arXiv subjects

Antonio De Felice

Publications and source records attributed to Antonio De Felice.

At least 19 recordsLinked to original sources

arXiv2026

Perturbations and stability of black holes with static scalar hair in general GLPV theories

We derive the background equations and complete odd- and even-parity quadratic actions for static, spherically symmetric black holes with radial scalar hair in general quartic-quintic Gleyzes-Langlois-Piazza-Vernizzi (GLPV) theories, including Horndeski. On regular, nondegenerate branches, the formulation applies across Killing horizons and yields local no-ghost conditions and radial and angular eikonal characteristics. Tensor modes in both parity sectors share the same squared radial speed. When the radial coordinate is timelike, generic antisymmetric mixing induces nonstandard large-multipole scaling for both even-parity angular branches. Requiring both branches to have finite, nonzero eikonal phase speeds selects the Horndeski-related compatibility condition between the quartic and quintic beyond-Horndeski functions. This condition aligns the covariant degeneracy directions and permits a local disformal map to Horndeski when regular and invertible. For a finite, nonzero scalar kinetic term at the horizon, every nontrivial branch of the exact quadratic GLPV black hole is locally unstable or has a degenerate tensor cone near a simple outer horizon. Apart from identified exceptions, this obstruction extends to general shift- and reflection-symmetric quadratic GLPV theories and the regular Horndeski-related branch of the quartic-quintic class. Finally, for scalar-Gauss-Bonnet black holes with a vanishing horizon scalar kinetic term, we construct analytic power-law quartic beyond-Horndeski deformations whose associated quintic function is fixed by the same condition. At sufficiently small coupling, all local no-ghost and high-frequency gradient-stability conditions hold throughout the exterior. For a linear Gauss-Bonnet coupling, we estimate an interior scale below which the background and stability expansions lose perturbative control; this does not imply a physical instability.

gr-qc↗

arXiv2026

Minimally modified gravity with Laplacian auxiliary constraints and an inflationary realization

We construct a minimally modified gravity theory that propagates only two tensorial gravitational degrees of freedom around a spatially flat Friedmann--Lemaître--Robertson--Walker (FLRW) background and admits a predictive cosmological perturbation theory. We first show that, in the original four-constraint construction, the homogeneous values of the Lagrange multipliers are not fully determined and nevertheless enter the quadratic tensor action, thereby obstructing cosmological predictivity. We remove this ambiguity by coupling the auxiliary constraints to spatial Laplacians of the multipliers. The multiplier sector then drops out of the homogeneous background equations while continuing to constrain the inhomogeneous scalar sector. The tensor dispersion relation generically contains both $k^{2}$ and $k^{4}$ contributions, whereas no propagating gravitational vector or scalar mode is present on the generic branch for which the constraint reduction is nondegenerate. We subsequently study a subclass whose gravitational Hamiltonian density is proportional to the lapse and contains cubic momentum invariants. In this subclass the $k^{4}$ tensor term vanishes and the lapse can be absorbed into a time redefinition at the background level. After coupling a canonical inflaton, the only propagating scalar mode is the inflaton fluctuation, with unit sound speed. For a quadratic potential, departures from general relativity shift the scalar spectral index, while a tensor speed $c_{T}>1$ suppresses the tensor-to-scalar ratio according to $r=16ε_{s}/c_{T}$. The parameter regions compatible with the observational bounds adopted in this work require a superluminal tensor speed; however, very large $c_{T}$ simultaneously reduces the tensor kinetic coefficient and may lower the perturbative cutoff, although determining the actual strong-coupling scale requires a nonlinear analysis.

astro-ph.CO↗

arXiv2026

Light propagation and intensity transport in metric-affine geometry

We study electromagnetic wave propagation in metric--affine geometries, where torsion and non-metricity may be present and the coupling between electromagnetism and spacetime is no longer unique. Rather than choosing a particular coupling prescription a priori, we construct electromagnetic sectors that preserve standard $U(1)$ gauge invariance and projective invariance of the affine connection as guiding symmetry principles. We introduce two representative models, one in which the Maxwell term is dressed by a scalar prefactor built from non-Riemannian invariants, and another in which the kinetic term is modified by a rank--four constitutive tensor acting as an anisotropic medium. We derive their geometric--optics limits and show that their couplings can modify the effective light cone, change the relation between field amplitude and intensity, induce polarization-dependent propagation, and generate birefringence and mode mixing. These results thus provide the formal basis for a broader phenomenological study connecting torsion and non-metricity with electromagnetic observables in concrete metric--affine backgrounds, including black-hole imaging, birefringent lensing, polarization observables, and departures from photon-number conservation.

