arXiv subjects

Milko Estrada

Publications and source records attributed to Milko Estrada.

At least 19 recordsLinked to original sources

arXiv2026

An Analytical Two Incompressible Fluid Star with a Mixed Ordinary Dark Matter Core and an Ordinary Matter Envelope

We construct an analytical relativistic two fluid star characterized by a mixed core, where ordinary matter and dark matter coexist as two independently conserved incompressible perfect fluids, and an envelope composed exclusively of ordinary matter. The fluids exchange neither matter nor energy and interact only through the common spacetime geometry, with the ordinary component extending across the core envelope interface while the dark component is confined to the core. Despite the mixed core--single-fluid envelope structure and the internal interface, the system remains analytically tractable, allowing us to obtain explicit expressions for the pressures and metric functions and to follow directly the effects of the dark-matter fraction and relative core size. We determine the physically admissible parameter space and derive a Buchdahl like critical compactness associated with the divergence of the central pressure, whose value depends on the relative dark matter density and the size of the mixed core. The Schwarzschild constant density star and its standard critical value, $2M/R=8/9$, are recovered in the corresponding one fluid limit. The mass--radius analysis further shows that configurations with the same global compactness can correspond to distinct internal matter distributions. Beyond providing an analytically controlled description of a core-confined second component, the construction offers a useful benchmark for identifying qualitative trends that may subsequently be examined in more realistic dark matter admixed neutron star models, whose detailed treatment lies beyond the scope of the present work.

gr-qc↗

arXiv2026

Regular fluid of strings black hole with non trivial core and asymptotic structure by gravitational decoupling

Cloud-of-strings (CS) geometries provide an effective description of one-dimensional string distributions. However, their central singularity cannot be removed through the standard regular black holes (RBH) mechanism based on an effective mass function, since the string sector contributes independently to the ultraviolet structure of the spacetime. Motivated by this observation, we investigate whether string-supported black holes can be consistently regularized while preserving the CS asymptotics and admitting a physically meaningful string-fluid interpretation. Using the gravitational decoupling method, we construct a RBH supported by an effective anisotropic string fluid. We show that the string sector deforms the de Sitter core, modifies the local topology of the spacelike slices, and introduces a longer-range correction dominating the usual Hayward/LQG term. The geometry admits non-extremal and extremal RBH, as well as a regular horizonless compact object. Moreover, the string parameter qualitatively modifies the thermodynamic evolution by shifting the Davies phase transition and the size of the black-hole remnant. Finally, the scalar quasinormal-mode spectrum exhibits systematic changes in both the oscillation frequencies and damping rates. These results show that regularizing string supported black holes is a physically distinct problem, with the matter sector governing the ultraviolet structure, thermodynamics, and dynamical response of the spacetime.

gr-qc↗

arXiv2026

Geometric Saturation of the Scale Function in Black-Bounce Spacetimes: Spherical, Planar, and Hyperbolic Transverse Sections

We present a new construction for black-bounce spacetimes based on a deformation of the scale function that acts as a geometric saturation rather than introducing a prescribed constant minimal length, as in conventional black-bounce models. This function encodes the gravitational information of the underlying vacuum counterpart, keeping the geometric quantities of the deformed metric finite in the short-distance region where tidal forces and curvature invariants of the vacuum geometry diverge. The deformation saturates at short scales, suppressing curvature divergences without requiring a potentially unstable de Sitter core. The resulting regular geometries with spherical, planar, and hyperbolic transverse sections describe regular black holes (RBHs), extremal RBHs, and traversable wormholes. A key result is that the deformed geometry remains finite at $l=0$ in all regimes, while, depending on the parameter space, the bounce, identified with the minimum of $R(l)$, may remain in the short-distance region or shift to a larger finite value of $l$. Thus, the deformation regularizes the central region and, in some regimes, also modifies the global spacetime profile. Spherical and planar RBHs satisfy the standard energy conditions near the bounce, showing that a geometric bounce does not necessarily require exotic matter sources in this region. In the hyperbolic case, the energy conditions depend more strongly on the mass parameter, being satisfied near the bounce for positive-mass RBHs but violated for negative-mass and extremal configurations. The hyperbolic sector also admits regular negative-mass black holes. For wormhole geometries, the WEC and NEC are violated near the throat, as expected.

