arXiv subjects

Kourosh Nozari

Publications and source records attributed to Kourosh Nozari.

At least 19 recordsLinked to original sources

arXiv2026

Higher Dimensional Loop Quantum Black hole in de Sitter Spacetime: Quasinormal Modes and Shadow Signatures

We investigate the dynamical and optical properties of a higher-dimensional loop-quantum-corrected black hole in a de Sitter background. We first analyze the horizon structure and identify the admissible nonextremal black-hole domain bounded by the extremal and Nariai configurations, ensuring the existence of distinct inner, event, and cosmological horizons for the parameter sets considered. We then examine the scalar effective potential and show that the loop-quantum correction deforms the classical scattering barrier primarily in the strong-field region while preserving its characteristic single-barrier structure. The quasinormal modes of massless scalar perturbations are computed using time-domain evolution with Prony extraction, the matrix method, and the WKB approximation, showing good agreement among the three approaches. The time-domain waveform and its Prony and matrix-frequency reconstructions provide an additional direct consistency check of the extracted ringdown spectrum. We find that loop quantum corrections induce moderate shifts in the quasinormal spectrum, whereas the spacetime dimensionality has a much stronger impact, leading to higher oscillation frequencies and damping rates. The negative imaginary parts of all modes indicate dynamical stability against massless scalar perturbations within the explored parameter range. Comparison with the corresponding classical black-hole backgrounds shows that the quantum-corrected quasinormal spectrum remains continuously connected to the classical photon-sphere branch, with the loop correction producing quantitative rather than qualitative changes.

gr-qc↗

arXiv2026

Phenomenological Rotating Extension of Black Holes with Primary Scalar Hair: Shadow Signatures in Beyond Horndeski Gravity

The Event Horizon Telescope (EHT) image of M87* provides a direct test of strong-field gravity, measuring an angular shadow diameter $θ_{d}=42\pm 3~μ\mathrm{as}$ and a circularity deviation $ΔC\leq 0.1$. Such observations allow quantitative tests of the Kerr paradigm and of possible deviations from the no-hair theorem. In scalar-tensor extensions of gravity, black holes may possess primary scalar hair, introducing an additional independent parameter beyond mass and spin. In this work, we construct a rotating configuration inspired by black hole solutions with primary scalar hair in beyond Horndeski gravity and analyze their photon regions and shadow formation. We show that the scalar hair parameter $Q$ induces characteristic modifications of the shadow, and in particular negative $Q$ enlarges the shadow and reduces its oblateness, while positive $Q$ shrinks and enhances its distortion. Adopting M87* as a representative case within this framework and imposing the EHT bounds on $θ_{d}$ and $ΔC$, we identify the viable $(a,Q)$ parameter space. We find that current observations do not exclude rotating black holes with primary scalar hair, although the allowed region is significantly restricted for $Q>0$. Finally, the scalar-hair-induced deviations are of order $\mathcal{O}(μ\mathrm{as})$, placing them near the sensitivity threshold of present instruments and within reach of next-generation horizon-scale imaging.

gr-qc↗

arXiv2026

Reheating after the Higgs Inflation

Since the discovery of the Higgs particle in the LHC, numerous inflationary models utilize it as the inflaton. In this study, after establishing that Higgs inflation can be a viable model in the context of unimodular gravity, we address a scenario related to reheating, which plays a crucial role in connecting the inflationary era to a hot Big Bang universe. Also, we calculate essential parameters such as the temperature and the e-folds number according to the equation of the state parameter ($ω_{re}$) in this regime of reheating. The physics behind the reheating process are extremely imprecise at present, and we have only a very limited range of acceptable values for $ω_{re}$. We analyze the compatibility with Planck2018, BICEP/Keck2021, DESI2024, and ACT array bounds and find that within this model, $ω_{re}$ is restricted to the range $ -1 \leq ω_{re} \leq \frac{1}{6} $.

