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Sara Saghafi

Publications and source records attributed to Sara Saghafi.

At least 19 recordsLinked to original sources

Effective reheating in Gauss--Bonnet inflation with $\mu(\phi,X)$ coupling

We study effective reheating in a scalar--Gauss--Bonnet inflationary model with a phase-space-dependent coupling $\mu(\phi,X)$, 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 $\lambda_{\rm GB}$ 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 $\lambda_{\rm GB}$ 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

Probing Quantum Gravity through Chaotic Orbits and Strong-Field Effects in Kerr Black Holes Embedded in Perfect Fluid Dark Matter

We study the nonlinear photon dynamics in quantum improved rotating black hole surrounded by perfect fluid dark matter (PFDM) using several methods of analysis, including Poincar\'e sections, Lyapunov exponents, Kolmogorov-Sinai (KS) entropy and weighted Birkhoff averages (WBA). In particular, we examine the effect of quantum-improved parameter $(\tilde{\omega})$ and PFDM parameter $(\zeta)$ on the null geodesic motion hence stability of circular orbit. Poincar\'e sections illustrate the transition from regular to chaotic motion as these parameters increase, characterized by the deformation and fragmentation of invariant tori and the emergence of scattered chaotic regions in phase space. The stability properties of null circular orbits are quantified through the Lyapunov indicators, revealing that both the quantum improvement parameter and the PFDM parameter enhance the sensitivity of photon trajectories to initial conditions. The KS entropy provides an independent measure of dynamical complexity and confirms the growth of chaotic behavior with increasing quantum and PFDM corrections. Additionally, the WBA method offers a robust quantitative criterion for distinguishing regular and chaotic orbits and allows a detailed mapping of the phase-space structure. The results demonstrate that the combined effects of quantum gravity corrections and PFDM significantly modify the effective potential governing photon motion, leading to a rich mixed phase-space structure with coexisting regular and chaotic regions. These findings underscore the crucial role of quantum and dark matter contributions in shaping photon dynamics near rotating black holes and suggest possible observational implications on black hole shadows and gravitational lensing in strong-field regimes.

gr-qc

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

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 $\Delta$ by an effective Gauss-Bonnet-like parameter $\alpha$, such that the spacetime reduces to the standard Kerr Newman solution in the limit $\alpha \to 0$. 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 $\alpha$ 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 $\alpha$ 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

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

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 \(\mu(\phi,X)\) 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

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 $\theta_{d}=42\pm 3~\mu\mathrm{as}$ and a circularity deviation $\Delta C\leq 0.1$. 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 $\theta_{d}$ and $\Delta C$, 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 $\mathcal{O}(\mu\mathrm{as})$, placing them near the sensitivity threshold of present instruments and within reach of next-generation horizon-scale imaging.

gr-qc

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

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 {\alpha}. 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

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

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

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

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

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

Higher-Dimensional MOG dark compact object: shadow behavior in the light of EHT observations

Consideration of extra spatial dimensions is motivated by the unification of gravity with other interactions, the achievement of the ultimate framework of quantum gravity, and fundamental problems in particle physics and cosmology. Much attention has been focused on the effect of these extra dimensions on the modified theories of gravity. Analytically examining astrophysical phenomena like black hole shadows is one approach to understand how extra dimensions would affect the modified gravitational theories. The purpose of this study is to derive a higher dimensional metric for a dark compact object in STVG theory and then examine the behavior of the shadow shapes for this solution in STVG theory in higher dimensions. We apply the Carter method to formulate the geodesic equations and the Hamilton Jacobi method to find photon orbits around this higher dimensional MOG dark compact object. We investigate the effects of extra dimensions and the STVG parameter alpha on the black hole shadow size. Next, we compare the shadow radius of this higher dimensional MOG dark compact object to the shadow size of the supermassive black hole M87, which has been realized by the Event Horizon Telescope (EHT) collaborations, in order to restrict these parameters. We find that extra dimensions in the STVG theory typically lead to a reduction in the shadow size of the higher dimensional MOG dark compact object, whereas the effect of parameter alpha on this black hole shadow is suppressible. Remarkably, given the constraints from EHT observations, we find that the shadow size of the four dimensional MOG dark compact object lies in the confidence levels of the EHT data. Finally, we investigate the issue of acceleration bounds in higher dimensional MOG dark compact object in confrontation with EHT data of M87.

gr-qc

Extended Uncertainty Principle: A Deeper Insight into the Hubble Tension?

