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S. Davood Sadatian

Publications and source records attributed to S. Davood Sadatian.

17 recordsLinked to original sources
arXiv2026

Bouncing Scenario in the $f(T)$ Modified Gravity Model with Dynamical System Analysis

In this work, we study nonsingular bouncing cosmology in quadratic modified teleparallel gravity $f(T)=T+βT^{2}$ within a flat FLRW background. Using dimensionless variables, we derive modified Friedmann equations and reformulate the dynamics as a 2D autonomous system. Phase-space analysis identifies saddle, unstable, and stable critical points, with a scalar field-dominated late-time attractor under specific potential conditions. A genuine nonsingular bounce is realized via a regular scale factor, finite and positive throughout, with Hubble parameter satisfying $H(t_b)=0$, $\dot{H}(t_b)>0$, ensuring a smooth transition from contraction to expansion without singularity. The torsion scalar, effective energy density, and geometry remain finite. The deceleration and effective equation of state parameters show a temporary phantom-like regime near the bounce, violating the null energy condition as required. The reconstructed bounce aligns with the dynamical system's qualitative structure, asymptotically approaching a matter-dominated saddle while avoiding unstable stiff-matter solutions. Our results confirm that quadratic $f(T)$ gravity offers a self-consistent and viable framework for regular bouncing cosmology.

gr-qc↗
arXiv2024

Modified Compton Effect and CMB Anisotropy

Recent satellite observations have revealed significant anisotropy in the cosmic microwave background (CMB) radiation, a phenomenon that had previously been detected but received limited attention due to its subtlety. With the advent of more precise measurements from satellites, the extent of this anisotropy has become increasingly apparent. This paper examines the CMB radiation by reviewing past research on the causes of CMB anisotropy and presents a new model to explain the observed temperature anisotropy and the anisotropy in the correlation function between temperature and E-mode polarization in the CMB radiation. The proposed model is based on a modified-generalized Compton scattering approach incorporating Loop Quantum Gravity (LQG). We begin by describing the generalized Compton scattering and then discuss the CMB radiation in the context of processes occurring at the last scattering surface. Our findings are derived from the latest observational data from the Planck satellite (2018). In our model, besides the parameters available in the Planck data for the standard model ($Λ$CDM), we introduce two novel parameters: $δ_{L}$, the density of cosmic electrons, and $M^2$, a parameter related to the modified-generalized Compton scattering effects. The results indicate that, based on the 2018 Planck data, small values were obtained for $δ_{L}$ and $M^{2}$, $δ_{L}=1.63\pm0.08(10^{-13})$ and $M^2=2.28\pm0.34(10^{-4})$, showing no significant deviation from the standard model. Moreover, increasing the values of $δ_{L}$ and $M^{2}$ leads to an increase in the range of fluctuations in the CMB temperature anisotropy power spectrum and the correlation function between temperature and E-mode polarization for multipoles $l<500$ until the first peak.

gr-qc↗
arXiv2024

Modified Gravity Model $f(Q,T)$ and Wormhole Solution

We investigate wormhole solutions using the modified gravity model $f(Q,T)$ with viscosity and aim to find a solution for the existence of wormholes mathematically without violating the energy conditions. We show that there is no need to define a wormhole from exotic matter and analyze the equations with numerical analysis to establish weak energy conditions. In the numerical analysis, we found that the appropriate values of the parameters can maintain the weak energy conditions without the need for exotic matter. Adjusting the parameters of the model can increase or decrease the rate of positive energy density or radial and tangential pressures. According to the numerical analysis conducted in this paper, the weak energy conditions are established in the whole space if $α< 0$, $12.56 < β< 25.12$ or $α> 0$, $β> 25.12$. The analysis also showed that the supporting matter of the wormhole is near normal matter, indicating that the generalized $f(Q,T)$ model with viscosity has an acceptable parameter space to describe a wormhole without the need for exotic matter.