gr-qc↗

arXiv2026

Defocusing dark energy: Raychaudhuri diagnostics beyond $w<-1/3$ and the phantom divide

In general relativity, cosmic acceleration is timelike defocusing of the comoving congruence and requires a negative total active gravitational mass density, $\mathcal{M}_{\rm tot}=ρ_{\rm tot}+3p_{\rm tot}<0$. The criterion $w\equiv p/ρ<-1/3$ diagnoses sector repulsion only for $ρ>0$: the inequality reverses for $ρ<0$, and ratio variables are ill-defined at $ρ=0$ even when the stress-energy tensor is finite. For sign-changing effective dark energy (DE), as in $Λ_{\rm s}$CDM-type histories, we instead use the signed density $ρ_{\rm de}$ and two branch-independent combinations. The regular null energy condition (NEC) boundary $\mathcal{I}_{\rm de}=ρ_{\rm de}+p_{\rm de}=0$ replaces the phantom divide $w_{\rm de}=-1$, while $\mathcal{M}_{\rm de}=ρ_{\rm de}+3p_{\rm de}<0$ governs sector-level Raychaudhuri repulsion. For a separately conserved DE sector with a smooth negative-to-positive density crossing of finite odd order $n$ at $z_\dagger$, we prove that $\mathcal{I}_{\rm de}$ and $\mathcal{M}_{\rm de}$ are negative in a punctured neighborhood and non-positive at the crossing, while $w_{\rm de}$ develops a kinematic pole with universal residue $n(1+z_\dagger)/3$. If $\mathcal{M}_{\rm de}>0$ at some sufficiently high redshift, continuity requires at least one repulsion boundary $z_{\rm rep}>z_\dagger$: the sector is already repulsive while $ρ_{\rm de}<0$. We derive the exact range of $ρ_{\rm de}'(z_\dagger)$ for acceleration at the crossing with the total NEC satisfied. Under the stated single-impulse and stationary-point assumptions, the deceleration parameter has one or three sign-changing zeros. A smooth $Λ_{\rm s}$CDM profile, an exponential infrared $f(T)$ model, and the minimal phantom brane illustrate the results. These results motivate organizing late-time inference around $(ρ,p,\mathcal{I},\mathcal{M})$ rather than around $w$ alone.

gr-qc↗

arXiv2026

Static regular black holes in Horndeski theories: analytic no-go and nonanalytic obstructions

Regular black holes in Horndeski theories must have stable horizons and regular centers. We study static, spherically symmetric, asymptotically flat configurations with a time-independent scalar. The horizon branch on which the scalar kinetic term $X$ remains nonzero is generically obstructed by divergent propagation speeds or ghost/gradient instabilities, aside from special degeneracies. On the regular branch, where $X$ vanishes at the horizon, analyticity at the relevant $X=0$ endpoints reduces the leading scalar equation to finite sets of Taylor coefficients. For nondegenerate shift-symmetric theories this gives a nonperturbative current no-hair theorem: the scalar is constant and the metric is Schwarzschild, hence centrally singular for nonzero ADM mass. For non-shift-symmetric positive-power couplings, the corresponding exclusion applies to the perturbative branch continuously connected to Schwarzschild. We also classify marginal nonanalytic departures: covariant regularity fixes the scalar-Gauss-Bonnet chain as the unique marginal nonanalytic completion. Hairy black holes in this completion evade the analytic current step but remain centrally singular.

