arXiv subjects

Francisco Tello-Ortiz

Publications and source records attributed to Francisco Tello-Ortiz.

At least 19 recordsLinked to original sources

arXiv2026

Beyond the Dolgov-Kawasaki instability: Covariant scalaron stability, entropy transport, and inner horizons in f(R) gravity

The Dolgov-Kawasaki instability tests the local scalaron response in metric $f(R)$ gravity, but does not determine strong-field horizon dynamics. We separate these questions using $F=f_R$ and the Legendre potential $\mathcal U(F)=FR-f$. The scalaron restoring coefficient is $m_{\rm sc}^2=(F-Rf_{RR})/(3f_{RR})$, whereas exact spherical Wald-area transport identities cancel the scalaron Hessian. Their Einstein-frame interpretation distinguishes longitudinal kinetic focusing from potential-dependent cross-focusing. We prove that, at fixed constant-curvature background with tracefree matter, transport depends only on $(f,f_R)$, while the scalaron mass depends on the independently adjustable $f_{RR}$. Reissner-Nordström in $f(R)=R+αR^2$ gives an exact discriminator: identical background transport coexists with opposite scalaron mass signs. A charged solution with varying $F$ and a cosmological check test the identities beyond this constant-$F$ sector. For a nonzero massive scalaron tail transmitted to a nondegenerate Cauchy horizon, a conditional fixed-background estimate gives a nonintegrable contribution to generalized entropy focusing. This estimate does not establish nonlinear mass inflation or a particular extendibility class. The results provide a layered framework for coupling regularity, background transport, scalaron stability, and nonlinear inner-horizon evolution.

gr-qc↗

arXiv2026

Where the Gauss--Bonnet coupling enters the Bondi--Sachs hierarchy

We integrate the Bondi--Sachs hierarchy of scalar--Gauss--Bonnet gravity with an arbitrary coupling function $f(Φ)$ and a radiating massless scalar, and determine the first order in $1/r$ at which the coupling $λ$ becomes visible in each field equation. We obtain the asymptotic falloff of the effective Gauss--Bonnet stress tensor together with its leading coefficients in covariant form on the sphere: every component decays between one and three powers of $1/r$ faster than the slowest falloff compatible with a smooth null infinity. As a consequence, (i) the Bondi mass-loss formula and the evolution equation of the angular-momentum aspect are those of the minimally coupled Einstein--scalar system, a statement that is exact in $λ$ under the assumption of a smooth expansion; (ii) the coupling generates no logarithmic terms through the orders considered, even though the Horndeski representation of the theory involves logarithmic functionals; and (iii) the coupling first enters a dynamical equation in the evolution of the $O(r^{-3})$ coefficient of the angular metric, through the cross term $-2λf'(\vp_0)\,\vp_1\,\partial_u N_{AB}$ between the asymptotic scalar charge aspect and the time derivative of the news. In Newman--Penrose form, the evolution equation of the leading coefficient of $Ψ_0$ acquires the source $-12λf'(\vp_0)\,\vp_1\barΨ_4^0$, while those of $Ψ_1$ and $Ψ_2$ are unchanged. The Gauss--Bonnet invariant decays as $r^{-6}$ and affects the scalar hierarchy only from the evolution of the $r^{-5}$ coefficient onward. For vacuum black-hole spacetimes, where the scalar is sourced by the coupling itself, all these effects are of order $λ^2$; if $f'(\vp_0)=0$ they are pushed one further order in $1/r$.

gr-qc↗

arXiv2026

On-shell Homogeneous Curvature Modes in Quadratic $F(R,\mathcal G)$ Cosmology

5We study homogeneous cosmological perturbations in the quadratic model $F(R,\mathcal G)=R+αR^2+β\mathcal G^2$. Linearization of the modified Friedmann equation together with dust conservation yields a second-order equation for the homogeneous Hubble perturbation, or equivalently a third-order equation for the matter contrast. We then restrict the asymptotic analysis to backgrounds that satisfy the zeroth-order equations of the same quadratic theory. Besides the low-curvature dust branch, which approaches $H=2/(3t)$, the model admits a nontrivial vacuum de Sitter solution with $βH_{\rm dS}^6=1/96$. Its homogeneous curvature modes are stable only when $α<-1/(6H_{\rm dS}^2)$. At high curvature the background equation also admits a vacuum big-rip asymptotic branch beginning with $H=7/(t_s-t)$; the first corrections are fixed by the quadratic couplings. The corresponding $\mathcal G^2$-dominated perturbation has one divergent homogeneous mode. The simultaneous Einstein limit is taken only as a consistency check and recovers the standard dust behavior.

