arXiv subjects

Amin Rezaei Akbarieh

Publications and source records attributed to Amin Rezaei Akbarieh.

At least 19 recordsLinked to original sources

arXiv2026

High-Frequency Gravitational-Wave Transduction in a SQUID-Terminated Superconducting Cavity

High-frequency gravitational waves in the MHz--GHz range require detection strategies beyond conventional interferometers. We study the response of a SQUID-terminated superconducting microwave cavity as a narrowband parametric transducer. The cavity boundary conditions and the flux dependence of the Josephson inductance are used to derive the quarter-wave mode spectrum and the first-order eigenfrequency response to changes in the physical line length, phase velocity, and SQUID inductive length. By projecting the perturbed dynamics onto the static cavity modes, we show that the resonance-frequency response and the photon-pair-production response are generally distinct. We therefore introduce two independent kernels, $R_n^ω$ and $R_n^{\rm pair}$, which coincide only when changes in mode normalization and spatial profiles can be neglected. Near $Ω_{\rm GW}\simeq 2ω_n$, the isolated-mode dynamics reduce to a Mathieu-type parametric amplifier, allowing the photon number, quadrature variances, gain, and instability threshold to be obtained in the presence of dissipation. Spontaneous photon production scales quadratically with the gravitational-wave strain and is extremely small for representative parameters, whereas phase-sensitive responses with a coherent probe can scale linearly with strain. The framework therefore provides a theoretical description of narrowband gravitational-wave transduction, while a quantitative sensitivity estimate requires device-specific calibration of the mechanical--electromagnetic response, verification of mode isolation, and a complete treatment of loss and noise.

gr-qc↗

arXiv2026

Baby-universe state dependence of the wormhole-induced gravi-axion mass gap

Euclidean wormholes can break the axion shift symmetry, and when this symmetry is broken, a periodic potential is created for the axion, and the curvature of this potential at a minimum point can give the axion mass. In this work, we take seriously the assumption that the mere existence of a wormhole does not mean that the axion mass is uniquely determined, and that more information is needed to determine it. We first show that since for a Giddings--Strominger wormhole with symmetry $O(4)$, the gravitational Pontryagin density is zero, the dynamical Chern--Simons interaction can affect quantum effects, including the determinant phase and fermionic selection rules, without changing the classical wormhole action. We also conclude that in a fixed $α$ sector, the axion can have a specific mass, but this mass depends on the baby-universe state and is not a universal value. Then, for the mixed case of $α$ sectors that give a distribution of conditional masses, we show that the lower edge of the averaged spectral measure is determined by the essential infimum of the conditional mass squared over the $α$ distribution. Finally, we conclude that even if almost all individual sectors are massive and the mean mass squared is also non-zero, if the support of the measure accumulates at the symmetric point $α=0$, the averaged spectral measure will be gapless. To clarify the discussion, we consider a clear example of this situation, which can be a Gaussian baby-universe state, and show that in it the spectral threshold starts at zero and there is no isolated massive pole. Therefore, predicting the mass and cosmology of a gravi-axion from a wormhole without specifying the baby-universe state is not a unique, state-independent prediction.

hep-th↗

arXiv2026

Dynamical and Observational Analysis of Generalized Nash's Theory of Gravity

We investigate cosmic evolution in generalized Nash's theory of gravity involving the quadratic Ricci invariant $χ=R_{μν}R^{μν}$. The analysis is divided into two complementary branches. First, we study the power-law family $f(R,χ)=R^α+βχ$ as a reduced autonomous system in a flat FLRW background. Because the adopted variables become singular at the Einstein--Hilbert limit $α=1$, the phase-space analysis is restricted to $α\neq1$, with $α=2$ used as a representative quadratic benchmark. This benchmark contains radiation-like boundary configurations, restricted scaling saddles, and de Sitter-like accelerating endpoints (a stable node away from $α=2$ and non-hyperbolic at the benchmark itself), but not a complete regular radiation-to-matter-to-de Sitter sequence. Second, we constrain the regular observational branch $f_{\rm obs}(R,χ)=R-2Λ+βχ$, which reduces exactly to flat $Λ$CDM when $β\to0$. The Hubble rate is obtained from the reduced $Λ$CDM-connected background branch, integrated over $0\le z\le10$ and matched at higher redshift to a standard radiation+matter+$Λ$ background. Using SNe~Ia, BAO, and Planck~2018 compressed CMB distance priors, we find an expansion history very close to $Λ$CDM, with the quadratic correction tightly constrained around the nested standard-model limit. The resulting bound on $β$ should be interpreted as a background-level constraint within this reduced prescription, not as a perturbation-level viability test of the full higher-derivative theory.