gr-qc↗

arXiv2026

Evading Cosmological Strong Coupling in Non-minimally Coupled Vector Gravity

Recent analyses of Proca theories with non-minimal curvature couplings have uncovered an additional scalar degree of freedom with an identically vanishing propagation speed, $c_s^2=0$, signaling a scale-dependent strong-coupling problem. In this {\it Letter}, we show that an additional derivative interaction can lift this zero-speed degeneracy and restore a non-degenerate quadratic dynamics for the scalar perturbations. In an open region of parameter space, the scalar kinetic matrix is positive definite and the high-frequency propagation speeds are real and positive. At quadratic order, this removes the specific signature of strong coupling associated with the $c_s^2=0$ mode. We also uncover a non-uniform limit as the temporal vector condensate approaches zero: at fixed nonzero $A_0$, the formal extreme-ultraviolet regime develops a ghost, while the kinetic structure of the exactly vanishing-condensate branch is different. The momentum scale at which this ghost appears is pushed toward increasingly high values as $A_0\to0$. Whether the ghost scale ultimately lies above the EFT cutoff depends on an independent determination of the EFT cutoff. Finally, we identify a stable de Sitter fixed point with $A_0\neq0$, surrounded by a finite region in which the scalar no-ghost and high-frequency stability conditions remain satisfied.

hep-th↗

arXiv2026

Data-Driven Discovery of a Simple Phantom-Crossing Dark Energy Parametrization

We develop a data-driven reconstruction programme for the dark-energy equation of state within VCDM, a minimally modified gravity framework in which both background and linear perturbations can be consistently evolved across the phantom divide. Using CMB, BAO, and type-Ia supernova data, we first perform a Bayesian spline reconstruction of $w(a)$, finding a preference for smooth, monotonic phantom-crossing trajectories. Bayesian evidence disfavors increasingly complex spline models, indicating that current observations exhibit a statistical preference for low-complexity dark-energy dynamics. Motivated by this result, we apply Exhaustive Symbolic Regression, an interpretable machine-learning technique that systematically searches over analytic expressions of fixed complexity, identifying the remarkably simple one-parameter form $w(a)={w_0}/{\sqrt a}$, which reproduces the reconstructed behaviour and fits the data at a level comparable to standard two-parameter parametrizations such as CPL. The model naturally crosses the phantom divide for $w_0<0$, suppresses early dark energy, and predicts a transient accelerating and phantom phase without a future big-rip singularity. As a one-parameter model, it is highly predictive, being a genuinely dynamical deformation of the cosmological constant rather than containing it as a limit. Bayesian model comparison yields mild-to-moderate support for this parametrization relative to standard two-parameter alternatives, and stronger evidence relative to $Λ$CDM. Our results suggest that current observations favour surprisingly simple dark-energy dynamics and illustrate how Bayesian reconstruction and symbolic regression can be combined into a principled model-discovery framework for cosmology.

astro-ph.CO↗

arXiv2026

Light rings, gravitational lensing, and ISCOs of exotic compact objects in Einstein-scalar-Maxwell theories

In Einstein-scalar-Maxwell theories with a coupling between the scalar field $ϕ$ and the electromagnetic field strength $F$ of the form $μ(ϕ) F$, we investigate the existence of exotic compact objects (ECOs) and their observational signatures in photon and massive-particle dynamics. For $μ(ϕ)$ diverging at the origin while all physical quantities remain finite, we demonstrate the existence of electrically charged ECOs with a shell-like structure whose density peaks at an intermediate radius. We compute their mass and radius, together with the scalar and vector field profiles, on a static and spherically symmetric background. We then examine the existence of light rings and place bounds on a model parameter by requiring the absence of a linearly stable light ring. Under this condition, photon echoes from ECOs are absent. We also compute the gravitational-lensing deflection angle and show that it attains a maximum for an impact parameter of the same order as the ECO radius. Finally, we study the parameter space in which innermost stable circular orbits of massive particles exist.

gr-qc↗

arXiv2026

Inspiral gravitational waveforms from charged compact binaries with scalar hair

We investigate gravitational waveforms from compact binary systems in Einstein-scalar-Maxwell (ESM) theories, where a scalar field $ϕ$ couples to a $U(1)$ gauge field $A_μ$ through a field-dependent function $μ(ϕ)$. In this framework, compact objects -- black holes (BHs), neutron stars (NSs), and exotic compact objects (ECOs) -- can carry both vector and scalar charges, with the latter arising as secondary hair induced by the former. Modeling the binary as electrically charged point particles with scalar-field-dependent masses, we derive the conservative dynamics in the near zone and compute the radiative fields in the far zone. The tensor waveform is modified through the effective dynamics and radiation-reaction-driven phase evolution, while scalar and vector modes introduce additional energy-loss channels. From the energy fluxes of tensor, scalar, and vector radiation, we construct the frequency-domain waveform using the stationary phase approximation. Dipole radiation sourced by differences in scalar and vector charge-to-mass ratios yields a leading $-1$ post-Newtonian correction. The deviation from general relativity is characterized by a single parameter $b$, which controls both amplitude and phase modifications. We further examine constraints from the orbital-period decay of binary pulsars, showing that current observations already place stringent bounds on $b$ for neutron star binaries. In addition, we evaluate $b$ for representative BH-BH, NS-NS, ECO-ECO binaries realized in ESM theories. Our results provide a unified framework for gravitational-wave signatures of charged compact binaries and offer a means of testing dark-sectorscalar and vector charges with current and future observations.