gr-qc↗

arXiv2026

Non-Schwarzschild black holes sourced by scalar-vector fields

In this work, we construct a static and spherically symmetric black hole solution sourced by a nonlinear scalar-vector sector within the framework of the Minimal Geometric Deformation (MGD) approach. Starting from a scalar-vector gravity theory in which the vector field is described by nonlinear electrodynamics, the gravitational field equations are integrated to obtain a non-Schwarzschild geometry. We show that the resulting family of solutions naturally splits into two black hole branches with distinct physical properties. While one branch satisfies the odd- and even-parity stability criteria, it violates the weak energy condition outside the event horizon. Conversely, the complementary branch exhibits a more satisfactory effective matter sector, admitting a real scalar-field reconstruction and a regular nonlinear electromagnetic coupling, although it does not satisfy the odd-parity stability criterion. The causal structure and stability properties of both black hole branches are analyzed and compared, whereas the matter reconstruction, geodesic motion and thermodynamic properties are investigated for the branch exhibiting the most satisfactory effective matter sector. These results reveal the rich structure of the MGD solution space and illustrate the interplay between dynamical stability, energy conditions and causal structure.

gr-qc↗

arXiv2026

Kaniadakis Holographic Dark Energy: UV$-$IR Duality, Horizon Thermodynamics and Cosmological Constraints

We investigate the cosmological and thermodynamic implications of holographic dark energy derived from the Kaniadakis deformation of the Bekenstein--Hawking entropy. In a spatially flat FLRW universe, the generalized entropy generates an effective dark-energy density with an infrared correction proportional to $H^{-2}$, naturally producing an ultraviolet--infrared structure in the cosmological dynamics. Using the Hayward--Kodama formulation of apparent-horizon thermodynamics, we derive a geometric equation of state and identify Van der Waals-type criticality, characterized by an inverted first-order phase transition and an inverted swallowtail Gibbs potential. We interpret this behavior as the thermodynamic signature of a geometric phase transition of the apparent horizon rather than standard fluid coexistence. We also consider an extended model with a $\dot H$ contribution inspired by the Granda--Oliveros cutoff and show that this unconventional critical behavior persists. At the perturbative level, the same ultraviolet--infrared competition controls the effective sound speed and dark-energy response through a dimensionless coupling $A(z)$, which acts as a geometric order parameter. In the minimal model, $A(z)$ may cross zero during matter domination, marking a transition between infrared- and ultraviolet-dominated regimes, whereas the extended scenario remains ultraviolet dominated. Finally, we confront both models with cosmic chronometers, PantheonPlus Type Ia supernovae, DESI DR2 baryon acoustic oscillations, and redshift-space distortion data. Bayesian comparison with $Λ$CDM, CPL, and standard holographic dark energy strongly disfavors the minimal model. The extended model yields the best maximum-likelihood fit among those considered, but remains disfavored relative to $Λ$CDM after accounting for its larger parameter volume.

gr-qc↗

arXiv2026

Black hole and wormhole branches in gravitational decoupling

Minimal Geometric Deformation (MGD) applied to a static Schwarzschild black hole seed generates a single decoupler function $h(r)$, obtained by solving the $θ$-sector field equations together with an equation of state. Once $h(r)$ is fixed, the resulting one-parameter family is controlled by the coupling strength $k$ through $F(r;k)=1+k\,h(r)$. We show that, whenever the deformation develops a simple outermost root that crosses the seed horizon, the same fixed decoupler leads to two mutually exclusive branches associated with different global completions: on one side of the critical coupling the deformed metric preserves the seed horizon as a black hole, whereas on the other side the root $r_*>2M$ lies in the exterior and cannot be interpreted as an interior modification of the black hole geometry. We prove that this root forces a loss of Lorentzian signature on the interval $(2M,r_*)$, so that no smooth extension of the exterior metric through the seed horizon $r=2M$ exists once $r_*$ lies outside it. Within the static, spherically symmetric class considered here, the corresponding smooth Lorentzian completion is a two-ended wormhole obtained by excising $(2M,r_*)$ and doubling the region $r\geq r_*$ across the minimal sphere $\mathcal T=\{r=r_*\}$. No topology change of any single spacetime is claimed or required: $k>k_c$ and $k<k_c$ simply correspond to two different, non-diffeomorphic manifolds, and Lemma~1 below shows that the metric itself dictates which of the two is the admissible completion for a given $k$. We compute the second homology group of both completions explicitly, $H_2(Σ_{\rm BH},\mathcal H)=0$ for the black hole exterior relative to its horizon and $H_2(Σ_{\rm WH})\cong\mathbb Z$ for the completed wormhole manifold, giving a discrete invariant that distinguishes the two branches.

