arXiv subjects

Radouane Gannouji

Publications and source records attributed to Radouane Gannouji.

At least 19 recordsLinked to original sources

arXiv2026

Higher Lovelock Curvature Terms Favor Local Nakedness in Dust Collapse

We show that higher-curvature Lovelock terms do not restore local cosmic censorship in spherical dust collapse, but instead promote the local visibility of central shell-focusing singularities. On the collapse branch with positive highest-order Lovelock coefficient $c_N$, the highest nonvanishing Lovelock order $N$ controls both the near-singularity collapse and the formation of trapped surfaces. In noncritical dimensions, $D-1-2N>0$, the apparent-horizon curve approaches the singularity curve with trapping exponent $β_N=(D-1)/(D-1-2N)$. Comparing this scale with the first nonvanishing correction $r^\ell$ to the singularity curve gives the local-visibility condition $\ell<β_N$, provided the singularity curve opens outward. Thus increasing $N$ enlarges the class of inhomogeneous initial data producing outgoing radial null rays from the central singularity. In the critical odd-dimensional branch, $D=2N+1$, no apparent horizon forms sufficiently close to the center, so any outward opening of the singularity curve gives local visibility. The locally visible singularities are Królak-strong along the emerging null rays, with Tipler strength reached at threshold. For bound and unbound collapse, the noncritical exponents are unchanged: the energy function modifies the opening of the singularity curve, while in the critical branch it enters the leading terminal collapse velocity.

gr-qc↗

arXiv2026

Neutron stars in Poincaré gauge gravity with quadratic torsion

We study static neutron stars in an algebraic sector of Poincaré gauge gravity with parity-even and parity-odd quadratic torsion invariants. Since torsion is non-propagating, the contorsion equation is algebraic and can be solved in terms of the spin current. For a Weyssenhoff fluid satisfying the Frenkel condition, the metric field equations reduce to ordinary Riemannian Einstein equations sourced by an effective fluid containing spin-squared corrections. We derive the effective energy density, radial pressure, and tangential pressure, allowing both isotropic and anisotropic spin correlations. In contrast with Einstein--Cartan theory, the coefficient of the effective spin-spin interaction is not fixed, but depends on the dimensionless quadratic-torsion couplings. In the Einstein--Cartan limit, using the metric definition of the stress-energy tensor, the unpolarized spin contribution gives $w_{\mathrm{spin}}=-1/3$. We then derive the corresponding modified Tolman--Oppenheimer--Volkoff equations and solve them numerically using the DD2 equation of state. For the positive effective spin-spin coupling branch considered here, the torsion correction makes the stellar configurations more compact, lowers the maximum mass, and reduces the binding energy relative to the general-relativistic sequence. For the smooth weak-polarization profiles considered, spin-correlation anisotropy has only a negligible effect on the mass--radius relation.

gr-qc↗

arXiv2026

Entanglement inequalities, black holes and the architecture of typical states

Using holographic realizations of the Araki-Lieb (AL) inequality, we show that typical pure states in large $N$ holographic CFTs possess two characteristic length scales determined solely by energy and conserved charges: a microscopic $L_{\mathrm{UV}}$ and an infrared $L_{\mathrm{IR}} > L_{\mathrm{UV}}$. Degrees of freedom between these scales effectively factorize -- one purifying the ultraviolet (scales $< L_{\mathrm{UV}}$) and the other the infrared sector (scales $> L_{\mathrm{IR}}$). Remarkably, the pure state factor including the ultraviolet sector is determined only by the energy and conserved charges up to exponentially suppressed corrections. Our results imply that all black holes in anti-de Sitter space can be isolated from an asymptotic region, the corona, that is formed by the inclusion of entanglement wedges for which the AL inequality is saturated, and an effective factorization emerges in the buffer region between the corona and the outer horizon. Crucially, we reproduce predictions of the eigenstate thermalization hypothesis and generalize them to rotating thermal ensembles.

hep-th↗

arXiv2026

Oppenheimer-Snyder Collapse in f(R) Gravity : Stalemate or Resolution?

