take the Galileon function as $G(ϕ)=G_{I}(ϕ)\left(1+G_{II}(ϕ)\right)$, where the part $G_{I}(ϕ)$ is motivated from the $α$-attractor inflationary scenario in its original non-canonical frame, and it ensures for the model to be consistent with the Planck 2018 observations at the CMB scales. The part $G_{II}(ϕ)$ is invoked to enhance the curvature perturbations at some smaller scales which in turn gives rise to PBHs formation. By fine-tuning of the model parameters, we find three parameter sets which successfully produce a sufficiently large peak in the curvature power spectrum. We show that these parameter sets produce PBHs with masses ${\cal O}(10)M_\odot$, ${\cal O}(10^{-5})M_\odot$, and ${\cal O}(10^{-13})M_\odot$ which can explain the LIGO events, the ultrashort-timescale microlensing events in OGLE data, and around $0.98\%$ of the current Dark Matter (DM) content of the universe, respectively. Additionally, we study the secondary Gravitational Waves (GWs) in our setup and show that our model anticipates the peak of their present fractional energy density as $Ω_{GW0} \sim 10^{-8}$ for all the three parameter sets, but at different frequencies. These predictions can be located well inside the sensitivity region of some GWs detectors, and therefore the compatibility of our model can be assessed in light of the future data. We further estimate the tilts of the included GWs spectrum in the different ranges of frequency, and confirm that spectrum follows the power-law relation $Ω_{GW0}\sim f^{n}$ in those frequency bands.

astro-ph.CO↗

arXiv2021

Primordial black holes formation in the inflationary model with field-dependent kinetic term for quartic and natural potentials

Within the framework of inflationary model with field-dependent kinetic term for quartic and natural potentials, we investigate generation of the primordial black holes (PBHs) and induced gravitational waves (GWs). In this setup, we consider a kinetic function as $G(ϕ)=g_I(ϕ)\big(1+g_{II}(ϕ)\big)$ and show that in the presence of first term $g_I(ϕ)$ both quartic and natural potentials, in contrast to the standard model of inflation, can be consistent, with the 68\% CL of Planck observations. Besides, the second term $g_{II}(ϕ)$ can cause a significant enhancement in the primordial curvature perturbations at the small scales which results the PBHs formation. For the both potentials, we obtain an enhancement in the scalar power spectrum at the scales $k\sim10^{12}~\rm Mpc^{-1}$, $10^{8}~\rm Mpc^{-1}$, and $10^{5}~\rm Mpc^{-1}$, which causes PBHs production in mass scales around $10^{-13}M_{\odot}$, $10^{-5}M_{\odot}$, and $10 M_{\odot}$, respectively. Observational constraints confirm that PBHs with a mass scale of $10^{-13}M_{\odot}$ can constitute the total of dark matter in the universe. Furthermore, we estimate the energy density parameter of induced GWs which can be examined by the observation. Also we conclude that it can be parametrized as a power-law function $Ω_{\rm GW}\sim (f/f_c)^n$, where the power index equals $n=3-2/\ln(f_c/f)$ in the infrared limit $f\ll f_{c}$.

astro-ph.CO↗

arXiv2021

Primordial black holes and induced gravitational waves in $k$-inflation

Recent observational constraints indicate that primordial black holes (PBHs) with the mass scale $\sim 10^{-12}M_{\odot}$ can explain most of dark matter in the Universe. To produce this kind of PBHs, we need an enhance in the primordial scalar curvature perturbations to the order of ${\mathcal{O}(10^{-2})}$ at the scale $ k \sim 10^{12}~\rm Mpc^{-1}$. Here, we investigate the production of PBHs and induced gravitational waves (GWs) in the framework of \textbf{$k$-inflation}. We solve numerically the Mukhanov-Sasaki equation to obtain the primordial scalar power spectrum. In addition, we estimate the PBHs abundance $f_{\text{PBH}}^{\text{peak}}$ as well as the energy density parameter $Ω_{\rm GW,0}$ of induced GWs. Interestingly enough is that for a special set of model parameters, we estimate the mass scale and the abundance of PBHs as $\sim{\cal O}(10^{-13})M_{\odot}$ and $f_{\text{PBH}}^{\text{peak}}=0.96$, respectively. This confirms that the mechanism of PBHs production in our inflationary model can justify most of dark matter. Furthermore, we evaluate the GWs energy density parameter and conclude that it behaves like a power-law function $Ω_{\rm GW}\sim (f/f_c)^n$ where in the infrared limit $f\ll f_{c}$, the power index reads $n=3-2/\ln(f_c/f)$.