physics.gen-ph↗

arXiv2026

Effective reheating in Gauss--Bonnet inflation with $μ(ϕ,X)$ coupling

We study effective reheating in a scalar--Gauss--Bonnet inflationary model with a phase-space-dependent coupling $μ(ϕ,X)$, in which a compact field-space feature is combined with a bounded kinetic gate. The modified inflationary background determines the pivot-scale quantities and the effective energy density at the end of inflation. These quantities are then used to derive the reheating duration $N_{\rm re}$ and temperature $T_{\rm re}$ through the thermal-history matching relation. We first perform two fixed-pivot reference scans by varying the overall Gauss--Bonnet strength $λ_{\rm GB}$ and the kinetic parameter $g_{_X}$ separately. For the reference parameter choices, increasing either parameter increases $N_{\rm re}$ and decreases $T_{\rm re}$ for the selected reheating equations of state. Additional benchmark calculations clarify how these variations depend on the dynamical regime of the model. In the $λ_{\rm GB}$ scan, the increase in $N_{\rm re}$ and the decrease in $T_{\rm re}$ persist, although both variations become strongly suppressed when the coupling is more localized or when the end of inflation is controlled more strongly by the E-model potential. In the $g_{_X}$ scan, stronger field-space localization and kinetic saturation can instead lead to a slight decrease in $N_{\rm re}$ and an increase in $T_{\rm re}$ as $g_{_X}$ is increased. When the bounded kinetic contribution is considered together with a weaker overall Gauss--Bonnet interaction, the resulting changes in the reheating quantities become nearly negligible. The fixed-pivot predictions of the representative and alternative benchmarks are compared with CMB constraints.These reheating constraints are then discussed for four representative values of the effective equation-of-state parameter, $\overline{w}_{\rm re}=-1/3,0,2/3,$ and $1$.

astro-ph.CO↗

arXiv2026

Thin Accretion Disks around Rotating Charged Black Holes in an Effective Higher-Curvature Spacetime

We investigate the structure and emission properties of a thin accretion disk around a rotating charged black hole described by an effective higher-curvature-inspired spacetime, constructed as a phenomenological deformation of the Kerr Newman geometry. In this framework, the deformation is introduced through a modification of the metric function $Δ$ by an effective Gauss-Bonnet-like parameter $α$, such that the spacetime reduces to the standard Kerr Newman solution in the limit $α\to 0$. Adopting a kinematical approach, we use test-particle motion to derive the specific energy, specific angular momentum, and angular velocity of circular orbits, and analyze the effects of the parameters $α$ and charge $Q$ on the innermost stable circular orbit (ISCO), radiative efficiency, radiation flux, temperature, and differential luminosity of the disk. We find that increasing $α$ shifts the ISCO inward and enhances the disk's radiation flux and temperature, while the presence of charge suppresses these quantities due to electrostatic effects. Our results demonstrate that effective higher curvature deformations of rotating black hole spacetimes can lead to observable deviations from the Kerr case, highlighting accretion disks as sensitive probes of strong-gravity effects without relying on a specific underlying gravitational theory.

gr-qc↗

arXiv2026

Beyond $f(ϕ)\mathcal{G}$: Gauss--Bonnet inflation with $μ(ϕ,X)$

Gauss--Bonnet inflation typically affects the dynamics over an extended portion of the trajectory, making it difficult to isolate a controlled imprint at CMB scales. We consider a trajectory-selective coupling $μ(ϕ,X)$ that gates the Gauss--Bonnet sector in phase space, enabling the higher-curvature contribution to be localized within a finite e-fold window while remaining negligible elsewhere. We identify stable inflationary solutions consistent with this localization and enforce standard ghost and gradient stability conditions for both scalar and tensor perturbations. For these viable backgrounds we compute pivot-scale observables and examine their dependence on the overall Gauss--Bonnet strength and on the kinetic gating. The framework offers a controlled route for realizing localized higher-curvature effects with predictable consequences for CMB-scale measurements.

astro-ph.CO↗

arXiv2025

Dimensional Phenomenology in Polymeric Quantization Framework

In this paper, we study the statistical mechanics within the polymer quantization framework in the semiclassical regime. We apply a non-canonical transformation to the phase space variables. Then, we use this non-canonical transformation to calculate the deformed density of states of the $2n$-dimensional phase space, which encompasses all polymer effects. In the next step, some thermodynamic features of a system of $n$-dimensional harmonic oscillators are studied by computing the deformed partition function. The results show that the number of microstates decreases because there is an upper bound on the momentum within the polymer framework. We found that in the high-temperature regime, when the thermal de Broglie wavelength is close to the Planck length, $n$ degrees of freedom of the system are frozen in this setup. In other words, there is an effective reduction in space dimensions from $n$ to $\frac{n}{2}$ in the polymeric framework, which also signals the fractional dimension for odd-dimensional oscillators.

gr-qc↗

arXiv2025

Accretion Process as a Probe of Extra Dimensions in MOG Compact Object Spacetimes