We study the Oppenheimer--Snyder (OS) collapse problem in metric $f(R)$ gravity by matching a homogeneous dust Friedmann--Lemaître--Robertson--Walker (FLRW) interior to a generalized Vaidya exterior across a timelike hypersurface. In metric $f(R)$ gravity, regular matching requires the continuity not only of the induced metric and extrinsic curvature, but also of the Ricci scalar and its normal derivative. These additional conditions generically exclude the usual Ricci-flat exteriors, such as the Schwarzschild solution. We show that, for an unrestricted generalized Vaidya exterior, the matching conditions fix the boundary data but do not uniquely determine the bulk extension, leaving open the possibility of a physical resolution of the collapse problem. However, once the exterior matter content is restricted to the generalized Vaidya form, the field equations impose a strong constraint, forcing $f_{,R}$ to be linear in the areal radius, $f_{,R}=A(v)\,r+B(v)$. For locally invertible $f_{,R}$ with $f_{,RR}\neq 0$, this sharply reduces the admissible class of exteriors, so that the matching data uniquely determine the exterior solution on each interval where the boundary map is locally invertible. We further show that, for generic viable $f(R)$ models, the branch with $A(v)\neq 0$ does not admit a global extension with finite asymptotic curvature, while the branch $A(v)=0$ places the interior on a constant-curvature sector. This excludes nontrivial dust collapse, although it does not rule out collapse for more general interior matter with constant trace. Thus, generalized Vaidya exteriors reopen the collapse problem at a formal level, but within the restricted matter sector considered here, the OS dust collapse problem remains unresolved and the physically acceptable branch is highly constrained.

gr-qc↗

arXiv2025

Linear stability of charged warm holes

Charged black holes are known to suffer from an interior instability associated with the presence of the Cauchy horizon. Recently, a hairy charged black hole was proposed that avoids the formation of a Cauchy horizon. It is natural to question whether this instability might manifest in the exterior solution. In this paper, we have analyzed the stability of this black hole. Our results show that vector perturbations are stable, along with the scalar sector for $l=0$. We have also computed the corresponding quasinormal modes (QNMs) and quasibound states (QBSs).

gr-qc↗

arXiv2025

Nonlinear dynamics in Horndeski gravity: a renormalized approach to effective gravitational coupling

This paper develops a renormalized perturbation theory framework for nonlinear structure formation in a broad class of modified gravity models that exhibit Vainshtein screening, with a focus on a viable subclass of Horndeski theories. We extend earlier perturbative methods, originally applied to DGP model, to construct a self-consistent treatment that captures both the linear modifications to gravity at large scales and the nonlinear screening effects at small scales. In the framework, the response of the gravitational potential to matter density fluctuations is characterized by renormalized propagators, leading to the definition of a nonlinear (or renormalized) effective gravitational constant. The paper details several numerical strategies to compute this renormalized gravitational constant. Numerical examples illustrate how the effective gravitational constant evolves with scale and redshift. These results are key to accurately predicting cosmological observables such as the matter power spectrum and bispectrum in modified gravity scenarios.

gr-qc↗

arXiv2025

Characteristic initial value problems for the Einstein-Maxwell-scalar field equations in spherical symmetry

The characteristic initial boundary problem is discussed in spherical symmetry for the Einstein-Maxwell-scalar field equations. It is formulated for an affine-null metric and the resulting field equations are cast into a hierarchical system of partial differential equations. The initial boundary value problem for a family of null hypersurfaces is specified for a timelike-null foliation at the central geodesic of spherical symmetry as well as for a double-null foliation where the corresponding boundary is a null hypersurface. For the latter, two distinct boundary value formulations arise -- one where the null boundary has zero Misner-Sharp mass and another one where the corresponding Misner-Sharp mass is nonzero. As an application, the nonextremal and the extremal Reissner-Nordström solution in null coordinates for a charged black hole and the Fisher-Janis-Newman-Winicour solution are derived.

gr-qc↗

arXiv2024

A model for cosmological perturbations in affine gravity

In this paper, we present the cosmological perturbation formalism for theories within the framework of affine gravity. These theories are distinguished by their connection, devoid of any metric. Our approach involves segregating perturbations into symmetric and antisymmetric components (related to torsion), each further decomposed into irreducible elements, namely scalars, pseudoscalars, vectors, pseudovectors, 2-tensors, and 3-tensors. Finally, we have fully addressed the gauge freedom in this context.

gr-qc↗

arXiv2023

Q-balls in K-field theory

We study the existence and stability of Q-balls in noncanonical scalar field theories, $K(|Φ|^2,X)$ where $Φ$ is the complex scalar field and $X$ is the kinetic term. We extend the Vakhitov-Kolokolov stability criterion to K-field theories. We derive the condition for the perturbations to have a well-posed Cauchy problem. We find that $K_{,X}>0$ and $K_{,X}+XK_{,XX}>0$ are necessary but not sufficient conditions. The perturbations define a strongly hyperbolic system if $(K_{,X}-2ϕ'^2 K_{,XX})(K_{,X}+2ω^2ϕ^2 K_{,XX}) > 0$. For all modifications studied, we found that perturbations propagate at a speed different from light. Generically, the noncanonical scalar field can lower the charge and energy of the Q-ball and therefore improves its stability.