gr-qc↗

arXiv2026

Mass inflation or scalaron rigidity: Cauchy horizons in metric f(R)

We develop a double-null framework for Cauchy-horizon dynamics in metric $f(R)$ gravity that separates ordinary mass inflation from scalaron cancellation and genuine escape from the blueshift instability. Starting from the exact spherical Hawking-mass transport equation, we show that a regular nondegenerate Cauchy horizon undergoes mass inflation whenever the effective longitudinal source is eventually nonnegative, uniformly dominates the mixed channel, and has a divergent blueshift-weighted integral. Hence bounded mass requires sufficiently strong blueshift-integrable suppression of this source or an independent competing channel. We then analyze the weaker branch in which the scalaron cancels only the leading Price-tail contribution. Under non-cancelling transverse and trace asymptotics, this forces $|R|\to\infty$ and an asymptotically linear high-curvature theory, $f(R)/R\to F_->0$, where $F_-\equiv\lim_{v\to\infty}f_R$. The curvature coefficient is fixed explicitly by the transverse asymptotics. For regular model classes with a finite high-curvature $f_{RR}$ limit one obtains $f_{RR}\to0$. More sharply, every eventually viable branch with $f_{RR}>0$ is forced to $R\to-\infty$ and $Λ_\infty>-1$; the standard scalaron mass parameter then diverges, while an additional differentiable rate condition yields $m_{\rm sc}^2\sim F/(3f_{RR})\to+\infty$. In the Einstein frame the scalaron null kinetic contribution is nonnegative, so the Jordan-frame cancellation cannot be interpreted as negative scalaron null energy. The result is a local rigidity classification rather than a global theorem of strong cosmic censorship.

gr-qc↗

arXiv2026

Area-product universality in multi-horizon black hole entropy

Black-hole entropy and multi-horizon area-product relations represent two apparently distinct manifestations of universality, arising respectively from microscopic statistics and classical geometry. We show that these structures are unexpectedly connected. Extending a minimum-assumptions statistical discretization to multiple Killing horizons, with distinct horizon sectors treated as statistically independent, we derive a general area-product rule. In particular, while the leading Bekenstein-Hawking entropy probes the sum of the horizon areas, the logarithmic correction probes their product. Consequently, whenever the classical area product is mass independent, the logarithmic entropy automatically inherits this universality. We demonstrate this for Reissner-Nordström and, under an area-based extension to rotating horizons, Kerr-Newman black holes. However, Reissner-Nordström-de Sitter provides a crucial three-horizon counterexample, for which the area-product rule survives while mass independence is generically lost for the physical horizons. Thus, statistical area-product selection and classical universality are distinct properties. Our results uncover a structural connection between microscopic entropy counting and classical multi-horizon geometry that is invisible at the level of the leading area law, suggesting that logarithmic corrections encode distinctive information about the global horizon structure.

gr-qc↗

arXiv2026

Gravitational Wave Standard Sirens as Probes of Lorentz Violation in Bumblebee Gravity

Gravitational-wave standard sirens provide a direct measurement of luminosity distance and therefore offer a new way to test gravity over cosmological scales. We use this idea to forecast the sensitivity of the Einstein Telescope (ET) to Lorentz violation in Bumblebee gravity, and we examine how the forecast changes when Type~Ia supernova information is added. A timelike Bumblebee vacuum expectation value can affect the cosmic expansion and, when it evolves with redshift, the propagation amplitude of gravitational waves. We study a constant-field case and an evolving-field case using mock ET catalogues with $10^3$ events together with a Pantheon+-like supernova sample. The supernova data substantially improve the background parameters: in the constant-field case the uncertainties in $H_0$ and $Ω_m$ decrease by a factor of about $4.4$, while in the evolving-field case they decrease by factors of about $1.6$ and $6.2$, respectively. By contrast, the Lorentz-violating parameter $\ell_0$ remains prior dominated, and the evolution index $β$ is constrained only by the gravitational-wave sector. The best forecast precision, $Δ\ell_0\simeq0.028$, is about $4.7\times10^{12}$ times weaker than the bound implied by GW170817. The principal result is therefore a quantified sensitivity gap rather than a forecast detection. We also express the prediction in the phenomenological $(Ξ,n)$ description of modified gravitational-wave propagation, allowing direct comparison with standard-siren studies of other gravity models.