gr-qc↗

arXiv2026

$Λ_{\rm s}$CDM cosmology from a type-II minimally modified gravity

We integrate $Λ_{\rm s}$CDM, a promising scenario for alleviating cosmological tensions, into VCDM, a type-II minimally modified gravity. This promotes the scenario to a fully predictive model (dubbed $Λ_{\rm s}$VCDM) that specifies the cosmological evolution self-consistently, including through the late-time AdS-to-dS transition epoch. In this theory, an auxiliary scalar field generates an effective cosmological constant with either a constant or a linear potential. This allows an abrupt mirror AdS-to-dS transition via a piecewise-linear potential with a sudden slope change. To remove the associated sudden singularity and ensure stable evolution, we smooth the junction using a blended sigmoid interpolant, obtaining rapid but continuous transitions. We identify two qualitatively distinct smooth mirror AdS-to-dS realisations of $Λ_{\rm s}$: (i) an agitated transition, in which the potential interpolates between equal-magnitude AdS and dS plateaus and $Λ_{\rm s}$ develops a central bump; and (ii) a quiescent transition, in which the potential remains continuous but changes slope across the transition layer, so that $Λ_{\rm s}(a)$ can remain monotone, with possible shallow shoulders, and a central bump is not automatic. Depending on type and sharpness, a finite-width transition can induce a transient accelerated-expansion interval ($\ddot a>0$) around $z\sim 1.5-2$, in addition to present-day acceleration, and, if the background enters a region where $V_{,ϕϕ}>2/3$, a nested super-acceleration episode. These distinct transient histories can imprint signatures on background and perturbation evolution. Our construction enables a self-consistent observational assessment of smooth $Λ_{\rm s}$CDM realisations and motivates multi-probe analyses to test transition dynamics and reassess cosmological tensions.

astro-ph.CO↗

arXiv2026

Vector Horndeski black holes in nonlinear electrodynamics

On a spherically symmetric and static background, we study the existence of linearly stable black hole (BH) solutions in nonlinear electrodynamics (NED) with a Horndeski vector-tensor (HVT) coupling, with and without curvature singularities at the center ($r=0$). Incorporating the electric charge $q_E$ and the magnetic charge $q_M$, we first show that nonsingular BHs can exist only if $q_M = 0$. We then study the stability of purely electric BHs by analyzing the behavior of perturbations in the metric and the vector field. Nonsingular electric BHs are unstable due to a Laplacian instability in the vector perturbation near the regular center. In the absence of the HVT coupling ($β=0$), singular BHs in power-law NED theories can be consistent with all linear stability conditions, while Born-Infeld BHs encounter strong coupling due to a vanishing propagation speed as $r \to 0$. In power-law NED and Born-Infeld theories with $β\neq 0$, the electric fields for singular BHs are regular near $r=0$, while the metric functions behave as $\propto r^{-1}$. Nevertheless, we show that Laplacian instabilities occur for regions inside the outer horizon $r_h$, unless the HVT coupling constant $β$ is significantly smaller than $r_h^2$. For $β\neq 0$, we also reconstruct the NED Lagrangian so that one of the metric functions takes the Reissner-Nordström form. In this case, there exists a branch where all squared propagation speeds are positive, but the ghost and strong coupling problems are present around the BH center. Thus, the dominance of the HVT coupling generally leads to BH instability in the high-curvature regime.

