arXiv subjects

Amir Hadi Ziaie

Publications and source records attributed to Amir Hadi Ziaie.

At least 19 recordsLinked to original sources

arXiv2026

Casimir traversable wormholes in Gauss-Bonnet gravity

In this work, we study Casimir wormhole solutions and investigate satisfaction of energy conditions in the framework of Einstein-Gauss-Bonnet (EGB) gravity. We firstly find traversable wormhole solutions supported by a general form for the Casimir energy density. Applying a specific redshift function, we choose suitable parameters that respect the energy conditions in the vicinity of the throat due to the presence of higher order curvature terms. Then, we check the energy conditions considering non-constant redshift function and for suitable choice of model parameters, we find that these conditions are satisfied at infinity as well as in the vicinity of the throat. The stability of the Casimir traversable wormhole solutions are investigated using the Tolman-Oppenheimer-Volkoff (TOV) equation. Finally, we study trajectory of null as well as timelike particles in the wormhole spacetime.

gr-qc↗

arXiv2026

Gravitational Collapse of an Inhomogeneous Fluid in Rastall Theory

We study spherically symmetric gravitational collapse of an inhomogeneous fluid with anisotropic energy momentum tensor (EMT) in Rastall gravity. Considering a linear equation of state (EoS) for the fluid profiles, i.e., $p_r=w_rρ$ and $p_θ=w_θρ$, we try to build and investigate non-singular collapse scenarios for which, the spacetime singularity that appears in the homogeneous case~\cite{ahz2019}, is absent. We therefore set the Rastall parameter in such a way that the effective radial pressure vanishes. This helps us to obtain a class of exact nonsingular solutions in which the matter shells undergo a {collapse} process in a contracting regime, reach a bounce point, and then enter an expanding phase. We further investigate formation of trapped surfaces during the dynamical evolution of the collapsing body. {It is found} that for the obtained solutions, {the} trapped surface formation can be avoided and consequently, the bounce event is not covered by the apparent horizon. Validity of weak energy condition (WEC) is also examined for the obtained solutions.

gr-qc↗

arXiv2026

Trace anomaly contributions to semi-classical wormhole geometries

We investigate wormhole solutions within the framework of the semi-classical Einstein equations in the presence of the conformal anomaly (or trace anomaly). These solutions are sourced by a stress-energy tensor (SET) derived from the trace anomaly, and depend on two positive coefficients, $α$ and $λ$, determined by the matter content of the theory and on the degrees of freedom of the involved quantum fields. For a Type B anomaly ($α=0$), we obtain wormhole geometries assuming a constant redshift function and show that the SET components increase with the parameter $λ$. In the case of a Type A anomaly ($λ=0$), we generalize previously known solutions, yielding a family of geometries that includes Lorentzian wormholes, naked singularities, and the Schwarzschild black hole. Using isotropic coordinates, we identify parameter choices that produce traversable wormhole solutions. Extending to the full trace-anomaly contribution, we solve the differential equation near the throat to obtain the redshift function and demonstrate that both the Ricci and Kretschmann scalars remain finite at the throat. We further analyze the trajectories of null and timelike particles, showing that the height and width of the effective potential for null geodesics increase monotonically with $α$, while the innermost stable circular orbit (ISCO) radius also grows with larger $α$. These results illustrate the rich interplay between trace anomaly effects and the structure and dynamics of wormhole spacetimes.

gr-qc↗

arXiv2025

Gamow Temperature in Tsallis and Kaniadakis Statistics

Relying on the quantum tunnelling concept and Maxwell-Boltzmann-Gibbs statistics, Gamow shows that the star-burning process happens at temperatures comparable to a critical value, called the Gamow temperature ({\tt T}) and less than the prediction of the classical framework. In order to highlight the role of the equipartition theorem in the Gamow argument, a thermal length scale is defined, and then the effects of non-extensivity on the Gamow temperature have been investigated by focusing on the Tsallis and Kaniadakis statistics. The results attest that while the Gamow temperature decreases in the framework of Kaniadakis statistics, it can be bigger or smaller than {\tt T} when Tsallis statistics are employed.

cond-mat.stat-mech↗

arXiv2025

Novel Brans-Dicke Wormhole Solutions in Non-Vacuum Spacetimes

In the present study we search for a new class of wormhole solutions in the framework of Brans-Dicke (BD) theory in the presence of anisotropic matter distribution. Considering a linear equation of state (EoS) between radial pressure and energy density profile we find exact static spherically symmetric solutions to the BD field equations which represent wormhole configurations. The solutions we obtain include both cases with zero and nonzero redshift functions, for which, the conditions on wormhole geometry together with the weak (WEC) and null (NEC) energy conditions put constraints on model parameters such as, the BD coupling and EoS parameters. These constraints also depend on other model parameters such as, the value of BD scalar field and energy density at the wormhole throat. The regularity of the obtained solutions is verified by calculating the Kretschmann scalar in order to ensure that curvature singularities are absent in the wormhole spacetime. We then find that BD wormholes in the presence of anisotropic matter can exist without violating NEC and WEC, either at the throat or across the entire spacetime.

