arXiv subjects

M. F. Fauzi

Publications and source records attributed to M. F. Fauzi.

8 recordsLinked to original sources

arXiv2026

A Quantum-Gravity-Motivated GUP Effective Metric

Recent critiques have addressed certain aspects of the generalized uncertainty principle (GUP) effective metric (Ong 2023). This study presents a scale-dependent quadratic GUP effective metric, constructed through analyses of the gravity-induced phase shift (COW experiment) and the Einstein-Bohr photon box Gedanken experiment. In contrast to the procedure outlined in (Xiang et al. 2018), the momentum-dependent metric is improved by introducing an interpolating function $Δp (r)$, which employs the effective distance concept to accurately capture the distinct behavior of $Δp (r)$ in both short and long distance regimes. The resulting effective metric exhibits the same structure as that derived from the Renormalization Group (RG) theory. However, the RG parameter $\hatγ$ can now be related to the dimensionless GUP parameter $β_0$, thereby distinguishing this metric from the RG-based approach. The corresponding effective metric prediction demonstrates internal consistency of the model, and phenomenologically the predictions are in agreement with some quantum black hole models in some limits. The effective metric improved black hole thermodynamics and shadow predictions compared to the heuristic approach and other proposed GUP effective metrics. Furthermore, the relationship between GUP and $f(R)$ gravity (D'Agostino et al. 2026) may provide a possible future route toward a more fundamental description of GUP.

gr-qc↗

arXiv2026

Photon Propagation and Black Hole Imaging in Kruglov Nonlinear Electrodynamics

We investigate the effective photon geometry associated with black holes in Kruglov nonlinear electrodynamics and its consequences for strong-field optical phenomena. This model constitutes a one-parameter generalization of Born-Infeld electrodynamics, interpolating between Maxwell theory and exponential electrodynamics through the parameter $q$. For a wide range of $q$, the spacetime geometry outside the event horizon remains close to the Reissner-Nordström solution, while photon propagation is governed by an effective geometry that depends sensitively on the nonlinear electrodynamics sector. We study the corresponding null geodesic structure through fully numerical calculations, focusing on photon spheres, light deflection, black hole shadows, and accretion-disk images. The effective geometry shows qualitatively distinct features depending on $q$. In particular, sufficiently small positive values of $q$ generate stable photon orbits outside the event horizon, together with significant modifications to the range of impact parameters supporting multiple photon trajectories. These effects produce observable modifications in the relativistic images, including systematic variations in the effective geometry. We also analyze the black hole shadow in relation to current horizon-scale constraints on Sgr~A*. Our results demonstrate that nonlinear electrodynamics can substantially modify photon propagation and relativistic image formation even when the underlying spacetime gometry remains close to the Maxwell electrodynamics case.

gr-qc↗

arXiv2026

Astrophysical environment around a black hole in the braneworld and its optical signatures

We investigate the impact of braneworld theory on the astrophysical environment surrounding a black hole. The black hole is sourced by localized matter from the bulk, which could describe both regular and singular (Schwarzschild) black hole. Employing an Einstein cluster description for the environment, we find that the anisotropic nature of the cluster, coupled with finite brane tension, leads to a weakening of gravity due to the quadratic and nonlocal corrections to the effective four-dimensional field equations. Consequently, this effect prevents horizon formation within the environment. Applying current constraints on the brane tension derived from neutron star observations, we demonstrate that this effect is particularly relevant for sub-stellar mass black holes embedded in compact environments. Furthermore, we investigate the optical signatures of finite brane tension in this scenario, specifically focusing on the black hole shadow and Einstein ring radii. We show that the Einstein ring radius decreases with a smaller brane tension, whereas the black hole shadow radius increases--somewhat contradicts the weakening gravity effects. Ultimately, these two observables may jointly serve to constrain the value of the brane tension in a very specific astrophysical scenarios.

hep-th↗

arXiv2026

Two descriptions of dark matter around a black hole: photon sphere, shadow, and lensing

We examine the observational discrepancies of two widely used models describing anisotropic (dark) matter distributions around a black hole, focusing on their photon spheres, shadow radii, and lensing observables. The models considered are the vacuum and Einstein cluster dark matter models, characterized by negative and zero radial pressure, respectively. The analysis reveals that these models display contrasting photon sphere behaviors. In particular, the Einstein cluster results in a more pronounced deviation in the shadow radius relative to the standard Schwarzschild black hole. Additionally, a distinctive lensing phenomenon associated with the matter halo is identified in both models.

