arXiv subjects

Kayoomars Karami

Publications and source records attributed to Kayoomars Karami.

At least 19 recordsLinked to original sources

arXiv2026

LQC inverse volume corrections inflation driven by fractional power law potentials in light of ACT observations

Here, we investigate the observational viability of fractional power law inflationary potentials, $V(\varphi) \propto \varphi^n$ ($n=$ 1/3, 2/5, 2/3), within the effective framework of Loop Quantum Cosmology (LQC) incorporating inverse volume corrections. By employing LQC modifications to the background dynamics and cosmological perturbation equations in the semi-classical regime, we analytically derive the scalar spectral index $n_{\rm s}$ and the tensor-to-scalar ratio $r$. These theoretical predictions are confronted with the latest high precision joint observational constraints, including ACT DR6, Planck 2018, DESI BAO, and BICEP/Keck datasets (P-ACT-LB-BK18). While steeper classical power law models are in severe tension with modern data, our results demonstrate that the inclusion of LQC inverse volume effects induces a prominent negative shift in $n_{\rm s}$. Consequently, for specific viable ranges of the quantum geometric parameters ($\sigma$ and $\delta$), the theoretical predictions are translated horizontally across the $r-n_{\rm s}$ plane. This mechanism successfully steers classically disfavored models back into the tightly constrained 68\% and 95\% CLs, significantly improving their consistency with precision cosmological data.

hep-th↗

arXiv2025

Revisited apparent horizon entropy and GSL in modified gravity

This work presents a universal and revisited formalism for the entropy of the apparent horizon in modified gravity to investigate the validity of the Generalized Second Law (GSL) of thermodynamics. This revisited horizon entropy is constructed directly from the modified Friedmann equations in a Friedmann-Robertson-Walker (FRW) universe. The resulting entropy relation contains, beside the standard Bekenstein-Hawking term, an additional integral contribution that encodes the effective energy density and pressure generated by deviations from general relativity. Using this universal entropy formula, a compact expression for the GSL is derived. This formalism is then applied to some viable $f(T)$ and $f(R)$ gravity models, in order to re-evaluate the validity of the GSL as a function of redshift. The analysis demonstrates that including the integral term in the revisited entropy can relatively improve the late-time validity of the GSL for some of these models while living others unchanged, thereby reinforcing the profound connection between thermodynamics and gravity.

gr-qc↗

arXiv2025

Structure formation in a non-canonical scalar field model of clustering dark energy

This paper examines the growth of dark matter and dark energy perturbations within a non-canonical scalar field model characterized by an exponential potential. Through dynamical system analysis, we identify critical points and track the background evolution of a spatially flat FLRW universe dominated by dark energy and pressureless dark matter. We systematically derive key cosmological quantities, including the Hubble parameter, deceleration parameter, density parameters, and the scalar field's equation of state, and explore their dependence on model parameters. Within the linear perturbation framework, employing the pseudo-Newtonian formalism, we compute the growth factor of matter density perturbations. To investigate the non-linear regime of structure formation, we employ the spherical collapse model and derive its key parameters. Building on these findings, we compute the function $f(z)\sigma_8(z)$ and the relative number density of halo objects exceeding a given mass threshold. Our results indicate that non-canonical scalar field models can effectively account for both background cosmic evolution and the growth of structure, offering potential insights into observational constraints and large-scale dynamics.

gr-qc↗

arXiv2025

Loop quantum inflation with inverse volume corrections in light of ACT data

Within the framework of loop quantum cosmology (LQC), we investigate the effect of inverse volume corrections on the low scale spontaneously broken supersymmetric (SB SUSY) and exponential inflationary potentials. The LQC modifications to the Friedmann equations and cosmological perturbation parameters are employed to assess the observational viability of these models against recent data from the Atacama Cosmology Telescope (ACT). Our results indicate that in contrary to the standard model of inflation, in the presence of inverse volume corrections in LQC, the prediction of SB SUSY and exponential potentials in the $r-n_{\rm s}$ plane lie inside the 68\% confidence level interval of the ACT data.

gr-qc↗

arXiv2025

Reheating and relic gravitational waves as remedies for degeneracies of non-canonical natural inflation

Here, a natural non-canonical inflationary model based on a power-law Lagrangian is investigated. We analyze the scalar spectral index $n_{\rm s}$ and the tensor-to-scalar ratio $r$ of the model and identify their degeneracies with respect to the free parameters. Notably, $n_{\rm s}$ and $r$ show effective independence from the model parameters due to degeneracies in the slow-roll parameters that leads to unresolved parameter degeneracies. Employing the constraints on reheating parameters such as the reheating duration $N_{\rm{reh}}$, the reheating temperature $T_{\rm{reh}}$, and the equation of state parameter $\omega_{\rm{reh}}$, is found to be insufficient to fully break these degeneracies. However, the relic gravitational wave spectrum provides a way to break degeneracy with respect to the non-canonical parameter $\alpha$, degeneracy with respect to the potential parameter $f$ persist. Finally, we specify the allowed ranges for the inflationary duration $N$ and the parameter $\alpha$, in light of the latest observational data. These results highlight the role of relic gravitational waves in refining inflationary models and illustrate the challenges in fully resolving parameter degeneracies.