astro-ph.CO↗

arXiv2021

Primordial black holes formation and secondary gravitational waves in nonminimal derivative coupling inflation

We study the possibility of the Primordial Black Holes (PBHs) formation with the aim of finding a considerable fraction of Dark Matter (DM), using the gravitationally enhanced friction mechanism which arises from a nonminimal derivative coupling between the scalar field and the gravity. Assuming the nonminimal coupling parameter as a special function of the scalar field and considering the potential of natural inflation, we find three parameter sets that produce a period of ultra slow-roll inflation. This leads to sufficiently large enhancement in the curvature power spectra to form PBHs. We show that under the gravitationally enhanced friction mechanism, PBHs with a mass around ${\cal O}\big(10^{-12}\big)M_\odot$ can constitute around $96\%$ of the total DM and so this class of PBHs can be taken as a great candidate for DM. We further study the secondary Gravitational Waves (GWs) in our setting and show that our model predicts the peak of the present fractional energy density as $Ω_{GW0} \sim 10^{-8}$ at different frequencies for all the three parameter sets. This value lies well inside the sensitivity region of some GWs detectors at some frequencies, and therefore the observational compatibility of our model can be appraised by the data from these detectors.

gr-qc↗

arXiv2021

Resonant Absorption of Kink MHD Waves in Inclined and Asymmetric Coronal Loops

This paper separately evaluates the effects of inclination and asymmetry of solar coronal loops on the resonant absorption of kink magnetohydrodynamic (MHD) oscillations. We modelled a typical coronal loop by a straight and axisymmetric cylindrical magnetic flux tube filled with cold plasma. We solved the dispersion relation numerically for different values of the longitudinal mass density stratification. We show that, in inclined and asymmetric loops, the frequencies and their corresponding damping rates of the fundamental and first-overtone modes of kink oscillations are smaller in comparison with semi-circular uninclined loops with the same lengths. The results also indicate that, the period ratio $P_1/P_2$, increases with increasing the inclination of the loop, but it decreases less than $2\%$ while imposing the asymmetry to each loop side, up to $9.66\%$ of the loop length. The ratio of each mode frequency to its corresponding damping rate remain unchanged approximately while the inclination or the asymmetry imposed. Hence, we conclude that these ratios are reliable for inferring the physical parameters of coronal loops and coronal medium, regardless of the loop shape or the state of its inclination. In addition, in contrast with the effect of asymmetry which is not significant on the period ratio $P_1/P_2$, when an observed oscillating loop has a smaller apex height, the state of its inclination is an important factor that should be considered, especially when the period ratio $P_1/P_2$, is taken into consideration for coronal seismology.

astro-ph.SR↗

arXiv2021

The effect of flow on resonant absorption of slow MHD waves in magnetic flux tubes

In this paper, we study kink and sausage oscillations in the presence of longitudinal background flow. We study resonant absorption of the kink and sausage modes in the slow continuum under magnetic pore conditions in the presence of flow. we determine the dispersion relation then solve it numerically, and find the frequencies and damping rates of the slow kink and sausage surface modes. We also, obtain analytical solution for the damping rate of the slow surface mode in the long wavelength limit. We show that in the presence of plasma flow, resonance absorption can result in strong damping for forward waves and can be considered as an efficient mechanism to justify the extremely rapid damping of slow surface sausage waves observed in magnetic pores. Also, the plasma flow reduces the efficiency of resonance absorption to damp backward waves. Furthermore, for the pore conditions, the resonance instability is avoided in our model.