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arXiv2026

Reissner Nordstrom black holes with integrable singularity interiors supported by string distributions

The Reissner Nordstrom (RN) black hole is characterized by two well known pathologies: a central singularity and an inner horizon associated with instabilities and a potential loss of predictability. In this work, we show that the RN exterior geometry can arise from an interior spacetime containing an integrable singularity but no inner horizon. In this scenario, tidal forces remain finite near the origin, allowing nondestructive radial infall, while the conventional description in terms of a pointlike mass is replaced by an extended matter distribution. To illustrate this possibility, we provide explicit realizations of such an interior region based on string distributions, namely a cloud of strings (CS) and a newly defined fluid of strings (FS). While the standard cloud of strings model leads to a divergence in the conserved energy associated with timelike Killing vectors, the proposed FS model can be interpreted as a geometrically screened version of the string cloud distribution and admits configurations that, when extended to infinity, describe black holes with finite conserved energy. Physical consistency between the interior region and the RN exterior geometry requires the continuity of temperature across the interface, implying thermal equilibrium between the two regions, while discontinuities in the tangential pressure can signal gravitational phase transitions. These results determine the physical conditions under which string based interior distributions can consistently generate the RN exterior geometry and clarify the circumstances under which phase transitions at the event horizon may arise.

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arXiv2025

Black hole solutions with a linear equation of state in Ho\v{r}ava gravity and Einstein--{\ae}ther theory

We provide a procedure to obtain black hole (BH) solutions in Ho\v{r}ava gravity and Einstein--{\ae}ther theory (HG--EA) for the spherically symmetric (SS) case with a static {\ae}ther. This procedure consists of first specifying the form of the equation of state (EoS), rather than prescribing an energy density profile. The usual EoS for the static and SS case, $\rho = -p_r$, is no longer satisfied due to the presence of the HG--EA terms. We study three linear EoS associated with: an analogue charged BH, a non-trivial extremal BH, and an ultra-relativistic stiff fluid, respectively. The HG--EA terms lead to exotic behaviors, both in the physical properties of the solutions and in their thermodynamics. In Case I, the matter sources can be interpreted as an exotic anisotropic matter distribution, giving rise to an effective electric-potential term in the geometry. In Case II, we obtain a non-trivial extremal BH solution for which the event horizon is $n_{\text{odd}}$-fold degenerate. In Case III, we find a solution with a non-trivial repulsive potential, where the influence of the HG--EA terms at short scales leads to the formation of a BH remnant whose horizon encloses a central singularity (instead of a de Sitter core as occurs in regular BHs

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arXiv2025

Localized five-dimensional rotating brane-world black hole Analytically Connected to an to an AdS$_5$ boundary

We provide a method to describe the geometry of an analytic, exponentially localized $5D$ rotating braneworld black hole, using the $5D$ Janis Newman algorithm in Hopf coordinates. The induced metric on the brane matches the standard $4D$ Kerr spacetime. Two curvature singularities arise: one confined to the $3$-brane at $z = r = 0$, and another that, on the brane, reproduces the Kerr singularity at $r = 0$, $\bar{\theta} = \pi/2$. The inner and event horizons, together with the stationary limit hypersurfaces, extend into the extra dimension in a pancake-like shape. We describe their behavior in the bulk. The energy momentum tensor represents a source transitioning from an anisotropic, non diagonal structure to a vacuum with negative cosmological constant. Thus, the localized black hole connects to an AdS$_5$ boundary. The geometry is supported by a non-diagonal anisotropic fluid in the bulk, requiring no matter on the brane. To evaluate the energy conditions, we use a one form from the dual basis that yields a diagonal energy momentum tensor.

gr-qc↗

arXiv2025

Five dimensional rotating and Quintessence black hole and their hypershadows

We present a new five-dimensional rotating quintessence black hole solution. To obtain this, we employ the $5D$ version of the Janis Newman algorithm, which incorporates the Hopf bifurcation. The variation of the quintessence parameter $w_q$ causes the geometry to transition from a regular rotating universe surrounded by a cosmological horizon to a singular rotating geometry, which can represent a naked singularity, a singular extremal black hole, or a singular black hole with both an inner and an outer (event) horizon. We have also determined the properties of the ergosphere. For the study of the shadow, we followed a novel approach in which the $2D$ shadow observed by humans corresponds to cross sections of the $3D$ shadow. We analyzed how quintessence affects both the size and shape of the black hole shadow, showing that increasing the quintessence strength reduces the shadow radius, contrary to the known results in $4D$. We also propose a speculative methodology to test the shadow behavior in five dimensional scenarios, in light of the constraints provided by the Event Horizon Telescope (EHT) concerning the shadow of the four-dimensional supermassive black hole M87. We identify scenarios in which the theoretical $5D$ results could be consistent with these observational constraints. We have also tested the circularity deviation of our shadows, finding that the results satisfy the bound $\Delta C \leq 0.1$ both in the case with quintessence and in the limiting case without quintessence. Finally, we determine the energy conditions required to support the solution.