gr-qc↗
arXiv2022

Dark Energy Density and IS (Israel-Stewart) Bulk Viscosity Model

We investigate the thermodynamics of a dark energy bulk viscosity model as a cosmic fluid. In this regard, the two theories of Eckart and Israel-Stewart (IS) are the basis of our work. Therefore, we first investigate the thermodynamics of cosmic fluids in the dark energy bulk viscosity model and the general relationships. Then, we express the thermodynamic relationships of Eckart's theory. Due to the basic equations of Eckart's theory and Friedmann's equations, we consider two states, one is $p=-ρ$ (standard) and the other is $p\neq-ρ$ (non-standard). In the standard state, we define the pressure $(p)$, energy density $(ρ)$, and bulk viscosity coefficient $(ξ)$ of the cosmic fluid in terms of cosmic time and we obtain its relations. We also mention that in this standard state, because of $p=-ρ$, the value of $a(t)$ is zero, so $a(t)$ is not defined in this state. But in the non-standard case $(p\neq-ρ)$ the bulk viscosity coefficient $(ξ)$ is zero and only the scale factor and pressure and energy density of the cosmic fluid is defined. We also consider two states of constant and variable bulk viscosity coefficients and obtain three Hubble constant parameters and scale factors in terms of cosmic time, and energy density in terms of the scale factor. In the state of variable bulk viscosity coefficient, we consider the viscosity coefficient as the power-law from energy density $(ξ=αρ^{s})$, which is $α>0$ and a constant. Following, we discuss the dissipative effects of cosmic fluids and examine the effects of energy density for dark energy in the Israel-Stewart(IS) theory. The results are comprehensively presented in two tables (1) and (2).

gr-qc↗
arXiv2022

Agegraphic Model based on the Generalized Uncertainty Principle

Many models of dark energy have been proposed to describe the universe since the beginning of the Big Bang. In this study, we present a new model of agegraphic dark energy ($NADE$) based on the three generalized uncertainty principles $KMM$ (Kempf, Mangan, Mann), Nouicer and $GUP^{*}$ ( higher orders generalized uncertainty principle).Using the obtained relations from three of types of $GUP$, in the form of three scenarios(Emergent,Intermediate,Logamediate), we consider three different eras of the universe evolution. Also we describe the evolution and expansion of the universe in each subsection. We will plot the obtained relations in these models for better comparatione.

gr-qc↗
arXiv2018

Generalization of the Randall-Sundrum Model Using Gravitational Model $F(T, Θ)$

In this letter, we explore a generalized model based on two scenarios including the Randall-Sundrum model and Gravity model $F(T,Θ)$. We first study the standard Randall-Sundrum Gravitational model and then add a function containing two parameters as torsion and trace energy-momentum tensor to the main action of the model. Next, we derive the equations of the generalized model and obtain a new critical value for the energy density of the brane. The results showed that inflation and the dark energy dominated stage can be realized in this model.

physics.gen-ph↗
arXiv2014

Is Time Inhomogeneous ?

In this article, we discuss probability of inhomogeneous time in high or low energy scale of physics. Consequently, the possibility was investigated of using theories such as varying speed of light (VSL) and fractal mathematics to build a framework within which answers can be found to some of standard cosmological problems and physics theories on the basis of time non-homogeneity.

gr-qc↗
arXiv2013

Considering Late-Time Acceleration in some Cosmological Models

We study two cosmological models: A non-minimally coupled scalar field on brane world model and a minimally coupled scalar field on Lorentz invariance violation model. We compare some cosmological results in these scenarios. Also, we consider some types of Rip singularity solution in both models.

gr-qc↗
arXiv2012

Cosmological aspects of a vector field model

We have studied a DGP-inspired braneworld scenario where the idea of Lorentz invariance violation has been combined into a specifying preferred frame that embed a dynamical normal vector field to brane. We propose the Lorentz violating DGP brane models with enough parameters can explain crossing of phantom divide line. Also we have considered the model for proper cosmological evolution that is according to the observed behavior of the equation of state. In other view point, we have described a Rip singularity solution of model that occur in this model.

gr-qc↗
arXiv2009

Comparison of Frames: Jordan vs Einstein Frame for a Non-minimal Dark Energy Model