The idea of extra spatial dimensions arises from attempts to unify gravity with other fundamental interactions, develop a consistent theory of quantum gravity, and address open problems in particle physics and cosmology. Considerable attention has been devoted to understanding how such dimensions modify gravitational theories. One way to probe their impact is through the analytical study of astrophysical processes such as black hole accretion. Since accretion efficiently converts gravitational energy into radiation, this makes it a powerful tool to test modified gravity (MOG) theories and higher-dimensional frameworks via the behavior of dark compact objects like black holes, neutron stars, and white dwarfs. In this work, we investigate the dynamics of neutral particles around a higher-dimensional, regular, spherically symmetric MOG compact object, focusing on the innermost stable circular orbit (ISCO), energy flux, temperature, and differential luminosity. We further analyze the accretion of a perfect fluid onto the same object, deriving analytical expressions for the four-velocity and proper energy density of the inflowing matter. Our findings show that extra dimensions reduce the ISCO radius while enhancing the corresponding flux and temperature. Finally, by comparing the effective disk temperature $T_{\text{eff}}$ with Event Horizon Telescope (EHT) observations of Sgr A*, we argue that MOG and higher-dimensional corrections to the accretion disk properties could be close to the current threshold of detectability.

gr-qc↗

arXiv2025

Shadow of Extreme Compact Charged Objects in Consistent 4-Dimensional Einstein-Gauss-Bonnet Gravity

In order to better describe gravitational phenomena on both very small and cosmological scales, there have been constant attempts to generalize and expand the theory of General Relativity (GR) since its inception. The Einstein Gauss Bonnet (EGB) theory is one such extension that adds spacetime corrections related to curvature. Since the standard Gauss Bonnet term is purely topological, it does not contribute to the field equations in four dimensions. To get around this restriction, however, an invariant four dimensional limit has been developed. In this work, we study Extreme Compact Charged Objects (ECCOs), which can resemble black holes, in a gravity framework that is compatible with Einstein Gauss Bonnet in four dimensions. Our main goal is to compare theoretical predictions with Event Horizon Telescope (EHT) observational data in order to constrain the Gauss Bonnet coupling constant α. In order to achieve this, we investigate important optical characteristics like the shadow, light bending angle, and other associated observables, as well as the geodesic structure of ECCO spacetimes in EGB gravity. Finally, we apply these findings to constrain the Gauss Bonnet constant.

gr-qc↗

arXiv2025

Optical Signatures of Einstein-Euler-Heisenberg AdS/dS Black Holes in the light of Event Horizon Telescope

Recent observations of the supermassive black holes $ M87^{*} $ and Sgr A$^{*}$ by the Event Horizon Telescope (EHT) have sparked intensified interest in studying the optical appearance of black holes (BHs). Inspired by this, we carry out a study on the optical features of Einstein-Euler-Heisenberg-Anti de Sitter/de Sitter (EEH-AdS/dS) BHs, including the trajectories of photons, shadow geometrical shape, energy emission rate, and deflection of light in this spacetime. Since, due to the nonlinear electrodynamics effects, photons propagate along null geodesics in an effective metric rather than the background metric, we first derive the effective metric of the EEH-AdS/dS BH. Then we study the null geodesics of the resulting effective metric and eventually compute the size of the EEH-AdS/dS BH shadow. To validate our results, we confront our results with the extracted information from EHT data of the supermassive BHs $ M87^{*} $ and estimate lower bounds for the shadow radius.

gr-qc↗

arXiv2025

Circular orbits and accretion disk around a deformed-Schwarzschild black hole in loop quantum gravity

In this paper, we study the motion of neutral and electrically charged particles in the vicinity of a deformed-Schwarzschild black hole inspired by Loop Quantum Gravity (LQG). To examine the motion of an electrically charged test particle, we propose an expression for electromagnetic 4-potential that contains the impacts of loop quantum gravity. This electromagnetic 4-potential satisfies approximately the covariant Maxwell's equations to first order in the loop quantum effects. We explore the effects of the loop quantum correction parameter on the particle geodesics. We investigate the innermost stable circular orbits (ISCOs) for both neutral and electrically charged particles in detail, demonstrating that the loop quantum parameter significantly influences on the ISCO radius, causing it to shrink. Finally, we explore the accretion disk around the loop quantum black hole. We delve into the electromagnetic radiation flux, temperature, differential luminosity, and the spectral luminosity as radiation properties of the accretion disk in detail. We show that the loop quantum correction parameter shifts the profile of the electromagnetic flux and accretion disk temperature towards the central object, leading to a slight increase in these quantities.

gr-qc↗

arXiv2025

Investigating QED Effects on the Thin Accretion Disk Properties Around Rotating Euler-Heisenberg Black Holes

