arXiv subjects

Maickol Muñoz-Palma

Publications and source records attributed to Maickol Muñoz-Palma.

4 recordsLinked to original sources

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

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

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↗