gr-qc↗

arXiv2025

$\alpha$-attractor inflation modified by GUP in light of ACT observations

Here, the $\alpha$-attractor inflation is investigated within a framework incorporating a minimal measurable length, as implemented by the Generalized Uncertainty Principle (GUP). The GUP modifications to the Friedmann equations and cosmological perturbation parameters are employed to assess the model observational viability against the Atacama Cosmology Telescope (ACT) data. Our results indicate that in the $r-n_s$ plane, the predictions of the standard $\alpha$-attractor model ($\beta=0$) lies near the $2\sigma$ boundary of joint observations. More interestingly enough is that in the presence of GUP effect, the predictions of the model for the GUP parameter $\beta \gtrsim O(10^{13})$ shifts into the $68\%$ CL interval. This value for $\beta$ is in well agreement with upper bounds on the GUP parameter deduced from cosmological analysis as well as quantum and gravitational experiments.

astro-ph.CO↗

arXiv2024

Mutated hilltop inflation in light of Planck/ACT observations

Here, a single field inflationary model driven by a mutated hilltop potential, a subclass of the hilltop models of inflation, is investigated. To constrain the parameter space, we employ the latest $r-n_{\rm s}$ constraints from Planck 2018, BICEP/Keck 2018, and the Atacama Cosmology Telescope (ACT) data, alongside reheating parameters $N_{\rm{re}}$, $T_{\rm{re}}$, and $\omega_{\rm{re}}$, and the model independent bound on the radiation dominated (RD) era $N_{\rm{rd}}$. Furthermore, the relic gravitational wave (GW) spectrum within the sensitivity domains of future GW detectors are analyzed. By combining CMB, reheating, RD era, and GW constraints, we find for the Planck+BK18 data that the inflationary duration is confined to $46 \leq N \leq 56$ (95\% CL) and $48.1 \leq N \leq 56$ (68\% CL). Moreover, the model parameter $\alpha$ is confined to $0.161 \leq \alpha \leq 0.890$ (95\% CL) and $0.217 \leq \alpha \leq 0.815$ (68\% CL). Inclusion of the ACT data further tighten the constraints to $54 \leq N \leq 56$ (95\% CL) and $0.29 \leq \alpha \leq 0.62$ (95\% CL), thereby enhancing the precision and robustness of the model predictions.

astro-ph.CO↗

arXiv2024

Primordial black holes generated by fast-roll mechanism in non-canonical natural inflation

In this work, a new fast-roll (FR) mechanism to generate primordial black holes (PBHs) and gravitational waves (GWs) in generalized non-canonical natural inflation is introduced. In this model, choosing a suitable function for non-canonical mass scale parameter $M(\phi)$ gives rise to produce a cliff-like region in the field evolution path. When inflaton rolls down the steep cliff, its kinetic energy during a FR stage increases in comparison with a slow-roll (SR) stage. Hence, seeds of PBH production are born in this transient FR stage. Depending on the position of the cliff, appropriate cases of PBHs for explaining total dark matter (DM), microlensing effects, LIGO-VIRGO events and NANOGrav 15 year data can be formed. The density spectrum of GWs related to one case of the model lies in the NANOGrav 15 year domain and behaves like $\Omega_{\rm GW_0}\sim f^{5-\gamma}$. The spectral index $\gamma=3.42$ for this case satisfies the NANOGrav 15 year constraint. Moreover, regarding reheating considerations, it is demonstrated that PBHs are born in the radiation-dominated (RD) era. Furthermore, viability of the model in light of theoretical swampland criteria and observational constraints on cosmic microwave background (CMB) scales are illustrated.

gr-qc↗

arXiv2024

Breaking the degeneracy of non-canonical quartic inflation by reheating considerations

