arXiv subjects

M. A. Ramzanpour

Publications and source records attributed to M. A. Ramzanpour.

11 recordsLinked to original sources

arXiv2026

Interacting Holographic Dark Energy in $f(Q)$ Gravity: Cosmological Evolution and Gravitational Wave Signatures

In this paper, an interactive Holographic Dark Energy (HDE) model is studied in the framework of modified gravity $f(Q)$. By adopting a power parameterization for the Hubble parameter, the field equations are reconstructed and the evolution of the universe at the background level and tensor perturbations are investigated. Then, using observational data $H(z)$, the model parameters are constrained and the dynamical behavior of dark energy throughout the history of the universe is analyzed. Also, the study of the evolution of energy density, pressure and the Equation of State (EoS) parameter of dark energy shows that dark energy in the late universe naturally tends to a region close to the cosmological constant behavior, while in the past it followed a distinct dynamical evolution. Stability analysis based on the speed of sound also indicates that the model has good classical stability around the present era. In addition, the compatibility of the current values of the relative density parameters of matter and dark energy with the observational constraints confirms the ability of the model to reproduce the main features of the observed universe. Next, the propagation of gravitational waves in the cosmological context of the model is investigated. The results show that the corrections due to $f(Q)$ gravity and the interaction between matter and dark energy can affect the evolution of tensor perturbations and produce signatures distinct from the standard scenario. Overall, the findings of this study indicate that the interactive HDE in the gravitational framework $f(Q)$ can provide a consistent framework for describing the cosmic acceleration and studying the cosmological consequences of gravitational waves.

gr-qc↗

arXiv2025

Stability and Thermodynamics of a Generalized Power-Law Dark Energy Model

We investigate a generalized power-law dark energy equation of state of the form $p = wρ- βρ^m$ in a flat FLRW universe, analyzing its dynamical stability and thermodynamic consistency. The model exhibits a rich phase space structure, with an effective cosmological constant $ρ^* = [(1+w)/β]^{1/(m-1)}$ emerging as a stable attractor for $(w < -1,~ m > 1)$. Notably, the universe evolves from an early de Sitter phase ($w \to -1$) to a late-time de Sitter-like one with phantom crossing ($w(z) < -1$), aligning with DESI observations. Dynamical analysis reveals that the $m > 1$ regime avoids ghost instabilities while accommodating phantom behavior, with $m = 2$ providing particular theoretical advantages. Thermodynamically, the Generalized Second Law holds when the null energy condition $ρ+ p \geq 0$ is satisfied, which naturally occurs for $ρ\geq ρ^*$. The model's compatibility with both observational data and fundamental thermodynamic principles suggests it as a viable framework for describing late-time cosmic acceleration, resolving tensions associated with phantom crossing while maintaining entropy dominance of the cosmological horizon.

gr-qc↗

arXiv2025

Correspondence between particle creation and dark components interaction in the context of $f(\mathcal{G})$ gravity

In this paper, we explore the particle creation scenario in the context of $f(\mathcal{G})$ gravity in flat-FLRW metric. For this purpose, from the perspective of thermodynamics and considering an adiabatic universe, we obtain the modified continuity equation in terms of the dynamic number of particles $N$. On the other hand, we obtain Friedmann's equations in $f(\mathcal{G})$ gravity and then write down the continuity equations of the components of matter and dark energy, taking into account the interaction between them. In what follows, by establishing a correspondence between the particle production scenario and $f(\mathcal{G})$ gravity, we obtain the cosmological parameters in terms of $N$. After that, we find cosmological solutions using power-law cosmology and compare them with Hubble parameter data. Finally, we fit the current model with Hubble data and plot the best fit in terms of the redshift parameter.

gr-qc↗

arXiv2024

Modified Uncertainty Principle with Cosmological Constant: More Insights on Dark Energy and Chandrasekhar Limit