gr-qc↗

arXiv2025

Singularity Avoidance in Gravitational Collapse of an Inhomogeneous Fluid in Rastall Gravity

Various types of inhomogeneous collapse models in general relativity (GR) lead to the formation of spacetime singularities either visible or hidden by a spacetime horizon. Our aim in the present work is to search for nonsingular models in Rastall gravity that arise as the final outcomes of spherically symmetric gravitational collapse of an inhomogeneous matter cloud. We firstly assume linear equations of state (EoS) for radial and tangential pressure profiles, i.e., $p_r=w_rρ$ and $p_θ=w_θρ$, then we set the Rastall parameter in such a way that the effective pressure in radial direction vanishes and examine the conditions under which the spacetime singularity can be avoided. We find exact nonsingular collapse solutions for which the collapsing cloud reaches a minimum physical radius at a finite amount of time and then rebounds to an expanding phase where the matter shells start moving away from each other. The solutions we obtain respect the weak energy condition (WEC), which is important for the physical validity of the model.

gr-qc↗

arXiv2024

Novel Casimir wormholes in Einstein gravity

In the context of General Relativity (GR), violation of the null energy condition (NEC) is necessary for existence of static spherically symmetric wormhole solutions. Also, it is a well-known fact that the energy conditions are violated by certain quantum fields, such as the Casimir effect. The magnitude and sign of the Casimir energy depend on Dirichlet or Neumann boundary conditions and geometrical configuration of the objects involved in a Casimir setup. The Casimir energy may act as an ideal candidate for the matter that supports the wormhole geometry. In the present work, we firstly find traversable wormhole solutions supported by a general form for the Casimir energy density assuming a constant redshift function. As well, in this framework, assuming that the radial pressure and energy density obey a linear equation of state, we derive for the first time Casimir traversable wormhole solutions admitting suitable shape function. Then, we consider three geometric configurations of the Casimir effect such as (i) two parallel plates, (ii) two parallel cylindrical shells, and (iii) two spheres. We study wormhole solutions for each case and their property in detail. We also check the weak and strong energy conditions in the spacetime for the obtained wormhole solutions. The stability of the Casimir traversable wormhole solutions are investigated using the Tolman-Oppenheimer-Volkoff (TOV) equation. Finally, we study trajectory of null as well as timelike particles along with quasi-normal modes (QNMs) of a scalar field in the wormhole spacetime.

gr-qc↗

arXiv2024

Casimir Wormholes in Brans-Dicke Theory

In recent years there has been a growing interest in the field of wormhole physics in the presence of the Casimir effect. As this effect provides negative energy density, it can be utilized as an ideal candidate for the exotic matter required for creating a traversable wormhole. In the context of modified theories of gravity such as Brans-Dicke (BD) theory \cite{BDTH}, wormhole geometries {have} been vastly investigated{. However}, the scientific literature is silent on the issue of BD wormholes in the presence of Casimir energy. Our aim in the present study is to seek for static spherically symmetric solutions representing wormhole configurations in BD theory with Casimir energy as the supporting matter. The Casimir {setup} we assume comprises two electrically neutral, infinitely large parallel planes placed in a vacuum. We then consider the Casimir vacuum energy density of a scalar field in such a configuration with Dirichlet {and} mixed boundary conditions. In the former case, the corresponding Casimir force is attractive and in the latter this force is repulsive. We present exact zero tidal force wormhole solutions as well as those with non-vanishing redshift function for both types of Casimir energies. The conditions on wormhole solutions along with the weak (WEC) and null (NEC) energy conditions put constraints on the values of BD coupling parameter. These constraints are also subject to the value of BD scalar field at the throat and the throat radius. We therefore find that BD wormholes in the presence of Casimir energy can exist without violating NEC and WEC (for the repulsive Casimir force). Finally, we examine the equilibrium condition for the stability of the obtained solutions using Tolman-Oppenheimer-Volkoff (TOV) equation.