gr-qc↗

arXiv2026

Dynamical Casimir Effect and Vacuum Friction in the Near-Horizon Geometry of a Black Hole

We investigate the Dynamical Casimir Effect (DCE) for a relativistic scalar field confined within a cavity possessing moving boundaries in the (1+1)-dimensional near-horizon geometry of a black hole. By applying a coordinate transformation, we map the moving-boundary problem to an equivalent acoustic metric with static boundaries, allowing for an exact canonical Hamiltonian formulation. We find that the local gravitational redshift fundamentally alters the vacuum structure, and the dynamical boundary motion induces time-dependent mode-mixing. When a boundary moves, it scatters the fluctuations of the ambient Hartle-Hawking state, generating a flux of created particles. Crucially, because the coordinate speed of light relative to the Killing time $t$ vanishes as one approaches the event horizon, we establish that maintaining physical, subluminal boundary motion requires the mechanical oscillation amplitude to scale proportionally with the proper distance to the horizon. Consequently, the effective Mach number of the moving mirror approaches zero in the near-horizon limit. Using a rigorous small-amplitude perturbative expansion and proper canonical operator normalization, we demonstrate that the transition probability into the field is heavily suppressed by a conformal geometric factor. Furthermore, we account for the Bose-enhancement caused by the thermal Hawking bath. While the thermal presence introduces infrared density-of-states enhancement, it remains insufficient to overcome the kinematic damping. Finally, we conclude that the extreme spacetime curvature acts to protect the near-horizon vacuum; the transition probability vanishes as the boundary approaches the event horizon, indicating a geometric and kinematic suppression of particle creation in the strong-gravity limit.

gr-qc↗

arXiv2025

Gravitational Memory Effect in the Massless Vector Field

We analyze the infrared structure and memory effects of a massless vector tensor theory with non minimal curvature coupling in asymptotically flat spacetimes. Using Bondi Sachs expansions, we identify the independent radiative data and derive the effective Bondi mass aspect, whose balance law receives an additional positive definite flux from the vector sector. This leads to modified displacement, spin, and center of mass memory (CM) expressions, where the gravitational contributions retain their General Relativity (GR) form and the vector field enters only through well defined flux terms. We also describe persistent vector memory effects associated with the leading angular vector mode, which are gauge invariant but do not affect the leading tidal observables. The BMS transformations act kinematically as in GR. Tensor vacua remain supertranslation degenerate, whereas the vector vacuum, defined by the vanishing vector field, is nondegenerate. All results reduce continuously to GR when the coupling is removed, isolating the precise channels through which vector curvature interactions modify the infrared dynamics.

gr-qc↗

arXiv2025

Exact Solutions for a Teleparallel Cosmological Model with Vector Field via Noether Symmetry

We study a cosmological model in the framework of teleparallel gravity, where a vector field $A_μ$ is non-minimally coupled to the torsion scalar $T$ in a flat Friedmann-Robertson-Walker (FRW) universe. Using the Noether symmetry approach, we identify specific forms for the coupling function $g(ξ)$ and the potential $V(ξ) = V_0 ξ^n$, with $ξ\equiv A_μA^μ$. The method allows us to find exact analytical solutions for the scale factor $a(t)$ and the scalar function $ξ(t)$. General solutions for arbitrary values of $n$ are derived, but special cases such as $n = 1$ and $n = 3$ are studied separately due to their distinct behavior. For $n = 1$, the model describes a transition from decelerated to accelerated expansion, and depending on the value of the model parameter, a transition to a phantom phase is also possible, which is consistent with phantom dark energy. For the special case $n=3$, this model first experiences a deceleration phase and then enters a stable accelerating phase, such that the acceleration parameter $q(t)$ changes from positive to negative values and the equation of state parameter $ω_ξ(t)$ tends to $-1$ at late times, similar to quintessence dark energy. Unlike the case $n=1$, in this case, no transition to the phantom phase is observed. Therefore, this model can describe the behavior of the universe during periods of dark energy dominance.