Numerous studies have shown that generalized uncertainty principle (GUP) removes the Chandrasekhar limit, which can be restored using a negative GUP parameter. This study indicates that observational phantom dark energy also requires an extended uncertainty principle (EUP) parameter with the opposite sign. Altering the signs of the GUP and EUP parameters without a physical rationale is questionable. We demonstrate that incorporating cosmological constant in a modified uncertainty principle (MUP) can address the sign change in GUP and EUP within a unified framework. The main advantage of MUP is that the sign change of the cosmological constant is acceptable and geometrically meaningful. To achieve this, we first derive a modified equation of state from the MUP framework and second test it with observational data for dark matter and dark energy. Importantly, the proposed MUP parameter, which is proportional to the cosmological constant with positive and negative signs, aligns with dark energy observations and restores the Chandrasekhar limit for stars. Finally, we will show that the Chandrasekhar mass limit provides an upper bound of $\leq 10^{-32}{\rm m^{-2}}$ for the cosmological constant, consistent with the observational value of $Λ_{\rm obs}=10^{-52}{\rm m^{-2}}$.

gr-qc↗

arXiv2024

Massive Dirac particles based on gapped graphene with Rosen-Morse potential in a uniform magnetic field

We explore the gapped graphene structure in the two-dimensional plane in the presence of the Rosen-Morse potential and an external uniform magnetic field. In order to describe the corresponding structure, we consider the propagation of electrons in graphene as relativistic fermion quasi-particles, and analyze it by the wave functions of two-component spinors with pseudo-spin symmetry using the Dirac equation. Next, to solve and analyze the Dirac equation, we obtain the eigenvalues and eigenvectors using the Legendre differential equation. After that, we obtain the bounded states of energy depending on the coefficients of Rosen-Morse and magnetic potentials in terms of quantum numbers of principal $n$ and spin-orbit $k$. Then, the values of the energy spectrum for the ground state and the first excited state are calculated, and the wave functions and the corresponding probabilities are plotted in terms of coordinates $r$. In what follows, we explore the band structure of gapped graphene by the modified dispersion relation and write it in terms of the two-dimensional wave vectors $K_x$ and $K_y$. Finally, the energy bands are plotted in terms of the wave vectors $K_x$ and $K_y$ with and without the magnetic term.

cond-mat.mes-hall↗

arXiv2024

Incorporating the Cosmological Constant in a Modified Uncertainty Principle

This study explores the cosmological constant problem and modified uncertainty principle within a unified framework inspired by a void-dominated scenario. In a recent paper~\cite{Yusofi:2022hgg}, voids were modeled as spherical bubbles of similar average sizes, and the surface energy on the voids' borders was calculated across various scales in a heuristic manner. We show that this results in a significant discrepancy of approximately $\mathcal{O}(+122)$ between the cosmological constant values from the minimum to the maximum radii of bubbles. Furthermore, when considering the generalized form of the uncertainty principle with both minimum and maximum lengths, i.e. $ΔX ΔP \geq \frac{\hbar}{2} \frac{1}{1- βΔP^2} \frac{1}{1- αΔX^2}$, a similar order of discrepancy is observed between $α_{\rm max}$ and $α_{\rm min}$, indicating that $α\proptoβ^{-1}\proptoΛ\propto{\rm length}^{-2}~(m^{-2})$. As a primary outcome of this finding, we offer a novel uncertainty principle that incorporates a non-zero cosmological constant.

gr-qc↗

arXiv2022

Cosmological Constant Problem and $H_0$ Tension in Void-dominated Cosmology

In this work, we study the cosmological constant problem and Hubble tension in the void-dominated cosmology scenario \cite{Yusofi:2022hgg}. For this goal, we will first consider the cosmic voids in the cosmic web as interconnected ideal spherical bubbles at the early Planck scale and late large scale. By heuristic calculations for each cosmic void, we obtain particular mass density and cosmological constant that are the same order of magnitude as the entire universe on any scale. The values obtained for the cosmological constant vary from scale to scale. As a result, it will be shown that there is a roughly $\sim 10^{22}$ difference between the cosmological constant at the present and Planck scales. Finally, it will be shown that the slight difference between the surface tension of the cosmic bubbles may explain the tension between local and global measurements of $H_{\rm 0}$.

astro-ph.CO↗

arXiv2022

Observational Hubble parameter data constraints on the interactive model of $f(T)$ gravity with particle creation