physics.plasm-ph↗

arXiv2020

Phase Mixing of Kink MHD Waves in the Solar Corona: Viscous Dissipation and Heating

Magnetohydrodynamic (MHD) kink waves have been observed frequently in solar coronal flux tubes, which makes them a great tool for seismology of the solar corona. Here, the effect of viscosity is studied on the evolution of kink waves. To this aim, we solve the initial value problem for the incompressible linearized viscous MHD equations in a radially inhomogeneous flux tube in the limit of long wavelengths. Using a modal expansion technique the spatio-temporal behavior of the perturbations is obtained. We confirm that for large Reynolds numbers representative of the coronal plasma the decrement in the amplitude of the kink oscillations is due to the resonant absorption mechanism that converts the global transverse oscillation to rotational motions in the inhomogeneous layer of the flux tube. We show that viscosity suppresses the rate of phase mixing of the perturbations in the inhomogeneous region of the flux tube and prevents the continuous building up of small scales in the system once a sufficiently small scale is reached. The viscous dissipation function is calculated to investigate plasma heating by viscosity in the inhomogeneous layer of the flux tube. For Reynolds numbers of the order of 10^6-10^8, the energy of the kink wave is transformed into heat in two to eight periods of the kink oscillation. For larger and more realistic Reynolds numbers, heating happens, predominantly, after the global kink oscillation is damped, and no significant heating occurs during the observable transverse motion of the flux tube.

astro-ph.SR↗

arXiv2019

The effect of magnetic twist on resonant absorption of slow sausage waves in magnetic flux tubes

Observations show that twisted magnetic flux tubes are present throughout the sun's atmosphere. The main aim of this work is to obtain the damping rate of sausage modes in the presence of magnetic twist. Using the connection formulae obtained by Sakurai et al. (1991), we investigate resonant absorption of the sausage modes in the slow continuum under photosphere conditions. We derive the dispersion relation and solve it numerically and consequently obtain the frequencies and damping rates of the slow surface sausage modes. We conclude that the magnetic twist can result in strong damping in comparison with the untwisted case.

astro-ph.SR↗

arXiv2018

Galileon intermediate inflation

We investigate intermediate inflation in the framework of a Galileon scalar field. To this aim, we first obtain the inflationary observables, including the scalar spectral index, the tensor-to-scalar ratio, the running of the scalar spectral index, as well as the non-Gaussianity parameters. Then, we examine the observational viability of the intermediate inflation within the framework of a Galileon scenario. Our results show that although the prediction of intermediate inflation in the standard framework is completely ruled out by the Planck 2015 observations, it can be put inside the allowed regions of the Planck 2015 TT,TE,EE+lowP data in the Galileon setting. Moreover, we determine the parameter space of the Galileon intermediate inflation for which the model is consistent with the Planck 2015 data. Besides, we derive the consistency relation in the Galileon scenario, and find that it differs from the standard inflation. We also estimate the running of the scalar spectral index and find that it is in well agreement with the 95\% CL constraint of the Planck 2015 results. Finally, we evaluate the local, equilateral, orthogonal, and enfolded non-Gaussianity parameters, and conclude that not only is the shape of non-Gaussianity approximately close to the equilateral one, but that it also satisfies the 68\% CL bound from the Planck 2015 data.

gr-qc↗

arXiv2018

Resurrecting the Power-law, Intermediate, and Logamediate Inflations in the DBI Scenario with Constant Sound Speed