The Einstein Euler Heisenberg (EEH) black hole model represents an extension of classical black hole solutions in general relativity by incorporating quantum electrodynamic (QED) corrections. These corrections are introduced through the inclusion of the Euler-Heisenberg Lagrangian, which accounts for the nonlinear effects of QED in the presence of strong electromagnetic fields. This study investigates the observational properties of a thin accretion disk surrounding the electrically charged rotating EEH black hole. By exploring the influence of the spin parameter and charge on key dynamical quantities such as the energy, angular momentum, angular velocity, and the innermost stable circular orbit (ISCO) of a test particle it becomes possible to analyze the radiative flux, temperature distribution, and differential luminosity of the thin accretion disk in the spacetime of the charged rotating EEH black hole. The results are compared to those of Kerr and Kerr Newman black holes in General relativity, revealing that QED corrections are found to increase the ISCO radius. Specifically, for a fixed electric charge, an increasing spin parameter leads to a larger ISCO radius compared to the standard Kerr black holes as a result of additional electromagnetic corrections introduced by the Euler Heisenberg theory. Conversely, when the spin parameter is held constant, an increase in the electric charge reduces the ISCO radius. Additionally, thin accretion disks around charged EEH rotating black holes exhibit higher temperatures and greater efficiency when the spin parameter is fixed and the electric charge is increased.

gr-qc↗

arXiv2025

Black holes surrounded by massive vector fields in Kaluza-Klein gravity

We present an exact black hole solution surrounded by massive vector fields predicted by Kaluza-Klein (KK) gravity. KK gravity in four dimensions (4D) is of particular interest, as it predicts a tower of particle states, including gravitons with spin-0 and spin-1 components, in addition to the massless spin-2 gravitons of general relativity. The extra degrees of freedom in the gravitational sector modify the law of gravity, allowing the theory to explain the effects attributed to dark matter in the universe. In this paper, we construct a black hole solution surrounded by massive spin-1 gravitons within KK theory. In addition to the influence of the massive vector fields, we incorporate an interaction term between the black hole and the massive vector field. The black hole solution is affected by the mass of the spin-1 graviton and an additional parameter that encodes corrections to Newton's constant, as well as the coupling between the massive vector field and the black hole mass. We find that the coupling between the massive vector field and the black hole mimics the effect of an electric charge. To this end, we investigate the accretion disk, quasinormal modes (QNMs), and the stability of the black hole spacetime. Finally, we use Event Horizon Telescope (EHT) observations of Sgr A* to constrain the black hole parameters.

gr-qc↗

arXiv2025

Observational Viability of Anisotropic Inflation Revisited

We investigate anisotropic inflation within the single-field model featuring an intermediate scale factor. Our analysis reveals that the anisotropic nature of the Friedmann equations in this framework affects the slow-roll parameters, which in turn influence key perturbation parameters. Using a numerical approach, we derive constraints on the intermediate parameter $β$ and the anisotropic parameter $c$. Our results show that the model is consistent with Planck2018 TT, TE, EE +lowE+lensing+BK14+BAO data at $68\%$ CL, for $0.84<β<1$ and $7.34<c<27.7$. At $95\%$ CL the consistency holds for $0.77<β<1$ and $7.17<c<28.9$. The model is also consistent with Planck2018 TT, TE, EE +lowE+lensing+BK18+BAO data, for $0.91<β<1$ and $8.00<c<27.4$ (at $68\%$ CL), and $0.88<β<1$ and $7.40<c<28.8$ (at $95\%$ CL). Additionally, we examine the reheating phase using these constraints on constraints on $β$ and $c$ and determine the observationally consistent ranges for the number of e-folds and the temperature during the reheating phase.

astro-ph.CO↗

arXiv2024

Accretion onto a Charged Black Hole in Consistent 4D Einstein-Gauss-Bonnet Gravity

In astrophysics, accretion is the process by which a massive object acquires matter. The infall leads to the extraction of gravitational energy. Accretion onto dark compact objects such as black holes, neutron stars, and white dwarfs is a crucial process in astrophysics as it turns gravitational energy into radiation. The accretion process is an effective technique to investigate the properties of other theories of gravity by examining the behavior of their solutions with compact objects. In this paper, we investigate the behavior of test particles around a charged four dimensional Einstein Gauss Bonnet black hole in order to understand their innermost stable circular orbit (ISCO) and energy flux, differential luminosity, and temperature of the accretion disk. Then, we examine particle oscillations around a central object via applying restoring forces to treat perturbations. Next, we explore the accretion of perfect fluid onto a charged 4D EGB black hole. We develop analytical formulas for four-velocity and proper energy density of the accreting fluid. The EGB parameter and the charge affect properties of the test particles by decreasing their ISCO radius and also decreasing their energy flux. Increasing the EGB parameter and the charge, near the central source reduces both the energy density and the radial component of the infalling fluid's four-velocity.