In this paper, we consider an open system from the thermodynamic perspective for an adiabatic FRW universe model in which particle creation occurs within the system. In that case, the modified continuity equation is obtained and then we correspond it to the continuity equation of $f(T)$ gravity. So, we take $f(T)$ gravity with the viscous fluid in flat-FRW metric, in which $T$ is the torsion scalar. We consider the contents of the universe to be dark matter and dark energy and consider an interaction term between them. The interesting point of this study is that we make equivalent the modified continuity equation resulting from the particle creation with the matter continuity equation resulting from $f(T)$ gravity. The result of this evaluation creates a relationship between the number of particles and the scale factor. In what follows, we write the corresponding cosmological parameters in terms of the number of particles and also reconstruct the number of particles in terms of the redshift parameter, then We parameterize the Hubble parameter derived from power-law cosmology with 51 data from the Hubble observational parameter. Next, we plot the corresponding cosmological parameters for the dark energy in terms of the redshift to investigate the accelerated expansion of the universe. In addition, by using the sound speed parameter, we discuss the stability analysis and instability analysis of the present model in different eras of the universe. Finally, we plot the density parameter values for dark energy and dark matter in terms of the redshift parameter.

gr-qc↗

arXiv2021

Extended Bose-Einstein condensate dark matter in viscous Gauss-Bonnet gravity

In this paper, we study the $F(R, G)$ gravity model with an interacting model by flat-FRW metric in a viscous fluid. We consider that the universe dominates with components of dark matter and dark energy. This means that the dark matter component derives from Extended Bose-Einstein Condensate (EBEC) and the components of dark energy arise from the $F(R, G)$ gravity. After obtaining the Einstein equation, the energy density and the pressure of dark energy are written in terms of the geometries of the curvature and the Gauss-Bonnet terms, and components of dark matter and viscous fluid. Also, the corresponding continuity equations are written with the presence of interaction terms. In what follows, we employ the EBEC regime instead of the normal dark matter by the dark matter Equation of State (EoS) as $p_{dm} = αρ_{dm} + βρ_{dm}^2$, which arises from the gravitational form. The EoS can be expressed from the perspective of the virial expansion, in which the first and second terms represent normal dark matter and quantum ground state. Next, the corresponding Friedmann equations reconstruct in terms of the redshift parameter, then by using the scenario of the power-law cosmology for the scale factor, we fit the present model with the Hubble amounts of 51 supernova data by the likelihood analysis. In that case, we acquire the cosmological parameters of dark energy in terms of the redshift parameter, and by plotting these graphs, we see that the universe is currently undergoing an accelerated expansion phase. Finally, we investigate the stability of the present model with the sound speed parameter.

gr-qc↗

arXiv2021

Viscous interacting and stability on dark matter Bose-Einstein condensation with modified Chaplygin gas

In this paper, the viscous cosmological dynamics are studied in the presence of dark matter Bose-Einstein Condensation (BEC) by curved-FRW background. For this purpose, we use the BEC regime rather than the normal dark matter (the cold dark matter or the barotropic dark matter) with the dark matter Equation of State (EoS) as $p_{dm} \propto ρ_{dm}^2$, which arises from the gravitational form. Therefore, we obtain the corresponding continuity equations with the existence of the universe components by considering an interacting model with modified Chaplygin gas. Afterward, we derive the energy density and the pressure of dark energy in terms of the redshift parameter. And then, by introducing a parametrization function and fitting it with 51 supernova data with the likelihood analysis, we find the cosmological parameters versus redshift parameter. In what follows, we plot the corresponding dynamic graphs proportional to redshift, and then we represent the universe is currently undergoing an accelerated expansion phase. Finally, we explore the stability and the instability of the present model with the sound speed parameter.

gr-qc↗

arXiv2020

Observational constraints and stability in viscous $f(T,\mathcal{T})$ gravity

In this paper, we study the $f(T,\mathcal{T})$ gravity model in the presence of the bulk viscosity by the flat-FRW metric. The field equation is obtained by teleparallel gravity with tetrad field. The universe components are considered as matter and dark energy which the dark energy component associates from the viscous $f(T,\mathcal{T})$ gravity. After calculating the Friedmann equations, we obtain the energy density, the pressure and the EoS of dark energy in terms of the redshift parameter. Afterward, we plot the corresponding cosmological parameters versus the redshift parameter and examine the accelerated expansion of the universe. In the end, we explore the system stability by a function called the speed sound parameter.

gr-qc↗