gr-qc↗

arXiv2025

Black holes inside cosmic voids

This study examines the gravitational and thermodynamic properties of static, spherically symmetric black holes within cosmic voids -- vast underdense regions of the universe. By deriving a novel solution based on a universal density profile for voids, we analyze its spacetime structure, which reveals two horizons: One of the black hole and the other related to the de Sitter-like behavior. As the void approaches a perfect vacuum, the black hole horizon diminishes, tending to that of the Schwarzschild solution, while the outer horizon increases. We also study the solution stability via sound speed of the fluid, as well as the thermodynamic properties, including Hawking temperature, evaporation time, entropy, and specific heat. Our results show that as the void empties, the Hawking temperature rises, shortening evaporation times. The entropy follows the area's law and specific heat exhibits a minimum for a given black hole size, indicating a thermal transition and highlighting the role of voids in the black hole evolution. These findings offer new insights into the relationship between local gravitational collapse and large-scale cosmic structure, enhancing our understanding of the black hole behavior in underdense environments. We also provide a glimpse of a potential thermodynamic interaction between the event horizon and the cosmological horizon.

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arXiv2025

Constructing Regular Lovelock Black Holes with degenerate vacuum and $\Lambda < 0$, using the gravitational tension. Shadow analysis

In \cite{Estrada:2024uuu}, a link between gravitational tension (GT) and energy density via the Kretschmann scalar (KS) was proposed to construct regular black holes (RBHs) in Pure Lovelock (PL) gravity. However, including a negative cosmological constant in PL gravity leads to a curvature singularity \cite{Cai:2006pq}. Here, we choose the coupling constants such that the Lovelock equations admit an $n$-fold degenerate AdS vacuum (LnFDGS), allowing us to construct an RBH with $\Lambda < 0$, where the energy density is analogous to the previously mentioned model. To achieve this, we propose alternative definitions for both the KS and GT. We find that, for mass parameter values greater than the extremal value $M_{\text{min}}$, our RBH solution becomes indistinguishable from the AdS vacuum black hole from inside the event horizon out to infinity. At small scales, quantum effects modify the geometry and thermodynamics, removing the singularity. Furthermore, due to the lack of analytical relationships between the event horizon, photon sphere, and shadow in LnFDGS, we propose a numerical method to represent these quantities.

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arXiv2024

A new representation of vacuum Lovelock solutions in $d = 2N+1$ dimensions: Black holes with an integrable singularity and regular black holes

In recent years, black hole (BH) solutions with an integrable singularity have garnered significant attention as alternatives to regular black holes (RBH). In these models, similarly to RBHs, an object would not undergo spaghettification when approaching the radial origin. Instead of the potentially unstable de Sitter core present in RBHs, an integrable singularity emerges where the Ricci scalar diverges while its volume integral remains finite. However, the construction of both RBH solutions and BHs with an integrable singularity typically requires the inclusion of specific forms of matter in the energy-momentum tensor. We demonstrate that, from a geometric perspective in the absence of matter, vacuum solutions in Lovelock gravity in $d=2N+1$ dimensions can be represented as vacuum BHs with an integrable singularity in Einstein-Gauss-Bonnet theory for $d=5$ and in cubic gravity for $d=7$. Meanwhile, the vacuum solution in quartic gravity in $d=9$ is described as a vacuum RBH with a nontrivial hyperboloidal cross-section. For all the aforementioned cases, we have determined the conditions that the parameters in the solutions must satisfy. Remarkably, in all discussed cases, there is no presence of an internal horizon near a potentially unstable de Sitter core.

gr-qc↗

arXiv2024

A way of decoupling the gravitational bulk field equations of regular braneworld black holes to suppress the bulk singularities

