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Diego Sáez-Chillón Gómez

Publications and source records attributed to Diego Sáez-Chillón Gómez.

At least 19 recordsLinked to original sources
arXiv2026

Optical Signatures of Black Holes Surrounded by a Generalized Cloud of Strings

In this work, we investigate the optical signatures of black holes surrounded by a generalized cloud of strings, described by the Letelier--Alencar spacetime. We first analyze null geodesics, showing that the string parameters can either increase or decrease the photon sphere radius and the critical impact parameter relative to their Schwarzschild values. Since the spacetime is not asymptotically flat, the critical impact parameter does not directly coincide with the physical shadow radius. We use this distinction to derive constraints on the model parameters from the shadow radius bounds of Sgr A* and M87*. In the weak-field regime, we calculate the light deflection angle using both the Gauss--Bonnet method and a perturbative expansion of the null geodesic equations, obtaining consistent results that contain a contribution from the asymptotically conical geometry in addition to the local gravitational deflection. We also examine the Shapiro time delay and identify a distance dependent contribution associated with the non Minkowskian asymptotic background. Finally, we study the optical appearance of geometrically and optically thin accretion disks and construct celestial sphere images through backward ray tracing. The transfer functions and lensing images show that the generalized cloud of strings modifies the null geodesic mapping in a nonuniform manner rather than producing a simple rescaling of the Schwarzschild image. In particular, configurations with a smaller shadow may still produce an outward displacement of the main Einstein ring image, demonstrating that different optical observables probe distinct aspects of photon propagation in this geometry.

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arXiv2026

From massless wormholes to massive black bounces: Shadows and multiple light rings

We develop a general framework for constructing massive black bounce geometries from massless wormhole seeds by introducing a position-dependent mass function while preserving the underlying areal-radius profile. We apply this procedure to the generalized Ellis-Bronnikov wormhole and obtain a new family of generalized Bardeen-like black bounce space-times, which continuously interpolates between traversable wormholes and regular black holes. We investigate the motion of massive particles and photons and show that, in contrast with the simpler orbital structure of the massless generalized Ellis-Bronnikov geometry, the generalized Bardeen-like space-time can exhibit multiple circular orbits and a rich light ring structure, including configurations with two unstable circular photon orbits separated by a stable one. We determine the corresponding photon sphere and shadow radii and compare our predictions with Event Horizon Telescope observations of Sgr A*, deriving observational constraints on the parameter space of the model. We further investigate the optical appearance produced by a geometrically and optically thin accretion disk through ray tracing. Multiple light rings generate characteristic nested structures in the high-resolution intensity profiles, whose relative brightness is strongly affected by the mass deformation and gravitational redshift. However, after modeling the finite angular resolution of the EHT with a Gaussian beam convolution, these fine structures are largely washed out, revealing a strong observational degeneracy between these exotic compact objects and standard black hole geometries at current EHT resolution.

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arXiv2026

Analyzing Quasi-normal modes spectrum in asymptotically de Sitter black bounces and their optical appearance

This work explores asymptotically de Sitter black-bounce spacetimes, obtained as a regular extension of the Schwarzschild-de Sitter geometry via the Simpson-Visser prescription. The resulting family of solutions provides a smooth interpolation between regular black holes, extremal configurations, and traversable wormholes within a single geometrical framework. In particular, we verify the regularity of the spacetime by showing that (some of) curvature invariants, including the Kretschmann and Ricci scalars, remain finite at the origin for any non-vanishing value of the regularization parameter $a$, ensuring geodesic completeness. An analysis of the effective stress-energy tensor reveals a violation of the null energy condition, which can be consistently interpreted in terms of a composite source involving a phantom scalar field coupled to nonlinear electrodynamics. Subsequently, we investigate the observational signatures of these geometries by studying: i) null geodesic propagation and ii) constructing the associated optical appearance. In particular, we determine the shadow radius and characterize the structure of photon rings through numerical ray-tracing procedure. Finally, we compute the spectrum of axial gravitational perturbations using a pseudospectral approach, obtaining the quasinormal mode frequencies and their overtones. Our results show a non-trivial interplay between the cosmological constant and the regularization scale $a$, which imprints itself on both the light-ring structure and the quasinormal spectrum. These features suggest potential observational discriminants that could distinguish these regular spacetimes from their singular counterparts in future gravitational-wave and high-resolution electromagnetic observations.