gr-qc↗

arXiv2026

Horizon-regular cross-focusing and inner-horizon obstructions in spherical f(R) gravity

We formulate a horizon-regular double-null criterion for regular inner marginal horizons in spherically symmetric metric $f(R)$ gravity. Using normalized outgoing and ingoing radial null vectors $\ell^μ$ and $n^μ$, with $n^μ$ affinely parametrized, we derive an exact evolution law for the area-weighted outgoing expansion $r^2θ_{(\ell)}$. Its source is controlled by the scalaron $F\equiv f_R>0$ and by a mixed quantity $\mathcal{P}_{\ell n}$ containing matter, scalaron derivatives, and the curvature potential. If $\mathcal{P}_{\ell n}\leq F/r^2$ along a regular ingoing null segment issuing from a nondegenerate future outer marginal sphere, then the outgoing expansion cannot return to zero, and no second regular marginal sphere of the same family can occur on that generator. Conversely, a nondegenerate future inner marginal sphere requires the reverse inequality, so an outer--inner pair necessarily entails a source reversal and an exact integral balance. No trapped-region assumption is required. In the static limit, the criterion reduces to a horizon-regular relation involving the radial derivative of the metric function and remains valid in the degenerate case under the stated regularity conditions. It reproduces the Reissner--Nordström classification and is verified in an exact charged, nonconstant-curvature $f(R)$ black hole with a nonconstant scalaron. The resulting Cauchy-horizon statement is conditional and applies only when the candidate boundary is also a regular nondegenerate future inner marginal horizon.

gr-qc↗

arXiv2026

Scalaron-modified null focusing and radial monotonicity in static f(R) gravity

We derive an exact radial monotonicity law for static, spherically symmetric spacetimes in metric $f(R)$ gravity. For $ds^2=A(r)dt^2-dr^2/B(r)-r^2dΩ^2$, the matter contribution and the scalaron Hessian combine into an effective radial-convergence numerator that fixes the derivative of $\Q=B/A$. On every connected static interval with $A>0$, $B>0$, and $f_R\equiv df/dR>0$, its sign therefore determines the monotonicity of $\Q$. The integrated identity retains the finite, generally nonzero value of $B/A$ at a regular nondegenerate Killing horizon; consequently, a fixed-sign convergence condition orders the horizon endpoint ratios rather than excluding two horizons. A zero-integral obstruction arises for equal endpoint values, including boundaries where $B\to0$ while $A$ remains finite and nonzero. Saturation is equivalent to $B/A=\mathrm{const}$, and in vacuum requires a scalaron profile linear in the areal radius. We illustrate the strict non-saturated branch, within the General-Relativity sector, using the exact constant-density stellar interior, and apply the equality and consistency diagnostics to the constant-$X$ and power-law solutions of Multamäki and Vilja. In particular, in the Schwarzschild--de Sitter two-horizon parameter range, one constant-$X$ solution crosses $f_R=0$ inside the complete static patch, while a direct substitution into the original radial equation exposes an unresolved exponent mismatch in the displayed power-law family. The results provide a model-independent static-sector diagnostic and a precise starting point for a future horizon-regular treatment of black-hole interiors.

gr-qc↗

arXiv2026

Regular black holes with Minkowskian cores: causal structure and observational degeneracy

We construct a new family of asymptotically flat static and rotating regular black holes characterized by a Minkowskian core and a finite two-horizon structure. The solutions are obtained within the framework of gravitational decoupling and provide an analytically tractable realization of non-singular black hole geometries with a regular interior and well-defined asymptotic properties. We investigate the optical appearance of the rotating spacetime through shadow observables and ray-traced images of geometrically thin, optically thin accretion disks. Despite the substantial differences between the interior geometry of these solutions and that of singular black holes, their optical signatures are found to remain remarkably close to those of Schwarzschild and Kerr spacetimes with the same asymptotic parameters. Our results show that markedly different black hole interiors may lead to nearly indistinguishable optical appearances, highlighting the challenges of probing the internal structure of compact objects using current shadow and imaging observations.