We construct a dark energy model where a scalar field non-minimally coupled to gravity plays the role of the dark component. We compare cosmological consequences of this non-minimal coupling of the scalar field and gravity in the spirit of the dark energy paradigm in Jordan and Einstein frames. Some important issues such as phantom divide line crossing, existence of the bouncing solutions and the stability of the solutions are compared in these two frames. We show that while a non-minimally coupled scalar field in the Jordan frame is a suitable dark energy component with capability to realize phantom divide line crossing, its conformal transformation in the Einstein frame has not this capability. The conformal transformation from Jordan frame to Einstein frame transforms the equation of state parameter of the dark energy component to its minimal form with a redefined scalar field and in this case it is impossible to realize a phantom phase with possible crossing of the phantom divide line.

gr-qc↗
arXiv2009

Bouncing Universe with a Nonminimally Coupled Scalar Field on a Moving Domain Wall

We study dynamics of a dark energy component nonminimally coupled to gravity on a moving domain wall. We use this setup to explain late-time accelerated expansion and crossing of the phantom divide line by the equation of state parameter of this non-minimally coupled dark energy component. By analyzing parameter space of the model, we show that this model accounts for accelerated expansion and crossing of the phantom divide line with a suitable fine-tuning of the nonminimal coupling. Then we study the issue of bouncing solutions in this framework.

gr-qc↗
arXiv2008

A Lorentz Invariance Violating Cosmology on the DGP Brane

We study cosmological implications of a Lorentz invariance violating DGP-inspired braneworld scenario. A minimally coupled scalar field and a single, fixed-norm, Lorentz-violating timelike vector field within an interactive picture provide a wide parameter space which accounts for late-time acceleration and transition to phantom phase of the scalar field.

gr-qc↗
arXiv2008

Late-time acceleration and Phantom Divide Line Crossing with Non-minimal Coupling and Lorentz Invariance Violation

We consider two alternative dark energy models: a Lorentz invariance preserving model with a nonminimally coupled scalar field and a Lorentz invariance violating model with a minimally coupled scalar field. We study accelerated expansion and dynamics of equation of state parameter in these scenarios. While a minimally coupled scalar field has not the capability to be a successful dark energy candidate with cosmological constant line crossing, a nonminimally coupled scalar field in the presence of Lorentz invariance or a minimally coupled scalar field with Lorentz invariance violation have this capability. In the later case, accelerated expansion and phantom divide line crossing are the results of interactive nature of this Lorentz violating scenario.

gr-qc↗
arXiv2008

Crossing of the Phantom Divided Barrier with Lorentz Invariance Violating Fields

We study possible crossing of the phantom divided barrier in a Lorentz invariance violating dark energy model. Lorentz invariance violation which is achieved by introducing a vector field in the action, incorporates directly in the dynamics of the scalar field and equation of state. This interesting feature allows us to study phantom divided barrier crossing in the context of Lorentz invariance violation. We show that for suitable choice of parameter space, equation of state can cross phantom divided barrier just by one scalar field and Lorentz violating vector field controls this crossing.

gr-qc↗
arXiv2007

Non-Minimal Inflation after WMAP3

The Wilkinson Microwave Anisotropy Probe (WMAP) three year results are used to constraint non-minimal inflation models. Two different non-minimally coupled scalar field potentials are considered to calculate corresponding slow-roll parameters of non-minimal inflation. The results of numerical analysis of parameter space are compared with WMAP3 data to find appropriate new constraints on the values of the non-minimal coupling. A detailed comparison of our results with previous studies reveals the present status of the non-minimal inflation model after WMAP3.

astro-ph↗
arXiv2006

Loop Quantum Gravity Modification of the Compton Effect

Modified dispersion relations(MDRs) as a manifestation of Lorentz invariance violation, have been appeared in alternative approaches to quantum gravity problem. Loop quantum gravity is one of these approaches which evidently requires modification of dispersion relations. These MDRs will affect the usual formulation of the Compton effect. The purpose of this paper is to incorporate the effects of loop quantum gravity MDRs on the formulation of Compton scattering. Using limitations imposed on MDRs parameters from Ultra High Energy Cosmic Rays(UHECR), we estimate the quantum gravity-induced wavelength shift of scattered photons in a typical Compton process. Possible experimental detection of this wavelength shift will provide strong support for underlying quantum gravity proposal.

gr-qc↗