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arXiv2026

Horizon singularity, energy conditions and shadows in time-dependent and spherically symmetric spacetime

Some dynamical spacetime solutions that describe ultra-compact objects are analysed. We show that by turning a static spacetime that describes a one-way wormhole into a dynamical time-dependent one, a singularity might arise in general. Several well-known wormhole solutions are studied, which are shown to become singular when promoted to a time-dependent wormhole. However, we show that one can construct a dynamical wormhole free of singularities. The corresponding Lagrangian that holds such dynamical spacetimes is also reconstructed, based on a two-scalar-field model free of ghosts. Energy conditions are also analysed, which might remain satisfied for a dynamical -singular- wormhole. Finally, the evolution of the optical appearance of such dynamical objects is simulated and compared to static solutions. Some interesting features emerge that point to a way to measure the time evolution of dynamical objects by looking at the photon ring structure around the object.

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arXiv2026

Observational signatures of negative mass wormholes through their shadows

Systems containing objects with negative mass (NMOs) are considered. Such a system consists of one object with positive mass and one NMO, where a bound state exists even though the force exerted by the NMO on the object with positive-mass is repulsive. Unlike standard binaries composed of positive-mass objects, the emitted gravitational waves exhibit decreasing frequency and amplitude over time. We propose a model that removes the ghost appearing in the construction of the Ellis-Bronnikov wormhole, a candidate for a NMO. Furthermore, we perform numerical simulations to obtain the optical appearance of such NMOs. The observed luminosity is also compared with the Schwarzschild black hole and with the Simpson-Visser wormhole, revealing clear differences in the photon ring substructure around the central object.

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arXiv2026

Tidal forces around the Letelier-Alencar cloud of strings black hole

In this work, we investigate relativistic tidal forces around a black hole sourced by a cloud of strings, described by the generalized Letelier-Alencar solution. We first review the original Letelier spacetime and its recent generalization, computing the Kretschmann scalar and showing that the generalized model exhibits a stronger curvature divergence at $r \to 0$ than both Letelier and Schwarzschild cases. We then analyze geodesic motion in this background. For massless particles, we focus on circular photon orbits, while for massive particles, we consider both radial infall and circular motion. We find that the radii of the photon sphere and of the innermost stable circular orbit increase with the cloud of strings parameter $g_s$ and decrease with the length scale $l_s$, and circular orbits cease to exist in certain regions of the parameter space. For radial motion, we compute the radial acceleration and the corresponding tidal forces. In this case, we show that an inversion between stretching and compression may occur, although this regime is typically hidden inside the event horizon. Once the tidal forces are known, we computed the behavior of the displacement vector in order to verify whether the usual stretching behavior induced by tidal forces is preserved. Finally, we study tidal forces for observers in circular motion, showing that the cloud of strings modifies the Keplerian frequency and the tidal force profile even at large distances, and that in this case there is no sign change of the tidal components.