gr-qc↗

arXiv2026

Exotic compact objects in Einstein-scalar-Maxwell theories

In k-essence theories within general relativity, where the matter Lagrangian depends on a real scalar field $ϕ$ and its kinetic term $X$, static and spherically symmetric compact objects with a positive-definite energy density cannot exist without introducing ghosts. We show that this no-go theorem can be evaded when the k-essence Lagrangian is extended to include a dependence on the field strength $F$ of a $U(1)$ gauge field, taking the general form ${\cal L}(ϕ, X, F)$. In Einstein-scalar-Maxwell theories with a scalar-vector coupling $μ(ϕ) F$, we demonstrate the existence of asymptotically flat, charged compact stars whose energy density and pressure vanish at the center. With an appropriate choice of the coupling function $μ(ϕ)$, we construct both electric and magnetic compact objects and derive their metric functions and scalar- and vector-field profiles analytically. We compute their masses and radii, showing that the compactness lies in the range ${\cal O}(0.01)<{\cal C}<{\cal O}(0.1)$. A linear perturbation analysis reveals that electric compact objects are free of strong coupling, ghost, and Laplacian instabilities at all radii for $μ(ϕ)>0$, while magnetic compact objects suffer from strong coupling near the center.

gr-qc↗

arXiv2025

Higgs-Portal Stueckelberg Dark Matter

We study dark photon dark matter $X$ associated with a dark $U(1)_X$ gauge symmetry. To evade laboratory and cosmological constraints on kinetic mixing with the Standard Model $U(1)_Y$, we assign a $Z_2$-odd dark parity to $X$ that forbids such mixing. The leading interactions then arise from gauge-invariant dimension-6 Higgs-portal operators, including both parity-even and parity-odd terms. We assume that the dark matter mass is generated via the Stueckelberg mechanism, which also induces a dimension-4 Higgs-portal operator $(H^\dagger H) X_μX^μ$ and additional dimension-6 operators. We investigate freeze-in production of $X$ from Higgs-pair annihilations after reheating, incorporating both gauge-invariant and Stueckelberg-induced operators. First, we consider the case in which the Wilson coefficients of the gauge-invariant dimension-6 operators, $C$ and $\tilde{C}$, are of order unity. We find that, in this case, the Stueckelberg contributions remain subdominant in dark matter production. This result follows from the requirement that the effective scale implied by perturbative unitarity must exceed the cutoff scale of the effective field theory. Next, we explore a more general situation in which $C$ and $\tilde{C}$ are smaller than unity. In this second case, Stueckelberg-induced operators can become comparable and lead to distinctive features in the dark-matter parameter space, including interference effects. For both cases, we show that there exists a wide parameter space consistent with the observed dark matter relic density.

hep-ph↗

arXiv2025

Nonsingular black holes and spherically symmetric objects in nonlinear electrodynamics with a scalar field

In general relativity with vector and scalar fields given by the Lagrangian ${\cal L}(F,ϕ,X)$, where $F$ is a Maxwell term and $X$ is a kinetic term of the scalar field $ϕ$, we study the linear stability of static and spherically symmetric objects without curvature singularities at their centers. We show that the background solutions are generally described by either purely electrically or magnetically charged objects with a nontrivial scalar-field profile. In theories with the Lagrangian $\tilde{\cal L}(F)+K(ϕ, X)$, which correspond to nonlinear electrodynamics with a k-essence scalar field, angular Laplacian instabilities induced by vector-field perturbations exclude all the regular spherically symmetric solutions including nonsingular black holes. In theories described by the Lagrangian ${\cal L}=X+μ(ϕ)F^n$, where $μ$ is a function of $ϕ$ and $n$ is a constant, the absence of angular Laplacian instabilities of spherically symmetric objects requires that $n>1/2$, under which nonsingular black holes with apparent horizons are not present. However, for some particular ranges of $n$, there are horizonless compact objects with neither ghosts nor Laplacian instabilities in the small-scale limit. In theories given by ${\cal L}=X κ(F)$, where $κ$ is a function of $F$, regular spherically symmetric objects are prone to Laplacian instabilities either around the center or at spatial infinity. Thus, in our theoretical framework, we do not find any example of linearly stable nonsingular black holes.