gr-qc↗

arXiv2026

Horizon formation from effective matter profiles in static spacetimes

The formation of event horizons is traditionally studied as the endpoint of gravitational collapse, where the matter distribution and the spacetime geometry are evolved simultaneously toward a black hole state. In this work, we consider the inverse problem: given a static geometry containing the simplest causally exposed singular structure, namely a timelike naked singularity, what are the minimal conditions on the surrounding matter required for the emergence of an event horizon? Within classical general relativity, we derive sufficient conditions for horizon formation in terms of the radial organization, compactness and finiteness of the matter distribution. These conditions are summarized by a simple geometric criterion that determines whether a static configuration becomes causally inaccessible to external observers. We further identify situations in which horizon formation necessarily fails, thereby characterizing both the existence and obstruction of causal cloaking in static spacetimes. Our results show that the emergence of an event horizon is not controlled solely by the total amount of matter, but rather by the way mass-energy is radially accumulated. This provides a minimal and geometrically transparent framework for understanding horizon formation as a causal transition in static spacetimes, independent of gravitational collapse or dynamical evolution.

gr-qc↗

arXiv2026

Field Sources for Dark Matter Black Holes

We investigate the field-theoretic realization of regular black holes sourced by dark matter halo profiles within nonlinear electrodynamics (NED) minimally coupled to gravity. Starting from a static, spherically symmetric geometry determined by a halo density profile $ρ(r)$, we reconstruct the associated mass function and derive the effective matter source supporting the spacetime. In the magnetic sector, the reconstruction is direct and yields a NED Lagrangian of the form $L(F)=-ρ(r(F))$, while in the electric sector the theory is obtained parametrically through the field equations. We analyze the admissibility and consistency of the reconstructed models by studying regularity at the origin, asymptotic behavior, and the relevant energy conditions. The formalism is applied to representative halo profiles, including the Einasto, Dehnen, Burkert, and pseudo-isothermal families. For halo distributions with finite central density, the resulting geometries naturally exhibit de Sitter cores and asymptotically Schwarzschild behavior, providing a controlled and physically transparent link between dark matter halo phenomenology and regular black-hole spacetimes. Our results show that a broad class of halo profiles admits an effective NED completion, offering a unified geometric and field-theoretic interpretation of regular black holes sourced by dark matter halos.

gr-qc↗

arXiv2026

Cosmological implications of Bumblebee theory on an FLRW background

We investigate some cosmological implications at background level of the Bumblebee model. The phase-space, the critical points and their stability are analyzed in detail applying well-established dynamical system techniques. What is more, upon comparison to available supernovae data, the best fit numerical value of the unique free parameter of the model is determined. We show graphically all the cosmological quantities of interest versus red-shift, such as the deceleration parameter, dark energy equation of state parameter, etc. The statefinders and the age of the Universe are also computed. Finally, a comparison to the $Λ$-CDM model is made as well.

gr-qc↗

arXiv2026

First-order phase transitions and cosmic evolution: thermodynamic approach to generalized holographic dark energy

Focusing on the description of cosmic evolution at late times, this study examines a generalized holographic dark energy (HDE) framework constructed via a polynomial expansion in the Hubble parameter, which includes contributions proportional to $H^{2}$, $H^{4}$, and $H^{6}$, introduced through a variable parameter within the standard holographic formula. The analysis is carried out in the context of a spatially flat Friedmann-Lemaître-Robertson-Walker (FLRW) Universe, consisting of non-interacting matter together with the HDE fluid. We obtain the full set of Friedmann equations to investigate cosmic evolution and then analyze the system to determine whether thermodynamic $P - v$ type phase transitions can occur.

gr-qc↗

arXiv2025

Cosmological FLRW phase transitions under exponential corrected entropy

This work considers how exponential corrections to the Bekenstein-Hawking entropy formula affect the thermodynamic behavior of the FLRW cosmological model. These corrections drastically change the form of the Friedman field equations inducing non-trivial phase transition behavior. For negative values of the trace parameter $α$, the system presents first-order phase transitions above the critical temperature, and for positive $α$, the system undergoes a reentrant phase transition. As these corrections are presumably relevant at the early Universe stage, to corroborate the presence of some potential vestige of this contribution in the current era, a study has been carried out comparing observational data and current values of the Hubble parameter.

hep-th↗

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 $r = 0$, $\barθ = π/2$. 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↗