Here, the quartic inflationary potential $V(\phi)=\frac{\lambda}{4}\phi^4$ within a non-canonical framework characterized by a power-law Lagrangian is investigated. We demonstrate that the predictions of this model align with the Planck 2018 observational data. We explore how the predictions of the model depend on the non-canonical parameter $\alpha$ and the number of $e$-folds $N$. Notably, the sound speed, non-Gaussianity parameter, scalar spectral index, and tensor-to-scalar ratio are all affected by variations in $\alpha$. However, the scalar spectral index exhibits a degeneracy with respect to variation in $\alpha$, which can be broken by incorporating reheating consideration. By applying a combination of theoretical and observational constraints on $(r-n_{\rm s})$, non-Gaussianity, and reheating parameters, we find that the duration of inflation is constrained to the range $55 \leq N\leq55.7$ $e$-folds for $60 \leq \alpha \leq 130$. Finally, we investigate relic gravitational waves and demonstrate that their energy density spectrum falls within the sensitivity range of gravitational waves detectors for this constrained range of $e$-folds.

gr-qc↗

arXiv2024

Primordial black holes in non-minimal Gauss-Bonnet inflation in light of the PTA data

Here, we investigate the formation of primordial black holes (PBHs) in non-minimal coupling Gauss-Bonnet inflationary model in the presence of power-law potentials. We employ a two part coupling function to enhance primordial curvatures at small scales as well as satisfy Planck measurements at the CMB scale. Moreover, our model satisfies the swampland criteria. We find PBHs with different mass scales and demonstrate that PBHs with masses around $\mathcal{O}(10^{-14})M_{\odot}$ can account for almost all of the dark matter in the universe. In addition, we investigate the implications of the reheating stage and show that the PBHs in our model are generated during the radiation-dominated era. Furthermore, we investigate the production of scalar-induced gravitational waves (GWs). More interestingly enough is that, for the specific cases $D_{\rm n}$ in our model, the GWs can be considered as a source of PTA signal. %evaluate the idea that the induced GWs propagating concurrently with the PBH production are the source of NANOGrav signal. Also, we conclude that the GWs energy density parameter at the nano-Hz regime can be parameterized as $\Omega_{\rm GW_0} (f) \sim f^{5-\gamma}$, where the obtained $\gamma$ is consistent with the PTA Observations.

gr-qc↗

arXiv2023

Primordial black holes and secondary gravitational waves from generalized power-law non-canonical inflation with quartic potential

Here, generation of PBHs and secondary GWs from non-canonical inflation with quartic potential have been probed. It is illustrated that, quartic potential in non-canonical setup with a generalized power-law Lagrangian density can source a consistent inflationary era with the latest observational data. Besides, we show that our model satisfies the swampland criteria. At the same time, defining a peaked function of inflaton field as non-canonical mass scale parameter $M(\phi)$ of the Lagrangian, gives rise to slow down the inflaton in a while. In this span, namely Ultra-Slow-Roll (USR) stage, the amplitude of the curvature perturbations on small scales enlarges versus CMB scales. It has been illustrated that, further to the peaked aspect of the chosen non-canonical mass scale parameter, the amount of $\alpha$ parameter of the Lagrangian has enlarging impact on the amplitude of the scalar perturbations. As a consequence of adjusting three parameter Cases of this model, three Cases of PBHs in proper mass scopes to explain LIGO-VIRGO events, microlensing events in OGLE data and DM content in its totality, could be produced. In the end, power-law behavior of the current density parameter of gravitational waves $\Omega_{\rm GW_0}$ in terms of frequency has been examined. Also, the logarithmic power index as $n=3-2/\ln(f_c/f)$ in the infrared regime is obtained.

gr-qc↗

arXiv2023

Primordial black holes in scalar field inflation coupled to the Gauss-Bonnet term with fractional power-law potentials

In this study, we investigate the formation of primordial black holes (PBHs) in a scalar field inflationary model coupled to the Gauss-Bonnet (GB) term with fractional power-law potentials. The coupling function enhances the curvature perturbations, then results in the generation of PBHs and detectable secondary gravitational waves (GWs). % We identify three separate sets of parameters for the potential functions of the form $\phi^{1/3}$, $\phi^{2/5}$, and $\phi^{2/3}$. By adjusting the model parameters, we decelerate the inflaton during the ultra slow-roll (USR) phase and enhance curvature perturbations. % Our calculations predict the formation of PBHs with masses of ${\cal O}(10)M_{\odot}$, which are compatible with LIGO-Virgo observational data. Additionally, we find PBHs with masses around ${\cal O}(10^{-6})M_{\odot}$ and ${\cal O}(10^{-5})M_{\odot}$, which can explain ultrashort-timescale microlensing events in OGLE data. % Furthermore, our proposed mechanism could lead to the formation of PBHs in mass scales around ${\cal O}(10^{-14})M_{\odot}$ and ${\cal O}(10^{-13})M_{\odot}$, contributing to approximately 99\% of the dark matter in the universe. % We also study the production of secondary GWs in our model. In all cases of the model, the density parameter of secondary GWs $\Omega_{\rm GW_0}$ exhibits peaks that intersect the sensitivity curves of GWs detectors, providing a means to verify our findings using data of these detectors. % Our numerical results demonstrate a power-law behavior for the spectra of $\Omega_{\rm GW_0}$ with respect to frequency, given by $\Omega_{\rm GW_0} (f) \sim (f/f_c)^{n}$. Additionally, in the infrared regime where $f\ll f_{c}$, the power index takes a log-dependent form, specifically $n=3-2/\ln(f_c/f)$.