We investigate the power-law, intermediate, and logamediate inflationary models in the framework of DBI non-canonical scalar field with constant sound speed. In the DBI setting, we first represent the power spectrum of both scalar density and tensor gravitational perturbations. Then, we derive different inflationary observables including the scalar spectral index $n_s$, the running of the scalar spectral index $dn_s/d\ln k$, and the tensor-to-scalar ratio $r$. We show that the 95\% CL constraint of the Planck 2015 T+E data on the non-Gaussianity parameter $f_{\rm NL}^{\rm DBI}$ leads to the sound speed bound $c_{s}\geq0.087$ in the DBI inflation. Moreover, our results imply that, although the predictions of the power-law, intermediate, and logamediate inflations in the standard canonical framework ($c_s=1$) are not consistent with the Planck 2015 data, in the DBI scenario with constant sound speed $c_s<1$, the result of the $r-n_s$ diagram for these models can lie inside the 68\% CL region favored by the Planck 2015 TT,TE,EE+lowP data. We also specify the parameter space of the power-law, intermediate, and logamediate inflations for which our models are compatible with the 68\% or 95\% CL regions of the Planck 2015 TT,TE,EE+lowP data. Using the allowed ranges of the parameter space of the intermediate and logamediate inflationary models, we estimate the running of the scalar spectral index and find that it is compatible with the 95\% CL constraint from the Planck 2015 TT,TE,EE+lowP data.

astro-ph.CO↗

arXiv2017

The effect of a twisted magnetic field on the phase mixing of the kink magnetohydrodynamic waves in coronal loops

There are observational evidences for the existence of twisted magnetic field in the solar corona. This inspires us to investigate the effect of a twisted magnetic field on the evolution of magnetohydrodynamic (MHD) kink waves in coronal loops. To this aim, we solve the incompressible linearized MHD equations in a magnetically twisted nonuniform coronal flux tube in the limit of long wavelengths. Our results show that a twisted magnetic field can enhance or diminish the rate of phase-mixing of the Alfvén continuum modes and the decay rate of the global kink oscillation depending on the twist model and the sign of the longitudinal ($k_z$) and azimuthal ($m$) wavenumbers. Also our results confirm that in the presence of a twisted magnetic field, when the sign of one of the two wavenumbers $m$ and $k_z$ is changed, the symmetry with respect to the propagation direction is broken. Even a small amount of twist can have an important impact on the process of energy cascade to small scales.

astro-ph.SR↗

arXiv2017

Resurrecting the exponential and inverse power-law potentials in non-canonical inflation

We study inflation within the framework of non-canonical scalar field. In this scenario, we obtain the inflationary observables such as the scalar spectral index, the tensor-to-scalar ratio, the running of the scalar spectral index as well as the equilateral non-Gaussianity parameter. Then, we apply these results for the exponential and inverse power-law potentials. Our investigation shows that although the predictions of these potentials in the standard canonical inflation are completely ruled out by the Planck 2015 observations, their results in non-canonical scenario can lie inside the allowed regions of the Planck 2015 data. We also find that in non-canonical inflation, the predictions of the aforementioned potentials for the running of the scalar spectral index and the equilateral non-Gaussianity parameter are in well agreement with the Planck 2015 results. Furthermore, we show that in the context of non-canonical inflation, the graceful exit problem of the exponential and inverse power-law potentials is resolved.

gr-qc↗

arXiv2017

Generalized Brans-Dicke inflation with a quartic potential

Within the framework of Brans-Dicke gravity, we investigate inflation with the quartic potential, $λφ^4/4$, in the presence of generalized Brans-Dicke parameter $ω_{\rm GBD}(φ)$. We obtain the inflationary observables containing the scalar spectral index, the tensor-to-scalar ratio, the running of the scalar spectral index and the equilateral non-Gaussianity parameter in terms of general form of the potential $U(φ)$ and $ω_{\rm GBD}(φ)$. For the quartic potential, our results show that the predictions of the model are in well agreement with the Planck 2015 data for the generalized Brans-Dicke parameters $ω_{\rm GBD}(φ)=ω_0φ^{n}$ and $ω_0e^{bφ}$. This is in contrast with both the Einstein and standard Brans-Dicke gravity, in which the result of quartic potential is disfavored by the Planck data.