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arXiv2026

Black bounce as a quantum correction from string T-duality: Thermodynamics, energy conditions, and observational imprints from EHT

Motivated by quantum gravity effects suggested by string theory, we investigate gravitational configurations sourced by an effective energy density inspired by T-duality. This density naturally introduces a minimal length scale $l_0$ that acts as an ultraviolet regulator, allowing the description of nonsingular geometries within a classical framework. By employing it as the matter source in the Einstein equations, we construct static and spherically symmetric spacetimes that interpolate smoothly between regular black holes and traversable wormholes, providing a geometric realization of the black bounce scenario. We examine the curvature invariants and confirm the absence of curvature singularities throughout the spacetime. The conditions for the existence of event horizons are analyzed in detail, which allows us to determine the causal structure of the solution. A comprehensive study of the geodesic motion is performed for both massive and massless particles, revealing the presence of photon circular orbits and an innermost stable circular orbit for massive particles. Using observational data from the Event Horizon Telescope, we constrain the minimal length parameter through the black hole shadow radius, finding that for $l_0 \lesssim 1.15\, M_{\text{ADM}}$ our solution remains consistent with observations within the $2σ$ confidence level. The optical appearance of spacetime is further investigated by considering a thin accretion disk surrounding the black bounce. From the heat capacity, we analyze the thermodynamic stability of the solution and identify the presence of a phase transition. Finally, we examine the energy conditions and discuss which of them are violated by the effective fluid supporting this geometry.

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arXiv2025

Light trajectories and optical appearances in asymptotically Anti-de Sitter-Schwarzschild and black string space-times

The Event Horizon Telescope (EHT) imaging of the central objects in the M87 and Milky Way galaxies provide compelling evidence that these objects are consistent with (Kerr) black holes. In view of these observations and the future expectations of Very Long Baseline Interferometry (VLBI) on which the EHT observations are based, an intensive research work has been carried out in the literature to simulating light trajectories and reconstructing the corresponding optical appearance for a wide array of modified black holes and ultra-compact objects. The corresponding images are directly affected not only by the background space-time geometry but also by the physics of the accretion disk, whose combination yields a characteristic fingerprint. In this paper, we consider such a fingerprint for objects which are not asymptotically flat but instead approach a Anti-de Sitter space-time. This assumption significantly influences light trajectories and, consequently, the corresponding images of the objects as seen by an observer at some distance, which can be used in future VLBI observations for testing alternatives of this kind to the Kerr paradigm. We illustrate our considerations with the examples of a Schwarzschild-Anti-de Sitter black hole and a black string, discussing their most notable departures from canonical, asymptotically-flat black hole space-times.

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arXiv2025

Tidal Stretching and Compression in Black Bounce Backgrounds

Black bounces are compact objects that combine the structures of regular black holes with those of wormholes. These spacetimes exhibit a rich causal structure and can differ fundamentally from usual black holes. In this work, we study the behavior of the tidal forces by considering different black bounce models. To this end, we start with the geodesic deviation equation and the tidal tensor, from which we compute the radial and angular components of the tidal forces. We find that these components are finite throughout the entire spacetime, including at the wormhole throats. Through the components of the displacement vector, we observe that, unlike the Schwarzschild case, a compression effect on bodies may occur in certain regions.

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arXiv2025

Shadows from thin accretion disks of parametrized black hole solutions

We discuss the optical appearance from thin accretion disks in parametrized black holes, namely, solutions characterized by an arbitrarily large number of parameters without any regards to the theory of the gravitational and matter fields they come from. More precisely, we consider the leading-order terms of the spherically symmetric Johanssen-Psaltis (JP) and Konoplya-Rezzolla-Zhidenko (KRZ) parametrizations after imposing constraints from asymptotic flatness and solar system observations. Furthermore, we use the inferred correlation, by the Event Horizon Telescope Collaboration, between the size of the bright ring (which is directly observable) and the size of the central brightness depression (which is not) of M87 and Sgr A$^*$ central supermassive objects, to constrain the parameters of the leading-order JP and KRZ solutions. Using ten samples of the Standard Unbound distribution previously employed in the literature to reproduce certain scenarios of General Relativistic HydroDynamical simulations, we produce images of four samples of JP and KRZ geometries enhancing and diminishing the shadow's size, respectively. Via a qualitative and quantitative analysis of the features of the corresponding photon rings and, in particular, of their relative brightness, we argue that it should be possible to distinguish between such parametrized solutions and the Schwarzschild geometry via future upgrades of very long baseline interferometry. We furthermore consider images of some naked objects within these parametrizations, and also discuss the role of inclination in comparing images of different black holes.