astro-ph.CO↗

arXiv2023

Primordial black holes in non-canonical scalar field inflation driven by quartic potential in the presence of bump

Here, generation of Primordial Black Holes (PBHs) from quartic potential in the presence of a tiny bump in non-canonical inflationary model has been inquired. It is demonstrated that, a viable inflationary era can be driven through the quartic potential in non-canonical framework with a power-law Lagrangian density. Furthermore, setting a suitable function of inflaton field as a correction term (like a bump) to the quartic potential, causes the inflaton to slow down for a while. In such a short time span, the amplitude of the scalar perturbations power spectrum on small scales grows up sufficiently versus CMB scales. In addition to the bump feature, the enhancing effect of the $\alpha$ parameter of the Lagrangian on the amplitude of the scalar power spectrum has been shown. Fine tuning of three parameter Cases of the model results in generating of three Cases of PBHs. In addition, we investigate the secondary Gravitational Waves (GWs) produced during generation of PBHs and show that their contemporary density parameter spectra $(\Omega_{\rm GW_0})$ can be tracked down by GWs detectors.

astro-ph.CO↗

arXiv2023

Generalized second law of thermodynamics in massive gravity

Here, we study the generalized second law (GSL) of thermodynamics in the framework of massive gravity. To do this, we consider a FRW universe filled only with matter and enclosed by the apparent horizon. In addition, we consider two models including generalized massive gravity (GMG) as well as dRGT massive gravity on de Sitter. For both models, we first study the dynamics of background cosmology and then explore the validity of GSL. We conclude that for the selected values of model parameters the GSL is respected.

gr-qc↗

arXiv2023

The Effect of Flow and Magnetic Twist on Resonant Absorption of Slow MHD Waves in Magnetic Flux Tubes

Observations show that there are twisted magnetic flux tubes and plasma flow throughout the solar atmosphere. The main purpose of this work is to obtain the damping rate of sausage modes in the presence of magnetic twist and plasma flow. We obtain the dispersion relation for sausage modes in slow continuity in an inhomogeneous layer under the conditions of magnetic pores, then we solve it numerically. For the selected density profile, the magnetic field, and the plasma flow as a function of radius across the inhomogeneous layer, we show that the effect of the twisted magnetic field on the resonance absorption at low speed of the plasma flow is greater than one at high speed.

astro-ph.SR↗

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

Primordial black holes ensued from exponential potential and coupling parameter in nonminimal derivative inflation model

Here, Primordial Black Holes (PBHs) creation from exponential potential has been inquired, through gravitationally raised friction emanated from the nonminimal coupling between gravity and field derivative setup. Setting a two-parted exponential function of inflaton field as coupling parameter, and fine-tuning of four parameter cases of our model, we could sufficiently slow down the inflaton owing to high friction during an ultra slow-roll phase. This empowers us to achieve enough enhancement in the amplitude of curvature perturbations power spectra, via numerical solving of Mukhanov-Sasaki equation. Thereafter, we illustrate the generation of four PBHs with disparate masses in RD era, corresponding to our four parameter cases. Two specimens of these PBHs with stellar ${\cal O}(10)M_{\odot}$ and earth ${\cal O}(10^{-6})M_{\odot}$ masses can be appropriate to explicate the LIGO-VIRGO events, and the ultrashort-timescale microlensing events in OGLE data, respectively. Another two cases of PBHs have asteroid masses around ${\cal O}(10^{-13})M_{\odot}$ and ${\cal O}(10^{-15})M_{\odot}$ with abundance of $96\%$ and $95\%$ of the Dark Matter (DM) content of the universe. Furthermore, we scrutinize the induced Gravitational Waves (GWs) ensued from PBHs production in our model. Subsequently, we elucidate that their contemporary density parameter spectra $(\Omega_{\rm GW_0})$ for all predicted cases have acmes which lie in the sensitivity scopes of the GWs detectors, thereupon the verity of our conclusions can be verified in view of deduced data from these detectors. At length, our numerical outcomes exhibit a power-law behavior for the spectra of $\Omega_{\rm GW_0}$ with respect to frequency as $\Omega_{\rm GW_0} (f) \sim (f/f_c)^{n} $ in the proximity of acmes position. As well, in the infrared regime $f\ll f_{c}$, the log-reliant form of power index as $n=3-2/\ln(f_c/f)$ is attained.

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↗