The standard cosmological model, known as the LambdaCDM model, has been successful in many respects, but it has some significant discrepancies, some of which have not been resolved yet. In measuring the Hubble-Lematre parameter, there is an apparent discrepancy which is known as the Hubble tension, defined as differences in values of this parameter measured by the Type Ia Supernovae (SNeIa) data (a model-independent method) and by the Cosmic Microwave Background (CMB) radiation maps (a model-dependent method). Although many potential solutions have been proposed, the issue still remains unresolved. Recently, it was observed that the Hubble tension can be due to the concept of uncertainty in measuring cosmological parameters at large distance scales through applying the Heisenberg Uncertainty Principle (HUP) in cosmological setups. Extending this pioneering idea, in the present study we plan to incorporate the Extended Uncertainty Principle (EUP) containing a minimal fundamental measurable momentum (or equivalently, a maximal fundamental measurable length) as a candidate setup for describing large-scale effects of Quantum Gravity (QG) to address the Hubble tension and constrain the EUP length scale. In this regard, by finding a relevant formula for the effective photon rest mass in terms of the present-time value of the Hubble-Lematre parameter, we see that discrepancies in the value of photon rest mass associated with the Hubble-Lematre parameter values estimated from model-independent and model-dependent methods perhaps is the cause of Hubble tension.

gr-qc

Asymptotically locally flat and AdS higher-dimensional black holes of Einstein-Horndeski-Maxwell gravity in the light of EHT observations: shadow behavior and deflection angle

Unification of gravity with other interactions, achieving the ultimate framework of quantum gravity, and fundamental problems in particle physics and cosmology motivate to consider extra spatial dimensions. The impact of these extra dimensions on the modified theories of gravity has attracted a lot of attention. One way to examine how extra dimensions affect the modified gravitational theories is to analytically investigate astrophysical phenomena, such as black hole shadows. In this study, we aim to investigate the behavior of the shadow shapes of higher-dimensional charged black hole solutions including asymptotically locally flat (ALF) and asymptotically locally AdS (ALAdS) in Einstein-Horndeski-Maxwell (EHM) gravitational theory. We utilize the Hamilton-Jacobi method to find photon orbits around these black holes as well as the Carter approach to formulate the geodesic equations. We examine how extra dimensions, negative cosmological constant, electric charge, and coupling constants of the EHM gravity affect the shadow size of the black hole. Then, we constrain these parameters by comparing the shadow radius of these black holes with the shadow size of M87* supermassive black hole captured by the Event Horizon Telescope (EHT) collaborations. We discover that generally the presence of extra dimensions within the EHM gravity results in reducing the shadow size of higher-dimensional ALF and ALAdS charged black holes, whereas the impact of electric charge on the shadow of these black holes is suppressible....

gr-qc

Accretion onto a static spherically symmetric regular MOG dark compact object

In astrophysics, the process of a massive body acquiring matter is referred to as accretion. The extraction of gravitational energy occurs as a result of the infall. Since it converts gravitational energy into radiation, accretion onto dark compact objects, e.g. black holes, neutron stars, and white dwarfs is an extremely significant process in the astrophysical context. Accretion process is a fruitful way to explore the features of modified gravity (MOG) theories by testing the behavior of their solutions associated with dark compact objects. In this paper, we study the motion of electrically neutral and charged particles moving in around a regular spherically symmetric MOG dark compact object to explore their related innermost stable circular orbit (ISCO) and energy flux. Then, we turn to investigate the accretion of perfect fluid onto the regular spherically symmetric MOG dark compact object. We obtain analytical expressions for four-velocity and proper energy density of the accreting fluid. We see that the MOG parameter increases the ISCO radius of either electrically neutral or charged test particles while it decreases the corresponding energy flux. Moreover, the energy density and the radial component of the four-velocity of the infalling fluid decrease by increasing the MOG parameter near the central source.

gr-qc

Shadow Behavior of the Quantum-Corrected Schwarzschild Black Hole Immersed in Holographic Quintessence

In this paper, we aim to explore the impact of the Planck scale corrections and the Holographic quintessence on the shadow behavior of non-rotating black holes. To do this, we consider the quantum-corrected Schwarzschild black hole surrounded by the quintessence field inspired by the Kazakov-Solodukhin and the Kiselev ideas, and we call this combination the Kazakov-Solodukhin-Kiselev (KSK) black hole. We conclude that the quintessence field as the candidate of dark energy in the black hole can be interpreted as Holographic quintessence. To find the geodesic equations of the black hole, we employ the Hamilton-Jacobi approach and also, the Carter procedure. We discover that the size of the shadow of this black hole, which depends on its central mass, is also determined by the Planck scale effects and Holographic quintessence.

hep-th