gr-qc↗

arXiv2017

Logamediate Inflation in f(T) Teleparallel Gravity

We study logamediate inflation in the context of $f(T)$ teleparallel gravity. $f(T)$-gravity is a generalization of the teleparallel gravity which is formulated on the Weitzenböck spacetime, characterized by the vanishing curvature tensor (absolute parallelism) and the non-vanishing torsion tensor. We consider an $f(T)$-gravity model which is sourced by a canonical scalar field. Assuming a power-law $f(T)$ function in the action, we investigate an inflationary universe with a logamediate scale factor. Our results show that, although logamediate inflation is completely ruled out by observational data in the standard inflationary scenario based on Einstein gravity, it can be compatible with the 68\% confidence limit joint region of Planck 2015 TT,TE,EE+lowP data in the framework of $f(T)$-gravity.

gr-qc↗

arXiv2016

Resonant absorption of kink magnetohydrodynamic waves by a magnetic twist in coronal loops

There is ample evidence of twisted magnetic structures in the solar corona. This motivates us to consider the magnetic twist as the cause of Alfven frequency continuum in the coronal loops, which can support the resonant absorption as a rapid damping mechanism for the observed coronal kink magnetohydrodynamic (MHD) oscillations. We model a coronal loop with a straight cylindrical magnetic flux tube which has constant but different densities in the interior and exterior regions. The magnetic field is assumed to be constant and aligned with the cylinder axis everywhere except a thin layer near the boundary of the flux tube which has an additional small magnetic field twist. Then, we investigate a number of possible instabilities that may arise in our model. In the thin tube thin boundary approximation, we derive the dispersion relation and solve it analytically to obtain the frequencies and damping rates of the fundamental (l=1) and first/second overtone (l=2,3) kink (m=1) MHD modes. We conclude that the resonant absorption by the magnetic twist can justify the rapid damping of kink MHD waves observed in coronal loops. Furthermore, the magnetic twist in the inhomogeneous layer can cause deviations from P1/P2=2 and P1/P3=3 which are comparable with the observations.

astro-ph.SR↗

arXiv2012

Entropy-corrected new agegraphic dark energy in Horava-Lifshitz cosmology

We study the entropy-corrected version of the new agegraphic dark energy (NADE) model and dark matter in a spatially non-flat Universe and in the framework of Hořava-Lifshitz cosmology. For the two cases containing noninteracting and interacting entropy-corrected NADE (ECNADE) models, we derive the exact differential equation that determines the evolution of the ECNADE density parameter. Also the deceleration parameter is obtained. Furthermore, using a parametrization of the equation of state parameter of the ECNADE model as $ω_Λ(z)=ω_0+ω_1 z$, we obtain both $ω_0$ and $ω_1$. We find that in the presence of interaction, the equation of state parameter $ω_0$ of this model can cross the phantom divide line which is compatible with the observation.

hep-th↗

arXiv2011

Holographic dark energy in Brans-Dicke theory with log correction

In the derivation of holographic dark energy density, the area law of the black hole entropy plays a crucial role. However, the entropy-area relation can be modified from the inclusion of quantum effects, motivated from the loop quantum gravity, string theory and black hole physics. In this paper, we study cosmological implication of the interacting entropy-corrected holographic dark energy model in the framework of Brans-Dicke cosmology. We obtain the equation of state and the deceleration parameters of the entropy-corrected holographic dark energy in a non-flat Universe. As system's IR cutoff we choose the radius of the event horizon measured on the sphere of the horizon, defined as $L=ar(t)$. We find out that when the entropy-corrected holographic dark energy is combined with the Brans-Dicke field, the transition from normal state where $w_D >-1 $ to the phantom regime where $w_D <-1 $ for the equation of state of interacting dark energy can be more easily achieved for than when resort to the Einstein field equations is made.

physics.gen-ph↗