gr-qc↗

arXiv2025

Thermodynamics of FLRW universe in Quadratic Gravity

In this paper, we investigate the thermodynamic aspects of quadratic gravity in a $D$-dimensional Friedmann-Lemaitre-Robertson-Walker (FLRW) universe. First, we derive the field equations and the effective energy-momentum tensor for quadratic gravity. Then, using these equations, we obtain the generalized Misner-Sharp energy within the framework of this model. We consider the thermodynamic behavior of the apparent horizon and derive the equations of state related to the pressure, temperature, and radius of the apparent horizon. Using the thermodynamic pressure, we obtain the critical points corresponding to phase transitions. We determine the critical temperature and critical radius in terms of model parameters, including the quadratic coupling and the cosmological constant. We also examine key thermodynamic quantities, such as Wald entropy, specific heat at constant pressure, enthalpy, Gibbs free energy. By examining the behavior of these quantities, we can gain insight into the thermodynamic stability of the quadratic gravity model. In particular, we find that quadratic terms change the stability conditions and can lead to new thermodynamic behaviors compared to general relativity.

gr-qc↗

arXiv2025

Gravitational memory effects in Tachyon gravity

In this paper, we explore how a massless Tachyon field, when non-minimally coupled to gravity, affects gravitational memory, soft theorems, and symmetries near null infinity. We set up the model using both the Jordan and Einstein frames, work out the equations of motion and study the behavior of fields at large distances with the Bondi-Sachs gauge. The Tachyon adds a ``breathing'' polarization mode to the gravitational radiation, which modifies the Bondi mass loss formula and leaves a trace in the displacement memory effect. We also derived the leading and subleading soft factors, involving the radiation of a soft graviton or a soft scalar.

gr-qc↗

arXiv2025

Generalized Misner-Sharp energy in $f(R,\mathcal{G})$ gravity

In this work, we explore the formulation of the Misner-Sharp energy within the framework of $f(R, \mathcal{G})$ gravity, a modified theory incorporating the Ricci scalar $R$ and the Gauss-Bonnet scalar $\mathcal{G}$. By extending the quasilocal energy definition to both static spherically symmetric spacetime and the dynamic Friedmann-Lemaitre-Robertson-Walker (FLRW) spacetime, we derive explicit expressions for the generalized Misner-Sharp energy using two complementary approaches: the integration method and the conserved charge method based on the Kodama vector. Our analysis shows that the Misner-Sharp energy expression in $f(R, \mathcal{G})$ gravity reduces to standard $f(R)$ gravity results when the Gauss-Bonnet term is absent, revealing how curvature modifications influence the geometric structure and dynamics of cosmic evolution. Furthermore, we investigate the thermodynamic properties at the apparent horizon associated with the FLRW background, and we find a connection to non-equilibrium thermodynamics unique to $f(R, \mathcal{G})$ gravity. These findings underscore the subtle and fundamental role of curvature corrections in determining the energy distribution and thermodynamic behavior of gravitational systems.

gr-qc↗

arXiv2025

Quantum circuit simulation of black hole evaporation and Maxwell demon interpretation

We suggest a quantum circuit model which simulates the black-hole evaporation process. In particular, Almheiri-Marolf-Polchinski-Sully (AMPS) paradox and the ER=EPR correspondence are reconsidered regarding our proposed model, which assumes a Maxwell's demon operating within a black hole interior. In other words, we form a quantum circuit, mimicking the behavior of the entanglement structure of the near-horizon region and the early Hawking radiation located far from the black hole. Furthermore, we indicate how the demon, by applying nonlocal correlations, can mediate via Einstein-Rosen bridges for the purpose of simulating the transfer of quantum information across the horizon without violating the monogamy of entanglement. Finally, the thermodynamic cost of the demon's operations regarding Landauer's principle is analyzed. This indicates that the information erasure has an energy comparable to the black hole entropy.