gr-qc↗

arXiv2025

Imaging the destruction of a rotating regular black hole

A regular black hole, unconstrained by the weak cosmic censorship conjecture, can exceed its critical spin limit and transition into a superspinar. In this paper, we investigate the observational appearance of a rotating regular black hole, specifically the Ghosh black hole and its superspinar counterpart, when surrounded by a thin accretion disk. The resulting images reveal distinct features: the black hole closely resembles its Kerr counterpart with slight deviations, while the superspinar configuration exhibits an inner photon ring structure. Furthermore, we investigate the image transition of the Ghosh black hole that has recently been destroyed by a collapsing null shell carrying a specific angular momentum. The results indicate that, apart from a possible sudden burst of light, the inner photon ring undergoes gradual transitions over time, with the transition times depending on the additional angular momentum gained by the black hole. Our findings also suggest that the transition timescale becomes significant for supermassive black holes, with masses at least less than about twice that of M87*.

gr-qc↗

arXiv2025

Horizonless star based on regular black hole with finite radius and its observational signatures

The horizonless configuration of regular black holes has recently attracted attention as a model for ultracompact stars. In this paper, we propose a new class of regular black hole models sourced by a de Sitter vacuum with a finite radius. We focus on studying its horizonless configuration, which is modified into an anisotropic gravastar by proposing an ansatz of equation of states. We confirm that an anisotropic gravastar approaching horizon formation must violate the dominant energy condition. We also found that the proposed object has an effectively similar structure as a frozen star on the time geometry at the extremal configuration. From the proposed model, we investigate the photon geodesics inside the object and predict the optical appearance of the object surrounded by a thin accretion disk. Our imaging results indicate that, assuming light does not interact with the object's interior, its optical appearance differs from that of a thin-shell gravastar. ``Chaotic" photon ring merges for $x>x_{m}$, where $x_{m}$ represents the minimum value required for the photon sphere to exist. In addition to its optical appearance, we investigate the axial gravitational perturbations emitted by this horizonless star. Notably, echo trains are found to exist for $x>x_{m}$, as determined by numerically solving the time-dependent Regge-Wheeler equation. By comparing the echo time with the GW170817 observation, we find that a frequency of 72 Hz can be achieved, albeit at the cost of requiring a relatively high value of $\ell$.

gr-qc↗

arXiv2024

Shadow images of regular black hole with finite boundary

Regular black hole is one of the bottom-up solutions designed to eliminate the singularity at the center of black holes. Its horizonless solution has gained interest recently to model ultracompact star. Despite interesting, this proposal is problematic due to the absence of a well-defined boundary. In this work, we introduce a novel regular black hole model inspired by the Hayward black hole, incorporating additional terms to define a clear and well-defined `surface' radius $R$. We analyze the null geodesics around the object, both horizonful and horizonless configurations, by studying the photon effective potential. We further simulate the shadow images of the object surrounded by a thin accretion disk. Our results indicate that for $R > 3M$ the horizonfull shadow differs slightly from that of a Schwarzschild black hole. In the horizonless configuration, we identify distinct inner light ring structures near the central region of the shadow image, which differ from those observed in horizonless Hayward black holes.

gr-qc↗

arXiv2024

Anisotropic gravastar as horizonless regular black hole spacetime and its images illuminated by thin accretion disk

A connection between regular black holes and horizonless ultracompact objects was proposed in~\cite{Carballo-Rubio:2022nuj}. In this paper, we construct a model of a horizonless compact object, specifically an anisotropic gravastar with continuous pressure, that corresponds to regular black hole spacetime in the appropriate limit. The construction begins by modeling an equation of state that satisfies the anisotropic gravastar conditions and transitions to the de Sitter ($p=-ε$) upon horizon formation. The spacetime structure is similar to the {\it Quantum Horizonless Compact Object} (QHCO) described in~\cite{Chen:2024ibc}. Within this model, we also generate images of the corresponding objects surrounded by a thin accretion disk. The resulting images reveal that assuming that the emitting matter exists only outside the object, the inner light ring structure closely resembles that of the horizonless configuration of a regular black hole and the QHCO, yet it exhibits a distinct light ring structure compared to the thin-shell gravastar model. However, the opposite occurs when emitting matter is taken into account inside the object.

gr-qc↗