gr-qc↗

arXiv2025

Dynamical dark energy parameterizations in VCDM

In the context of a theory of minimally modified gravity called VCDM, one can realize any cosmological behavior at the level of the homogeneous and isotropic background without introducing fatal instabilities for perturbations. Therefore, VCDM provides a theoretically-consistent and observationally-testable framework of dynamical dark energy parameterizations with or without phantom behaviors. In this paper, we propose the VCDM realizations of various phenomenological parameterizations present in the literature: the Chevallier-Polarski-Linder (CPL), Barboza-Alcaniz (BA), Jassal-Bagla-Padmanabhan (JBP), Exponential (EXP), and Logarithmic (LOG) models. Using the VCDM equations for cosmological perturbations, we test them against the recent cosmological datasets, Planck 2018 and DESI BAO DR2, and then discuss their implications.

gr-qc↗

arXiv2025

The non-minimal 3-form cosmology and the rise of the cuscuton

We consider the 3-form theory with non-minimal coupling to gravity in an expanding Universe. First, we assume that the background is homogeneous and isotropic, and that the three-form is coupled to both the Ricci scalar and the Ricci tensor. We show that in this case, it propagates three degrees of freedom: a scalar mode and two tensor ones. Then, we consider an anisotropic background that corresponds to a Bianchi Type I Universe, and set the coupling with the Ricci tensor to zero. We show that, similarly to the Proca theory with non-minimal coupling to gravity, this case leads to two branches for the background solutions - depending on the values of the 3-form. However, in contrast to the Proca case, we show that no extra modes appear. We explore the no-ghost conditions and speed of propagation for all three modes in both branches. Finally, we show that one of the branches can be written as a theory of a constrained scalar, coupled to a cuscuton field.

gr-qc↗

arXiv2025

Unexplored regions in teleparallel $f(T)$ gravity: Sign-changing dark energy density

While $f(T)$ gravity has shown considerable potential in addressing cosmological tensions, we explore previously overlooked solution spaces that hold further promise. We examine the case where the customary assumption of a strictly positive effective DE density may not apply, offering new possibilities. Focusing on $f(T) = T e^{T_*/T}$, we investigate cosmological solutions parametrized by the parameter $β= T_*/T_0$. This parameter uniquely determines $Ω_{\rm m0}$, and its sign plays a crucial role in characterizing deviations from the $Λ$CDM. We elaborate on the structural asymmetry between the positive- and negative-$β$ branches: while the $β_{+}$ leads to dynamics with modest departures from $Λ$CDM, the $β_{-}$ yields more pronounced and nontrivial deviations. Despite these deviations, the negative-$β$ branch can remain consistent with local gravity constraints through an effective chameleon-like mechanism. We also examine the model in the context of dynamical DE. Ensuring consistency with CMB data, the widely studied $β_{+}$ exhibits phantom behavior, while the previously overlooked $β_{-}$ features a sign-changing DE density that transitions smoothly from negative to positive values at $z_{\dagger} \sim 1.5$. Though the sign-changing DE leads to a larger-than-expected enhancement, we extend the analysis by incorporating $Λ$. This extension broadens the solution space consistent with the SH0ES measurement while maintaining consistency with CMB. Additionally, it introduces richer phenomenological possibilities, including the potential moderation or cessation of cosmic acceleration at very low redshifts, aligning with recent observational analyses, such as those from DESI BAO data. Our findings suggest that existing $f(T)$ models, as well as $f(Q)$ models, should be revisited in light of the novel theoretical insights presented here.

gr-qc↗

arXiv2025

Instability of regular planar black holes in four dimensions arising from an infinite sum of curvature corrections

In four-dimensional scalar-tensor theories derived via dimensional regularization with a conformal rescaling of the metric, we study the stability of planar black holes (BHs) whose horizons are described by two-dimensional compact Einstein spaces with vanishing curvature. By taking an infinite sum of Lovelock curvature invariants, it is possible to construct BH solutions whose metric components remain nonsingular at $r=0$, with a scalar-field derivative given by $ϕ'(r)=1/r$, where $r$ is the radial coordinate. We show that such BH solutions suffer from a strong coupling problem, where the kinetic term of the even-parity scalar-field perturbation associated with the timelike coordinate vanishes everywhere. Moreover, we find that these BHs are subject to both ghost and Laplacian instabilities for odd-parity perturbations near $r=0$. Consequently, the presence of these pathological features rules out regular planar BHs with the scalar-field profile $ϕ'(r)=1/r$ as physically viable and stable configurations.

gr-qc↗