gr-qc↗

arXiv2024

Late time cosmological solutions in $f(R,T)$ gravity in a viscous Universe

Considering the condition on conservation of energy momentum tensor (EMT), we study late time cosmological solutions in the context of $f(R,T)=R+αT^{n}$ gravity (where $α$ and $n$ are constants) in a flat FLRW spacetime. The present model discusses the case of a barotropic perfect fluid as the matter content of the Universe, along with the case when dissipative effects are taken into account. Briefly, assuming a single perfect fluid we find that the mentioned model is not capable of presenting an observationally consistent picture of the late time accelerated expansion of the Universe; nevertheless, the model leads to admissible solutions when the bulk viscosity is included. In this regard, a consistent setting is found considering the Eckart, Truncated and Full Israel-Stewart theories which determine the behavior of bulk viscosity. In the presence of bulk viscosity, the behavior of the deceleration parameter (DP) shows that the underlying model can describe an acceptable evolution even if the isotropic pressure of matter content of the Universe is negligible.

gr-qc↗

arXiv2022

Einstein Static Universe and its Stability in Generalized Rastall Gravity

The Einstein static (ES) state is a good candidate for describing the very early universe in terms of a regular cosmological model in which the Big Bang singularity is avoided. In the present study we propose an ES solution in the framework of generalized Rastall gravity (GRG), a modified version of original Rastall theory in which the coupling parameter is allowed to vary with respect to the spacetime coordinates. Introducing an ansatz for the Rastall parameter, existence and the corresponding stability of the solutions are investigated. We show that the GRG is capable of describing a stable singularity-free state for the universe. The problem of transition from an ES to an inflationary state is also addressed. We find that a time variation of the equation of state parameter from values lying in the range |w|<1/3 to the value w=-1/3 can give rise to such a phase transition. The vector and tensor perturbations around the ES solution are studied, as well. In the case of GRG, the vector perturbations remain frozen, nevertheless, the tenor perturbations can grow in such a way that the ES solution remains stable provided the ratio of Rastall gravitational constant to the Einsteinian one always exceeds a minimum value for each tensor mode.

gr-qc↗

arXiv2022

A dynamical system representation of generalized Rastall gravity

In this work we study the phase-space analysis of generalized Rastall gravity (GRG) which has recently been introduced as a modification to the original version of Rastall gravity (RG). In GRG, the coupling parameter assumes a dynamical feature and may play the role of dark energy (DE) which is responsible for the present accelerating expansion of the Universe. Our investigation shows that such a modification of General Relativity (GR) admits a stable critical point corresponding to the late time accelerated expansion of the Universe. Assuming both dark matter (DM) and an ultra-relativistic perfect fluid (radiation) as the cosmic ingredients, we find that the underlying model presents a viable sequence of cosmic evolution, beginning from radiation dominated era passing then through DM domination and finally reaches the late time DE dominated era. Moreover, taking into account the contribution due to the spatial curvature within the total energy density leads to a growing mode for it in such a way that its present values are consistent with those reported in recent observations. We also present a numerical simulation of the model which particularizes our dynamical system representation. These studies demonstrate the ability of GRG to describe the present accelerated expansion of the Universe.

gr-qc↗

arXiv2021

Wormholes in Poincarè gauge theory of gravity

In the present work, we {study} static spherically symmetric solutions representing wormhole configurations in Poincarè gauge theory ({\sf PGT}). The gravitational sector of the Lagrangian is chosen as a subclass of {\sf PGT} Lagrangians for which, the spin-$0^+$ is the only propagating torsion mode. The spacetime torsion in {\sf PGT} has a dynamical nature even in the absence of intrinsic angular momentum (spin) of matter, hence, {torsion can play a principal role in the case of usual spin-less gravitating systems. We therefore} consider a spin-less matter distribution with an anisotropic energy momentum tensor ({\sf EMT}) as the supporting source for wormhole structure {to obtain a class of zero tidal force wormhole solutions.} It is seen that the matter distribution obeys the physical reasonability conditions, i.e., the weak ({\sf WEC}) and null ({\sf NEC}) energy conditions either at the throat and throughout the spacetime. We further consider varying equations of state in radial and tangential directions via definitions $w_r(r)=p_r(r)/ρ(r)$ and $w_t(r)=p_t(r)/ρ(r)$ and {investigate} the behavior of state parameters {at the throat of wormhole}. We observe that our solutions allow for wormhole configurations without the need of exotic matter. Observational features of the wormhole solutions are also discussed utilizing gravitational lensing effects. It is found that the light deflection angle diverges at the throat (which indeed, effectively acts as a photon sphere) and can get zero and negative values depending on the model parameters.