hep-th↗

arXiv2022

Stability of generalized Einstein-Maxwell-scalar black holes

We study the stability of static black holes in generalized Einstein-Maxwell-scalar theories. We derive the master equations for the odd and even parity perturbations. The sufficient and necessary conditions for the stability of black holes under odd-parity perturbations are derived. We show that these conditions are usually not similar to energy conditions even in the simplest case of a minimally coupled scalar field. We obtain the necessary conditions for the stability of even-parity perturbations. We also derived the speed of propagation of the five degrees of freedom and obtained the class of theories for which all degrees of freedom propagate at the speed of light. Finally, we have applied our results to various black holes in nonlinear electrodynamics, scalar-tensor theories and Einstein-Maxwell-dilaton theory. For the latter, we have also calculated the quasinormal modes.

gr-qc↗

arXiv2021

Spontaneous symmetry breaking in the late Universe and glimpses of early Universe phase transitions à la baryogenesis

Spontaneous symmetry breaking is the foundation of electroweak unification and serves as an integral part of the model building beyond the standard model of particle physics and it also finds interesting applications in the late Universe. We review development related to obtaining the late cosmic acceleration from spontaneous symmetry breaking in the Universe at large scales. This phenomenon is best understood through Ginzburg-Landau theory of phase transitions which we briefly describe. Hereafter, we present elements of spontaneous symmetry breaking in relativistic field theory. We then discuss the "symmetron" scenario-based upon symmetry breaking in the late Universe which is realized by using a specific form of conformal coupling. However, the model is faced with "NO GO" for late time acceleration due to local gravity constraints. We argue that the problem can be circumvented by using the massless $λϕ^4$ theory coupled to massive neutrino matter. As for the early Universe, spontaneous symmetry breaking finds its interesting applications in the study of electroweak phase transition. To this effect, we first discuss in detail, the Ginzburg-Landau theory of first order phase transitions and then apply it to electroweak phase transition including technical discussions on bubble nucleation and sphaleron transitions. We provide a pedagogical expositions of dynamics of electroweak phase transition and emphasize the need to go beyond the standard model of particle physics for addressing the baryogenesis problem. Review ends with a brief discussion on Affleck-Dine mechanism and spontaneous baryogenesis. Appendixes include technical details on essential ingredients of baryogenesis, sphaleron solution, one loop finite temperature effective potential and dynamics of bubble nucleation.

gr-qc↗

arXiv2021

Negative cosmological constant in the dark sector?

We consider the possibility that the dark sector of our Universe contains a negative cosmological constant dubbed $λ$. For such models to be viable, the dark sector should contain an additional component responsible for the late-time accelerated expansion rate ($X$). We explore the departure of the expansion history of these models from the concordance $Λ$ Cold Dark Matter model. For a large class of our models the accelerated expansion is transient with a nontrivial dependence on the model parameters. All models with $w_X>-1$ will eventually contract and we derive an analytical expression for the scale factor $a(t)$ in the neighborhood of its maximal value. We find also the scale factor for models ending in a Big Rip in the regime where dustlike matter density is negligible compared to $λ$. We address further the viability of such models, in particular when a high $H_0$ is taken into account. While we find no decisive evidence for a nonzero $λ$, the best models are obtained with a phantom behavior on redshifts $z\gtrsim 1$ with a higher evidence for nonzero $λ$. An observed value for $h$ substantially higher than $0.70$ would be a decisive test of their viability.

astro-ph.CO↗

arXiv2020

Weak gravity on a $Λ$CDM background

We consider Horndeski modified gravity models obeying stability, velocity of gravitational waves $c_T$ equals $c$ and quasistatic approximation (QSA) on subhorizon scales. We assume further a $Λ$CDM background expansion and a monotonic evolution on the cosmic background of the $α$ functions as $α_i= α_{i0}~a^s$ where $i=M,B$, $a$ is the scale factor and $α_{i0}$ ($α_{M0}, α_{B0}$), $s$ are arbitrary parameters. We show that the growth and lensing reduced (dimensionless) gravitational couplings $μ\equiv G_{\rm growth}/G$, $Σ\equiv G_{\rm lensing}/G$ exhibit the following generic properties today: $Σ_0 < 1$ for all viable parameters, $μ_0<1$ (weak gravity today) is favored for small $s$ while $μ_0>1$ is favored for large $s$. We establish also the relation $μ\geq Σ$ at all times. Taking into account the $fσ_8$ and $E_G$ data constrains the parameter $s$ to satisfy $s\lesssim 2$. Hence these data select essentially the weak gravity regime today ($μ_0<1$) when $s<2$, while $μ_0>1$ subsists only marginally for $s\approx 2$. At least the interval $0.5\lesssim s \lesssim 2$ would be ruled out in the absence of screening. We consider further the growth index $γ(z)$ and identify the $(α_{M0},α_{B0},s)$ parameter region that corresponds to specific signs of the differences $γ_0-γ_0^{ΛCDM}$, and $γ_1-γ_1^{ΛCDM}$, where $γ_0\equiv γ\bigl|_{z=0}$ and $γ_1\equiv \frac{{\rm d}γ}{\rm d z}\bigl|_{z=0}$. In this way important information is gained on the past evolution of $μ$. We obtain in particular the signature $γ_0>γ_0^{ΛCDM}$ for $s<2$ in the selected weak gravity region.