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arXiv2025

Non-stationary wormholes with the presence of scalar fields and modified gravity

Some novel regular spacetimes are considered that show a non-stationary wormhole structure. A Simpson-Visser-like procedure is applied to reconstruct these regular spacetimes, free of time-like and space-like singularities. Such a procedure is also applied to describe a regular cosmological expansion, where the universe reaches a minimum scale and then rebounds. This type of regular spacetime is achieved by considering some scalar fields as sources, with the appropriate kinetic term and scalar potential. We show that all these sources turn out to be ghosts due to the wrong sign of the kinetic term. These ghosts can, however, be eliminated by constraints. Nevertheless, the same procedure is also explored in the framework of modified gravities, particularly within the so-called f(R) gravity, where a new wormhole spacetime is also obtained that does not require a ghost scalar field.

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arXiv2025

Quasi-normal modes and echoes of generalized black hole bounces and their correspondence with shadows

We study the quasi-normal modes (QNMs) of a family of generalized black bounces interpolating between regular black holes and traversable wormhole solutions according to a single extra parameter $a$. Firstly, working with a generic spherically symmetric space-time with arbitrary radial function and an anisotropic fluid matter source, the general equations for the gravitational waves are obtained. Then, we focus on such particular space-time metric and use the time-domain method to find the evolution of the QNMs with respect to the parameter $a$, finding larger frequencies and damped modes as $a$ grows. Furthermore we find that, for a gap in the values of $a$ for which no horizon is present but several photon spheres are, echoes are produced. Such echoes, which come from trapped modes in the potential well that are eventually leaked off for higher frequencies, appear as repetitions of the original wave but with modulated amplitude and decreased frequencies, and study their evolution with $a$. In addition, at the light of the correspondence recently discussed in the literature between QNMs and black hole imaging, we discuss the relation of the features of such echoes with those features (photon rings and shadows) of optical images from thin accretion disks. Despite working with simplified models and settings, our analysis provides useful insights on the usefulness of the correspondence for both gravitational waves and shadows.

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arXiv2025

Field Sources for Wormholes With Multiple Throats/Anti-throats

In this work, we investigate wormhole geometries with multiple throats and anti-throats in general relativity. The existence of these structures is identified through the analysis of minima and maxima in the area of the solution. Using embedding diagrams, we visualize the geometry and demonstrate that these objects exhibit a complex structure, distinct from standard single-throat wormholes. We further analyze the geodesic motion in such spacetimes. The solutions are derived from Einstein's equations by coupling a phantom scalar field to nonlinear electrodynamics, and we show that distinct scalar field profiles can generate the same spacetime geometry. Additionally, we examine the energy conditions and demonstrate that, for specific parameter choices, all energy conditions can be partially satisfied in certain regions of spacetime.

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arXiv2025

General spherically symmetric black bounces within non-linear electrodynamics

Over the last years, the search of new regular black bounce solutions has drawn a lot of attentional over the international community working in gravitation. Indeed, in the era of gravitational waves detections out of binary mergers and of the imaging of the plasma around supermassive black holes, the study of everywhere regular solutions has become a common trend given the unique opportunity posed by multi-messenger astronomy to test deviations from the Kerr family of solutions. Among them, in this paper we consider the black bounce paradigm introduced by Simpson and Visser in [JCAP \textbf{02}, 042 (2019)], and provide a general procedure for reconstructing static spherically symmetric black bounce-type solutions that might interpolate between regular black holes and wormholes. We show that even after imposing some smoothness and flatness conditions on the metric components, additional analysis is required to obtain a well-defined black bounce solution. Then, the corresponding matter Lagrangian is reconstructed by using non-linear electrodynamics and the energy conditions are studied.