gr-qc↗

arXiv2021

Dynamical wormholes in Lovelock gravity

In this study we present evolving wormhole configurations in third-order {Lovelock} gravity and investigate the possibility that these solutions satisfy the energy conditions. Using a generalized Friedmann-Robertson-Walker spacetime, we derive evolving wormhole geometries by considering a constraint on the Ricci scalar. In standard cosmological models, the Ricci scalar is independent of the radial coordinate $r$ and is only a function of time. We use this property to introduce dynamic wormhole solutions expanding in an inflationary cosmological background and explore the effects of higher-order {Lovelock} terms on the dynamics of such wormholes. Our analysis shows that for suitable third-order {Lovelock} coefficients, there are wormhole solutions that respect the weak energy condition {(WEC)}. In addition to this, we also present other wormhole solutions that satisfy the WEC throughout their respective evolution.

gr-qc↗

arXiv2021

Non-minimal coupling inspires the Dirac cosmological model

In the framework of the generalized Rastall theory (GRT), we study the ability of a non-minimal coupling between geometry and matter fields in order to provide a setting which allows for a variable G during the cosmic evolution. In this regard, the compatibility of this theory with Dirac hypothesis on the variations of G is investigated, and additionally, the possibility of obtaining the current accelerated universe is also addressed. In summary, our study indicates that, in GRT, having in hand the G profile, one may find the corresponding non-minimal coupling between the energy source and geometry and vise versa, in a compatible way with the current accelerated universe.

gr-qc↗

arXiv2021

A Note on Effects of Generalized and Extended Uncertainty Principles on Jüttner Gas

In recent years, the implications of the generalized (GUP) and extended (EUP) uncertainty prin-ciples on Maxwell-Boltzmann distribution have been widely investigated. However, at high energy regimes, the validity of Maxwell-Boltzmann statistics is under debate and instead, the Jüttner distribution is proposed as the distribution function in relativistic limit. Motivated by these considerations, in the present work, our aim is to study the effects of GUP and EUP on a system that obeys the Jüttner distribution. To achieve this goal, we address a method to get the distribution function by starting from the partition function and its relation with thermal energy which finally helps us in finding the corresponding energy density states.

gr-qc↗

arXiv2020

A Generalized Interacting Tsallis Holographic Dark Energy Model and its thermodynamic implications

The paper deals with a theoretical model for interacting Tsallis holographic dark energy (THDE) whose infrared (IR) cut-off scale is set by the Hubble length. The interaction $Q$ between the dark sectors (dark energy and pressureless dark matter) of the universe has been assumed to be non-gravitational in nature. The functional form of $Q$ is chosen in such a way that it reproduces well known and most used interactions as special cases. We then study the nature of the THDE density parameter, the equation of state parameter, the deceleration parameter and the jerk parameter for this interacting THDE model. Our study shows that the universe exhibits the usual thermal history, namely the successive sequence of radiation, dark matter and dark energy epochs, before resulting in a complete dark energy domination in the far future. It is shown the evolution of the Hubble parameter for our model and compared that with the latest Hubble parameter data. Finally, we also investigate both the stability and thermodynamic nature of this model in the present context.

gr-qc↗

arXiv2020

A connection between Rastall-type and f(R, T) gravities

Recently a Lagrangian formulation for Rastall Gravity (RG) has been proposed in the framework of f(R, T) gravity. In the present work we obtain Lagrangian formulation for the standard Rastall theory assuming the matter content is a perfect fluid with linear Equation of State (EoS). We therefore find a relation between the coupling constant of the Lagrangian, the Rastall gravitational coupling constant and the EoS parameter. We also propose a Lagrangian for the Generalized Rastall Gravity (GRG). In this case the Rastall parameter which is a constant is replaced by a variable one. More exactly, it appears as a function of the Ricci scalar and the trace of the Energy Momentum Tensor (EMT). In both mentioned models, the Lagrangians are constructed by a linear function of R and T.

gr-qc↗

arXiv2019

Traversable wormholes in Einsteinian cubic gravity

In the present work we investigate wormhole configurations described by a constant redshift function in Einstein-Cubic gravity ({\sf ECG}). We derive analytical wormhole geometries by assuming a particular equation of state ({\sf EoS}) and investigate the possibility that these solutions satisfy the standard energy conditions. We introduce exact asymptotically flat and anti-de Sitter (AdS) spacetimes that admit traversable wormholes. These solutions are obtained by imposing suitable values for the parameters of the theory so that the resulted geometries satisfy the weak energy condition ({\sf WEC}) in the vicinity of the throat, due to the presence of higher-order curvature terms. Moreover, we find that AdS solutions satisfy the {\sf WEC} throughout the spacetime. A description of the geodesic motion of timelike and null particles is presented for the obtained wormhole solutions. Also, using gravitational lensing effects, observational features of the wormhole structure are discussed.

gr-qc↗