arXiv subjects

B. Malekolkalami

Publications and source records attributed to B. Malekolkalami.

10 recordsLinked to original sources

arXiv2026

Barrow Holographic Dark Energy with a Ricci Cutoff in Power-Law $f(Q,L_m)$ Gravity: Late-Time Cosmological Dynamics

We investigate late-time Barrow holographic dark energy (BHDE) with Ricci cutoff in $f(Q,L_m)=Q+αQ^n-2L_m$ gravity. The $Q^n$ term modifies the symmetric-teleparallel geometry, while $Δ$ deforms holographic energy via entropy-area relation. Linear matter Lagrangian allows separate identification of geometric and holographic effects. Modified Friedmann equations with Ricci-cutoff BHDE yield a closed first-order equation for $E(z)=H(z)/H_0$, solved numerically. The model evolves from matter domination to late-time acceleration. Increasing $n$ makes $w_{\mathrm{DE},0}$ less negative, raises $z_t$, lowers $j_0$; increasing $Δ$ makes $w_{\mathrm{DE},0}$ and $q_0$ more negative, raises $z_t$ and $j_0$. Calibrating $\mathcal{C}_{B,\mathrm{ref}}$ with $q_{0,\mathrm{ref}}\simeq -0.527$ gives $w_{\mathrm{DE},0}\simeq -1.003$, $z_t\simeq0.442$, $j_0\simeq1.486$. Two-dimensional $(n,Δ)$ maps show opposite effects on $w_{\mathrm{DE},0}$ but same direction on acceleration onset. We compare expansion histories with 36 cosmic-chronometer measurements ($0.07\leq z\leq1.965$), using DESI covariance and asymmetric error at $z=0.8$. Comparison is conditional for fixed trajectories, without fitting; Hubble statistic varies modestly. Results provide a consistent background realization of Ricci-cutoff BHDE in $f(Q,L_m)$ gravity and motivate future perturbation and multi-probe studies.

physics.gen-ph↗

arXiv2026

Geodesics Structure and Light Deflection of Regular Phantom Black Hole

The geodesic structure of Regular Phantom Black Holes (\textbf{RPBH}) space--time is analyzed and discussed in three asymptotically cases: Flat, de Sitter (\textbf{dS}), and Anti--de Sitter (\textbf{AdS}). The impact of the important scale parameter $b$\footnote{This parameter determines the coupling strength between phantom field and gravity.} on the trajectory of particles is studied and investigated which can mimic repulsion or attraction. By virtue of Effective Potential (\textbf{EP}) tool, the circular orbits and their stability are discussed. Also, in the asymptotically flat spacetime, the angle of light deflection versus the scale parameter is presented.

gr-qc↗

arXiv2026

Geodesics Structure and Thermodynamic Properties of Gaussian Black Hole in Quadratic Ricci Scaler Gravity

The geodesic structure and thermal properties of Gaussian Black Holes (\textbf{GBH})s in modified and Einstein gravities are studied and compared. In the geodesic part, motion of a test particle (massive and massless) are discussed, specially properties of the circular motion are considered. In the thermodynamic part, the mass, entropy and temperature functions are considered and discussed. The local and global stability is also analyzed through the Heat Capacity (\textbf{HC}) and Gibbs Energy (\textbf{GE}). The results show the thermodynamic differences are more than geodesic ones in the two theories of gravity with the note that the modified gravity is more consistent with the physical world.

gr-qc↗

arXiv2024

Observational constraints on FLRW, Bianchi type I and V brane models

This study explores the compatibility of Covariant Extrinsic Gravity (CEG) with current cosmological observations. We employ the chi-square statistic and Markov Chain Monte Carlo (MCMC) methods to fit the FLRW and Bianchi type-I and V brane models to the latest datasets, including Hubble, Pantheon+ Supernova samples, Big Bang Nucleosynthesis (BBN), Baryon Acoustic Oscillations (BAO), and the structure growth rate, $fσ_8(z)$. Parameters for FLRW universe consist $\left(Ω^{\text{(b)}}_0, Ω^{\text{(cd)}}_0, Ω^{\text{(k)}}_0, H_0, γ, σ_8\right)$, while for the Bianchi model are $\left(Ω^{\text{(b)}}_0, Ω^{\text{(cd)}}_0, Ω^{(β)}_0, H_0, γ, Ω^{(θ)}_0, σ_8\right)$. We determine the best values for cosmological parameters. For the FLRW model, these values depend on the sign of $γ$: $γ> 0$ yields $γ=0.00008^{+0.00015}_{-0.00011}$, and $Ω^{\text{(k)}}_0=0.014^{+0.024}_{-0.022}$ and $γ< 0$ leads to $γ=-0.0226^{+0.0054}_{-0.0062}$, and $Ω^{\text{(k)}}_0=0.023^{+0.039}_{-0.041}$. In both cases $Ω^{\text{(k)}}_0>0$ represents a closed universe. Similarly, for the Bianchi type-V brane model, the parameter values vary with the sign of $γ$, resulting in $γ= 0.00084^{+0.00019}_{-0.00021}$, $Ω^{(β)}_0 =0.0258^{+0.0052}_{-0.0063} $, and $Ω^θ_0(\times 10^{-5} ) = 4.19^{+0.67}_{-0.75}$ (as with the density parameter of stiff matter) for $γ> 0$, and $γ= -0.00107^{+0.00019}_{-0.00020}$, $Ω^{(β)}_0 = 0.0259^{+0.0050}_{-0.0062} $, and $Ω^θ_0(\times 10^{-5} ) = 4.17^{+0.91}_{-0.98}$ for $γ< 0$. In both cases $Ω^{(β)}_0>0$, which represents the Bianchi type-V, because in the Bianchi type-I, $β=0$. Utilizing these obtained best values, we analyze the behavior of key cosmological parameters.