gr-qc↗

arXiv2025

Cosmology of Quasi-Dilaton Massive Gravity with Non-minimal Kinetic Coupling

In this study, we introduce an extension of the quasi-dilaton massive gravity theory and derive the field equations by varying the action with respect to the metric. This extension elucidates the dynamics of the system and demonstrates how it can encompass and recover previous cosmological models through different parameter values. We present the cosmological background equations to analyze self-accelerating solutions that can explain the late-time accelerated expansion of the Universe, driven by an effective cosmological constant arising from massive gravity. Besides, we apply the quasi-dilaton massive gravity theory with non-minimal kinetic coupling to a Type Ia Supernovae (SNIa) data set to test its viability. Our findings indicate that the theory is able to account for the observed acceleration of the expansion of the universe without invoking dark energy. In addition, we carry out a comprehensive perturbation analysis examining tensor, vector, and scalar perturbations independently. We derive the dispersion relation of gravitational waves in a Friedman-Lemaitre-Robertson-Walker (FLRW) cosmology and determine the stability conditions of the system. Such an analysis results in a sharper quasi-dilaton massive gravity theory with non-minimal kinetic coupling by ensuring the stability conditions of the system are maintained and that strong constraints on theory parameters are provided.

gr-qc↗

arXiv2024

The Cosmological Impact of Brane-Chern-Simons Massive Gravity

In this paper, we present a novel extension of massive gravity theory; the Brane-Chern-Simons massive gravity theory. We explore the cosmological implications of this theory by deriving the background equations and demonstrating the existence of self-accelerating solutions. Interestingly, our theory suggests the existence of self-accelerating mechanisms that originate from an effective cosmological constant, leading to intriguing possibilities for understanding the nature of cosmic acceleration. Furthermore, we perform a tensor perturbation analysis to investigate the propagation of gravitational waves in this framework. We derive the dispersion relation for gravitational waves and study their behavior in the Friedmann-Lemaître-Roberson-Waker cosmology within the context of Brane-Chen-Simons massive gravity. Utilizing the latest Union2 type Ia supernova dataset comprising $557$ SNIa events, we provide observational support for our theoretical framework, indicating that the Brane-Chern-Simons massive gravity theory is consistent with cosmological observations.

gr-qc↗

arXiv2024

Cosmological Study in Myrzakulov $F(R, T)$ Quasi-dilaton Massive Gravity

This study explores the cosmological implications of the Myrzakulov $F(R, T)$ quasi-dilaton massive gravity theory, a modification of the de Rham-Gabadadze-Tolley (dRGT) massive gravity theory. Our analysis focuses on the self-accelerating solution of the background equations of motion, which are shown to exist in the theory. Notably, we find that the theory features an effective cosmological constant corresponding to the massive graviton, which has important implications for our understanding of the universe's accelerated expansion. To assess the validity of the Myrzakulov $F(R, T)$ quasi-dilaton massive gravity theory, we employ two datasets: the Union2 Type Ia Supernovae (SNIa) dataset, consisting of 557 observations, and the Pantheon SNIa data, which includes 1048 SNe I-a events gathered from diverse SN I-a samples. Our results demonstrate that the theory is capable of explaining the accelerated expansion of the universe without requiring the presence of dark energy. This finding supports the potential of the Myrzakulov $F(R, T)$ quasi-dilaton massive gravity theory as an alternative explanation for the observed cosmic acceleration. Moreover, we investigate the properties of tensor perturbations within the framework of this theory and derive a novel expression for the dispersion relation of gravitational waves. Our analysis reveals interesting features of the modified dispersion relation in the Friedmann-Lemaître-Robertson-Walker (FLRW) cosmology, providing new insights into the nature of gravitational waves in the context of the Myrzakulov $F(R, T)$ quasi-dilaton massive gravity theory.

gr-qc↗

arXiv2024

Gravitational wave pulse and memory effects for hairy Kiselev black hole and its analogy with Bondi-Sachs formalism