We provide a methodology for decoupling the bulk gravitational field equations of braneworld black holes to suppress the bulk singularities. Thus, we provide a regular braneworld black hole setup. To achieve this, we apply a Minimal Geometric Deformation (MGD) with respect to a coupling constant $ \alpha $ to the $4D$ Minkowski spacetime embedded in an extra dimension. This results in a gravitational decoupling into a system $ \mathcal{A} $ with equations of motion of order $ \alpha^0 $ and a system $ \mathcal{B} $, related to the so-called Quasi-Einstein equations of order $ \alpha $. This methodology allows for the construction of a regular geometry everywhere. We outline the necessary constraints for eliminating singularities and provide a recipe for solving the equations of motion. Both the warp factor, the scalar field, and the potential obtained are smooth and free from Dirac delta singularities. A control parameter is introduced such that, in the limit $ b \to 0 $, the Randall-Sundrum (RS) setup is recovered, resulting in a transition from a thick brane to a thin brane. The asymptotic behavior of the curvature invariant $ \displaystyle \lim_{y \to \pm \infty} R_{5D}(r,y) $ is positive near the de Sitter core (for small $ r $), asymptotically negative for finite $ r > r_* $, and asymptotically flat at the $4D$ boundary as $ r \to \infty $. Although this work aims to suppress bulk singularities, it is expected that our methodology may be useful for future investigations related to the embedding of gravitational objects within other braneworld contexts.

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arXiv2024

Pure Lovelock Gravity regular black holes

We present a new family of regular black holes (RBH) in Pure Lovelock gravity, where the energy density is determined by the gravitational vacuum tension, which varies for each value of $n$ in each Lovelock case. Speculatively, our model may capture quantum effects through gravitational tension. In this way, a hypothetical analogy is drawn between the pair production ratio in the Schwinger effect and our energy density. A notable feature of our model is that the regular solution closely resembles the vacuum solution before reaching the event horizon. For odd $n$, the transverse geometry is spherical, with phase transitions occurring during evaporation, and the final state of this process is a remnant. For even $n$, the transverse geometry is non trivial and corresponds to a hyperboloid. In the case of $d=2n+1$ with even $n$, we find an RBH without a dS core and no inner horizon (whose presence has been recently debated in the literature due to the question of whether its presence is unstable or not), and no phase transitions. For $d > 2n + 1$ with even $n$, the RBH possesses both an event horizon and a cosmological horizon, also with no inner horizon present. The existence of the cosmological horizon arises without the usual requirement of a positive cosmological constant. From both numerical and analytical analysis, we deduce that as the event horizon expands and the cosmological horizon contracts, thermodynamic equilibrium is achieved in a remnant when the two horizons coincide.

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arXiv2024

Braneworld Black Bounce to Transversable Wormhole Analytically Connected to an asymptotically $AdS_5$ Boundary

We extend the recent approach from reference [1] to obtain complete and analytic solutions (both brane and bulk) of a Simpson-Visser (SV) geometry within a braneworld framework. The embedded geometry can represent a traversable wormhole (TWH), a one-way wormhole (OOWH), or a regular black hole (RBH). The resulting geometry is regular everywhere, eliminating any singularity or local de-Sitter core at the origin and on the brane location, where the regular geometry is given by the SV geometry. The throat of TWHs or OOWHs can extend into the extra dimension. The event horizon of RBH extends along the extra dimension, prompting speculation on the extension of entropy into this dimension. Although the induced geometry is characterized by tension, acting akin to a positive cosmological constant (thus potentially representing empty space), the induced four-dimensional geometry remains regular. There is no need to introduce additional energy sources on the brane to achieve this regularity. Hence, the brane's geometry, which may depict an RBH, TWH, or OWWH, is influenced by the geometric properties of the bulk

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arXiv2024

Braneworld Black Bounce to Transversable Wormhole

We provide a way for embedding a 4-dimensional geometry corresponding to the Simpson Visser (SV) spacetime which is capable of representing a traversable wormhole, a one-way wormhole, or a regular black hole into a Randall-Sundrum setup. To achieve this, we linearly deform the bulk geometry and the bulk matter distribution concerning a coupling constant. These deformations induce a transition from a $5D$ vacuum AdS state to an anisotropic matter distribution. The latter results in the induced geometry on the brane transitioning from a singular Schwarzschild spacetime to a regularized SV spacetime. Since there are no sources or matter fields on the brane, we can assert that the induced SV geometry on the brane arises from the influence of geometrical and matter deformations in the bulk. Thus, the central singularity is suppressed. We determine the cases where the energy conditions are either satisfied or violated. Our spacetime is asymptotically radial AdS, which is intriguing given the absence of a global AdS box that would prevent instability under larger wavelength perturbations. Therefore, it is no longer appropriate to claim that instability exists for very small perturbations near the AdS horizon. Thus, we propose that the stability of the solution can be analyzed by examining the speed of sound due to the presence of matter fields in the energy momentum tensor.