gr-qc↗

arXiv2020

Deflection of Light by a Rotating Black Hole Surrounded by "Quintessence"

We present a detailed analysis of a rotating black hole surrounded by "quintessence". This solution represents a fluid with a constant equation of state, $w$, which can for example describe an effective warm dark matter fluid around a black hole. We clarify the conditions for the existence of such a solution and study its structure by analyzing the existence of horizons as well as the extremal case. We show that the deflection angle produced by the black hole depends on the parameters $(c,w)$ which need to obey the condition $cw<0$ because of the weak energy condition, where $c$ is an additional parameter describing the hair of the black hole. In this context, we found that for $w\simeq 0.1$ (consistent with warm dark matter) and $c<0$, the deviation angle is larger than that in the Kerr spacetime for direct and retrograde orbits. We also derive an exact solution in the case of $w=-1/3$.

gr-qc↗

arXiv2020

Critical collapse in K-essence models

We study gravitational collapse in K-essence model with shift symmetry. For these models, we have the formation of two types of horizons, event and sonic. For the particular case $K(X)=X+βX^2$ we found three different regimes. In the weak field regime the scalar field disperses to infinity, in the very strong regime both horizons form at the same time and finally for the intermediate regime, the sonic horizon could form first or both horizons form at the same time. The threshold of formation of black hole is found in the regime where the sonic horizon forms first. We observe a universal behavior with scaling parameter $γ\simeq 0.51$. Interestingly this universal behavior is already encoded in the sonic horizon much before the black hole forms and therefore the emergence of the event horizon.

gr-qc↗

arXiv2020

Energy extraction and particle acceleration around a rotating dyonic black hole in $N=2$, $U(1)^2$ gauged supergravity

In the present paper, we explore various gravitational aspects such as energy extraction (via the Penrose process and Superradiance), particle collisions around a $\mathcal{N}=2$, $U(1)^2$ dyonic rotating black hole (BH) in the gauged supergravity model. The impact of the rotation parameter ($a$) and the gauge coupling constant ($g$) on the behaviour of horizon and ergoregion of the BH is studied. It is of interest to note that, compared with the extremal Kerr BH, the gauge coupling constant, under certain constraints, can enhance the maximum efficiency of energy extraction by the Penrose process almost double. Under the same constraints, we can extract approximately 60.75\% of the initial mass energy from the BH which is noticeably higher in contrast to the extremal Kerr BH. The limit of energy extraction in terms of the local speeds of the fragments is also examined with the help of the Wald inequality. We identify an upper limit on the gauge coupling constant up to which the phenomenon of Superradiance is likely to occur. Finally, we computed the center-of-mass energy ($E_{CM}$) of two particles with the same rest masses moving in the equatorial plane of the BH. Our study also aims to sensitize $E_{CM}$ to the rotation parameter and the gauge coupling constant for extremal and nonextremal spacetime as well. Especially, for the extremal case, an infinitely large amount of $E_{CM}$ can be achieved closer to the horizon which allows the BH to serve as a more powerful Planck-energy-scale collider as compared to Kerr and any other generalized BHs in the Kerr family explored so far in general relativity. However, $E_{CM}$ for the nonextremal spacetime is shown to be finite and has an upper bound.

gr-qc↗

arXiv2019

Large scale structures: from inflation to today: a brief report

We briefly review some selected topics gravitating around large scale structures. We derive from inflation the evolution of dark matter perturbations. The stress is put on the non-linear regime of structures formation, with a particular emphasis on relativistic effects, the Effective Field Theory approach, the role of dark energy and the possibility of inhomogeneous universes.

gr-qc↗

arXiv2019

Pure Lovelock black hole in the dimension, $d=3N+1$, is stable

In this paper we show that pure Lovelock static Schwarzschild's analogue black hole in dimensions $d>3N+1$, where $N$ is the degree of Lovelock polynomial action, is stable even though pure Gauss-Bonnet $N=2$ black hole is unstable in dimension $d<7$. We also discuss and compare quasinormal modes for pure Lovelock and the corresponding Einstein black hole in the same dimension. We find that perturbations decay with characteristic time which is weakly dimensional dependent as it depends only on the gravitational potential of the background solution, while frequency of oscillations however depend on the dimension. Also we show that spectrum of perturbations is not isospectral except in $d=4$.

gr-qc↗