The investigation of non-vacuum cosmological backgrounds containing black holes is greatly enhanced by the Kiselev solution. This solution plays a crucial role in understanding the properties of the background and its relationship with the features of the black hole. Consequently, the gravitational memory effects at large distances from the black hole offer a valuable means of obtaining information about the surrounding field parameter N and parameters related to the hair of the hairy Kiselev Black hole. This paper investigates the gravitational memory effects in the context of the Kiselev solution through two distinct approaches. At first, the gravitational memory effect at null infinity is explored by utilizing the Bondi-Sachs formalism by introducing a gravitational wave (GW) pulse to the solution. The resulting Bondi mass is then analyzed to gain further insight. Therefore, the Kiselev solution is being examined to determine the variations in Bondi mass caused by the pulse of GWs. The study of changes in Bondi mass is motivated by the fact that it is dynamic and time-dependent, and it measures mass on an asymptotically null slice or the densities of energy on celestial spheres. In the second approach, the investigation of displacement and velocity memory effects is undertaken in relation to the deviation of two neighboring geodesics and the deviation of their derivative influenced by surrounding field parameter N and the hair of hairy Kiselev black hole. This analysis is conducted within the context of a gravitational wave pulse present in the background of a hairy Kiselev black hole surrounded by a field parameter N.

gr-qc↗

arXiv2023

Dynamical non-locality in the near-horizon region of a black hole with quantum time

The formalization of the modular energy operator within the curved spacetime is achieved through the timeless approach proposed by Page and Wootters. The investigation is motivated by the peculiar behavior of the near horizon region of a black hole and its quantum effects, leading to a restriction of the study to the immediate vicinity. The focus lies on the perspective of a static observer positioned close to the horizon. This paper highlights the alteration of the modular energy's behavior in this region compared to flat spacetime. Furthermore, it is observed that the geometry of the spacetime influences the non-local properties of the modular energy. Moreover, within the event horizon of the black hole, the modular energy exhibits a completely distinct behavior, rendering its modular behavior imperceptible in this specific region.

gr-qc↗

arXiv2023

Massive gravity solution of Black Holes and Entropy Bounds

The dRGT massive gravity represent a comprehensive theory which properly describes massive graviton field. Latterly, the exact spherical solutions are identified for the black hole in the dRGT massive gravity theory. In this paper, we derive Bousso's D-bound entropy for the black hole solutions of dRGT massive gravity. By an entropic consideration which provides a criterion, it is demonstrated that the relation between the D-bound and Bekenstein entropy bound imposes some constraints on the structure parameters of black hole solutions in dRGT massive gravity.

gr-qc↗

arXiv2023

Accretion disk around regular black holes

Regular black holes are crucially important as approaches to solving the singularity problem, and in this paper, the accretion disk of Bardeen and Hayward models have been studied. For this purpose, we calculated the physical properties of black holes, including radiant energy, luminosity derivative, temperature, and conversion efficiency of accretion mass into radiation. The obtained results show that the non-zero-free parameters of regular black holes cause the radius of the innermost stable circular orbit of the disk to shift to smaller values. As a result of this displacement, we saw an increase in the profiles of radiant energy, luminosity derivative, and temperature. We also find that Bardeen and Hayward's black holes are more efficient in converting mass to radiation than Schwarzschild. Finally, we compared the free parameter of these two black holes with the spin of the rotating black hole and found that the Bardeen and Hayward black holes can mimic the slowly rotating Kerr black hole.

gr-qc↗

arXiv2022

Aether-quasi-dilaton massive gravity

Although quasi-dilaton massive gravity is a well-defined gravitational theory, it exhibits instabilities and suffers from the strong coupling problem. In this work we construct an extension of the theory, through the inclusion of the aether field. Focusing on flat Friedmann-Lemaitre-Robertson-Walker geometry, we show the existence of exact, self-accelerating solutions at the background level, characterized by an effective cosmological constant arising from the graviton mass. Additionally, we perform a detailed perturbation analysis, investigating separately the tensor, vector, and scalar perturbations, extracting the dispersion relation of gravitational waves, and determining the stability conditions for vector and scalar sectors. As we show, there are always regions in the parameter space in which the obtained solutions are free from ghost instabilities, as well as from the strong coupling problem. Hence, although the aether field does not play an important role in the background self-accelerating solutions, it does play a crucial role in the alleviation of the perturbation-related problems of the simple quasi-dilaton massive gravity.

gr-qc↗