hep-th↗

arXiv2024

Quantized $p$-form Gauge Field in D-dimensional de Sitter Spacetime

In this work, we utilize the dynamic invariant method to obtain a solution for the time-dependent Schr\"odinger equation, aiming to explore the quantum theory of a $p$-form gauge field propagating in $D$-dimensional de Sitter spacetimes. Thus, we present a generalization, through the use of $p$-form gauge fields, of the quantization procedure for the scalar, electromagnetic, and Kalb-Ramond fields, all of which have been previously studied in the literature. We present an exact solution for the $p$-form gauge field when $D=2(p+1)$, and we highlight the connection of the $p=4$ case with the chiral $N=2$, $D=10$ superstring model. We could observe particle production for $D \neq 2(p+1)$ because the solutions are time-dependent. Additionally, observers in an accelerated co-moving reference frame will also experience a thermal bath. This could have significance in the realm of extra-dimensional physics and presents the intriguing prospect that precise observations of the Cosmic Microwave Background might confirm the presence of additional dimensions.

hep-th↗

arXiv2023

New models of d-dimensional black holes without inner horizon and with an integrable singularity

Theoretically, it has been proposed that objects traveling radially along regular black holes (RBHs) would not be destroyed because of finite tidal forces and the absence of a singularity. However, the matter source allows the creation of an inner horizon linked to an unstable de Sitter core due to mass inflation instability. This inner horizon also gives rise to the appearance of a remnant, inhibiting complete evaporation. We introduce here a $d$-dimensional black hole model with Localized Sources of Matter (LSM), characterized by the absence of an inner horizon and featuring a central integrable singularity instead of an unstable de Sitter core. In our model, any object tracing a radial and timelike world-line would not be crushed by the singularity. This is attributed to finite tidal forces, the extendability of radial geodesics, and the weak nature of the singularity. Our LSM model enables the potential complete evaporation down to $r_h=0$ without forming a remnant. In higher dimensions, complete evaporation occurs through a phase transition, which could occur at Planck scales and be speculatively driven by the Generalized Uncertainty Principle (GUP). Unlike RBHs, our model satisfies the energy conditions. We demonstrate a linear correction to the conventional area law of entropy, distinct from the RBH's correction. Additionally, we investigate the stability of the solutions through the speed of sound.

gr-qc↗

arXiv2023

A study about black hole solutions with nonconstant transversal curvature and its conserved charges in Lovelock gravity

In this work, the analysis of some new static black hole solutions of Lovelock gravity with nonconstant curvature transverse section is presented. It will be shown that the finiteness of the charges and the action principle rely on the existence of constraints on the geometry of the transverse sections. Finally, in this context, some new sound solutions with nonconstant curvature transverse sections that deviate from the previously known geometries are discussed.

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arXiv2023

Dymnikova GUP-corrected black holes

We consider the impact of Generalized Uncertainty Principle (GUP) effects on the Dymnikova regular black hole. The minimum length scale introduced by the GUP modifies the energy density associated with the gravitational source, referred to as the Dymnikova vacuum, based on its analogy with the gravitational counterpart of the Schwinger effect. We present an approximated analytical solution (together with exact numerical results for comparison) that encompasses a wide range of black hole sizes, spanning from microscopic (Planckian and sub-Planckian) to macroscopic scales, whose properties crucially depend on the ratio between the de Sitter core radius and the GUP scale. The emergence of a wormhole inside de Sitter core in the innermost region of the object is one of the most relevant features of this family of solutions. Our findings demonstrate that these solutions remain singularity free, confirming the robustness of the Dymnikova regular black hole under GUP corrections. Regarding energy conditions, we find that the violation of the strong, weak, and null energy conditions which is characteristic of the pure Dymnikova case does not occur at Planckian scales in the GUP corrected solution. This contrast suggests a departure from conventional expectations and highlights the influence of quantum corrections and the GUP in modifying the energy conditions near the Planck scale.

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