gr-qc↗

arXiv2023

The null geodesics of charged and non-charged black hole in mimetic gravity

The null geodesics around the charged black hole spacetimes are investigated in the mimetic gravity framework when Einstein's gravity is coupled to a nonlinear electromagnetic field. The photon paths in nonlinear electrodynamics are geodesics of the effective metric which is determined by the background metric and the particular nonlinear theory considered. The nonlinear effects are represented by a quadruple moment and appear as a correction term to Reissner Nordstrom (RN) metric and Reissner Nordstrom-anti-de Sitter (RN-(A)dS) in cylindrical metric. Remarkably, the nonlinear effects prevent the circular orbits around black holes, and the presence of the mimetic field can change the repulsive character of dS space to the attractive one. Also, the effect of the mimetic parameter manifests itself in the stronger or weaker gravity of the black hole.

gr-qc↗

arXiv2023

Probing extra dimensions through cosmological observations of dark energy

We investigate the isometrically embedded Bianchi type-V cosmology braneworld model in a $D$-dimensional bulk space. The model provides a fluid of geometric dark energy (GDE) and unification of fundamental forces similar to the Kaluza--Klein (KK) theory. The Planck energy density, the fine structure constant, the muon mass, and the number of extra dimensions are all factors that determine the density of the induced GDE. The model also predicts that graviton has mass, which is determined by the induced cosmological constant (CC). Our results are compatible with observations of the standard model of cosmology if the Universe has 22 non-compact extra dimensions. Our model provides an alternative method for probing extra dimensions.

gr-qc↗

arXiv2022

Structure Formation in Dark Matter Particle Production Cosmology

We investigate a cosmological scenario in which the dark matter particles can be created during the evolution of the Universe. By regarding the Universe as an open thermodynamic system and using non-equilibrium thermodynamics, we examine the mechanism of gravitational particle production. In this setup, we study the large-scale structure (LSS) formation of the Universe in the Newtonian regime of perturbations and derive the equations governing the evolution of the dark matter overdensities. Then, we implement the cosmological data from Planck 2018 CMB measurements, SNe Ia and BAO observations, as well as the Riess et al. (2019) local measurement for $H_0$ to provide some cosmological constraints for the parameters of our model. We see that the best case of our scenario ($χ_{\rm tot}^{2}=3834.40$) fits the observational data better than the baseline $Λ$CDM model ($χ_{\rm tot}^{2} = 3838.00$) at the background level. We moreover estimate the growth factor of linear perturbations and show that the best case of our model ($χ_{fσ_{8}}^{2}=39.85$) fits the LSS data significantly better than the $Λ$CDM model ($χ_{fσ_{8}}^{2}=45.29$). Consequently, our model also makes a better performance at the level of the linear perturbations compared to the standard cosmological model.

astro-ph.CO↗

arXiv2020

Cosmological constraints on dark matter particle production rate

Gravitational particle production has been investigated by using Einstein's gravitational field equations in the presence of a cosmological constant. To study the mechanism of particle creation, the Universe has been considered as a thermodynamics system and non-equilibrium thermodynamics has been employed. In order to estimate the cosmological parameters with observational data, including SNe Ia, BAO, Planck 2015 and HST, we have chosen a phenomenological approach for the rate of particle creation. A non-zero particle production rate was obtained implying that the possibility of the particle production is consistent with recent cosmological observations. In the 1 sigma confidence interval, the ratio of Gamma/3H0 was obtained to be 0.0835.

gr-qc↗

arXiv2017

Quartic time-dependent oscillatons

In this paper, we will study some properties of oscillaton, spherically symmetric object made of a real time-dependent scalar field, Using a self- interaction quartic scalar potential instead of a quadratic or exponential ones discussed in previous works. Since the oscillatons can be regarded as models for astrophysical objects which play the role of dark matter, there- fore investigation of their properties has more importance place in present time of physics; research. Therefore we investigate the properties of these objects by Solving the system of differential equations obtained from the Einstein Klein Gordon (EKG) equations and will show their importance as new candidates for the role of dark matter in the galactic scales.

gr-qc↗

arXiv2014

Late time acceleration in a non-commutative model of modified cosmology

We investigate the effects of noncommutativity between the position-position, position-momentum and momentum-momentum of a phase space corresponding to a modified cosmological model. We show that the existence of such noncommutativity results in a Moyal Poisson algebra between the phase space variables in which the product law between the functions is of the kind of an $α$-deformed product. We then transform the variables in such a way that the Poisson brackets between the dynamical variables take the form of a usual Poisson bracket but this time with a noncommutative structure. For a power law expression for the function of the Ricci scalar with which the action of the gravity model is modified, the exact solutions in the commutative and noncommutative cases are presented and compared. In terms of these solutions we address the issue of the late time acceleration in cosmic evolution.

gr-qc↗