gr-qc↗

arXiv2024

Traces of Quantum Gravity Effects at Late time Cosmological Dynamics via Distance Measures

Inspired by the entropy-area relation of black hole thermodynamics, we study the thermodynamics of cosmological apparent horizon in a spatially flat Friedmann-Robertson-Walker (FRW) universe in the framework of an Extended Uncertainty Principle (EUP). The adopted EUP naturally admits a minimal measurable momentum (equivalently a maximal measurable length), as an infrared cutoff in the theory. We derive the modified Friedmann equations in this setup and explore some predictions of these equations for the late time universe via distance measures. We show that in this framework it is possible to realize the late time cosmic speed-up and transition to the phantom phase of the equation of state parameter of the effective cosmic fluid without recourse to any dark energy component or modified gravity. Inspection of various distance measures in this framework shows that an EUP with a negative deformation parameter suffices for the interpretation of the late time asymptotically de Sitter universe with standard non-relativistic matter.

gr-qc↗

arXiv2024

Shadow behavior of an EMSG charged black hole

Recent shadow images of Sgr A and M87 captured by Event Horizon Telescope (EHT) collaboration confirms the existence of black holes or their possible alternatives in the center of galaxies. On the other hand, the new image of Sgr A in polarized light suggests a Magnetic field spiraling at the Edge of the Milky Way Central Black Hole. Due to the gravitational lensing effect, the bending of light in the background geometry of the black hole casts a shadow. In recent years, black holes and their properties have been vastly studied in the framework of General Relativity and other modified theories of gravity. One of the possibilities to generalize GR is Energy Momentum Squared Gravity (EMSG). It is important to mention that EMSG modifies all matter field equations, adding some non linear terms to Maxwell equations. EMSG theory as a modified theory of gravity predicts an asymptotically de Sitter charged black hole whose shadow cast and other related characteristics have not been examined yet. Hence we consider the EMSG charged black hole and investigate the shadow shape of this kind of black hole solution in confrontation with EHT results. In the case of non linear electrodynamics, the path of the photon is null on some effective metric. By deriving the effective metric of an EMSG charged black hole we study the null geodesics of the effective metric in the Hamilton Jacobi method. we find the photon orbits and compute the shadow size of this black hole. Then we examine how electric charge and the coupling constant of the EMSG affect the shadow size of the black hole in a positively accelerated expanding universe (with a positive cosmological constant). We explore the viable values of these parameters constrained by EHT data by comparing the shadow radius of an EMSG charged black hole with the shadow size of Sgr A.

gr-qc↗

arXiv2024

Dissipative Quintessential Cosmic Inflation

In this paper we construct a dissipative quintessential cosmic inflation. For this purpose, we add a multiplicative dissipative term in the standard quintessence field Lagrangian. We consider the specific form of dissipation as the time integral including the Hubble parameter and an arbitrary function that describes the dissipative properties of the quintessential scalar field. Inflation parameters and observables are calculated under slow-roll approximations and a detailed calculation of the cosmological perturbations is performed in this setup. We consider different forms of potentials and calculate the scalar spectral index and tensor-to-scalar ratio for a constant as well as variable dissipation function. To check the reliability of this model, a numerical analysis on the model parameters space is done in confrontation with recent observational data. By comparing the results with observational joint datasets at 68% and 95% confidence levels, we obtain some constraints on the model parameters space, specially the dissipation factor with e-folds numbers N = 55 and N = 60. As some specific results, we show that the power-law potential with a constant dissipation factor and N = 60 is mildly consistent with observational data in some restricted domains of the model parameter space with very small and negative dissipation factor and a negligible tensor-toscalar ratio. But this case with N = 55 is consistent with observation considerably. For power-law potential and variable dissipation factor as $Q = αϕ^n$, the consistency with observation is also considerable with a reliable tensor-to-scalar ratio. The quadratic and quartic potentials with variable dissipation function as $Q = αϕ^n$ are consistent with Planck2018 TT, TE, EE+lowE+lensing data at the 68% and 95% levels of confidence for some intervals of the parameter n.

astro-ph.CO↗