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arXiv2024

Modified gravity/Dynamical Dark Energy vs $Λ$CDM: is the game over?

Over the last decades, tests on the standard model of cosmology, the so-called $Λ$CDM model, have been widely analysed and compared with many different models for describing dark energy. Modified gravities have played an important role in this sense as an alternative to $Λ$CDM model. Previous observational data has been always favouring $Λ$CDM model in comparison to any other model. While statistically speaking, alternative models have shown tehir power, fitting in some cases the observational data slightly better than $Λ$CDM, the significance and goodness of the fits were not significantly relevant to exclude the standard model of cosmology. In this paper, a generalisation of exponential $F(R)$ gravity is considered and compared with $Λ$CDM model by using the latest observational data. Also some well-known model independent parameterisations for the equation of state (EoS) of dark energy are explored. These scenarios are confronted with the renewed observational data involving the Pantheon plus datasets of supernovae type Ia, the Hubble parameter estimations, data from the cosmic microwave background and baryon acoustic oscillations, where the latter includes the data provided by Dark Energy Spectroscopic Instrument collaboration. Results of this analysis suggest that standard exponential $F(R)$ models provide much better fits than $Λ$CDM model, which is excluded at 4$σ$. Moreover, the parameterisations of the equation of state suggest a non-constant EoS parameter for dark energy, where $Λ$CDM model is also excluded at 4$σ$.

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arXiv2024

Boundary terms and on-shell action in Ricci-based gravity theories: the Hamiltonian formulation

Considering the so-called Ricci-based gravity theories, a family of extensions of General Relativity whose action is given by a non-linear function of contractions and products of the (symmetric part of the) Ricci tensor of an independent connection, the Hamiltonian formulation of the theory is obtained. To do so, the independent connection is decomposed in two parts, one compatible with a metric tensor and the other one given by a 3-rank tensor. Subsequently, the Riemann tensor is expressed in terms of its projected components onto a hypersurface, allowing to construct the $3+1$ decomposition of the theory and the corresponding Gauss-Codazzi relations, where the boundary terms naturally arise in the gravitational action. Finally, the ADM decomposition is followed in order to construct the corresponding Hamiltonian and the ADM energy for any Ricci-based gravity theory. The formalism is applied to the simple case of Schwarzschild space-time.

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arXiv2024

Propagation and emission of gravitational waves in the weak-field limit within the Palatini formalism

In the era of gravitational waves physics, when detections of wave fronts are increasing in number, sensibility, frequencies and distances, gravitational physics has entered a period of maximum activity and brilliance. This has open a new window where General Relativity can be challenged in both weak as strong-field regimes. In this paper, we focus on the analysis of gravitational waves propagation and emission in the weak-field regime for gravitational theories within the Palatini formalism. Our results show that gravitational waves propagation in vacuum matches General Relativity predictions as well as the multipolar expansion when considering weak sources, independently of the form of the gravitational action.

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arXiv2024

On black bounce space-times in non-linear electrodynamics

One of the main issues in gravitation is the presence of singularities in the most common space-time solutions of General Relativity, as the case of black holes. A way of constructing regular solutions that remove spacelike singularities consists in implement a bounce on such space-time, leading to what is usually known as black bounce space-times. Such space-times are known to describe regular black holes or traversable wormholes. However, one of the main issues lies on reconstructing the appropriate source that leads to such a solution. In this paper, a reconstruction method is implemented to show that such types of metrics can be well accommodated in non-linear electrodynamics with the presence of a scalar field. Some of the most important black bounces solutions are reconstructed in this framework, both in 3+1 as in 2+1 dimensions. For the first time in the literature, these solutions have an electrically charged source of matter from non-linear electrodynamics. Specific features are indicated that distinguish electric sources from magnetic ones, previously found for the same space-times.

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