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Khireddine Nouicer

Publications and source records attributed to Khireddine Nouicer.

At least 19 recordsLinked to original sources
arXiv2026

Spontaneous scalarization of neutron stars in teleparallel gravity with derivative torsional coupling

We study neutron star configurations in a teleparallel gravity model featuring a scalar field coupled to both matter and torsion. In the Einstein frame, the theory includes a derivative coupling between the scalar field and the torsion vector, together with a conformal matter coupling \(A(ϕ)=\exp(βϕ^{2}/2)\). Static and slowly rotating neutron-star solutions are constructed for realistic equations of state, focusing on the APR and MS1 equations of state. Scalarized solutions appear only within a finite range of central densities and correspond to localized deviations from the general-relativistic mass--radius and mass--central-density relations. The onset and extent of scalarization depend on the equation of state and on the strength of the derivative torsional interaction, which can either enhance or suppress scalarization relative to the general-relativistic scalarized branch. At high central densities, scalarization is quenched and the solutions approach the general-relativistic limit, remaining bounded even for strong torsional couplings. No scalarized solutions are found in the absence of matter coupling (\(β=0\)). The normalized scalar charge follows trends consistent with the global mass relations, indicating an intermediate scalarized regime suppressed at high compactness. For slowly rotating stars, the moment of inertia depends systematically on the torsional coupling and the equation of state, with stiffer equations yielding larger values. These results highlight the potential of neutron-star radius and rotational measurements to test teleparallel scalarization scenarios.

gr-qc↗
arXiv2024

Neutron Stars in Scalar Torsion Theories with Nonminimal Coupling

We study the existence and structure of static and slowly rotating neutron stars (NSs) in a particular truncation of scalar torsion theory with a scalar field $ ϕ$ non-minimally coupled to the torsion scalar, and a potential of the form $ V(ϕ)=-μ^2ϕ^2/2 +λϕ^4 /4 $. We derive the hydrostatic equilibrium equations in the static case and solve them numerically for both interior and exterior regions using the appropriate boundary conditions near the center and far from the star. We plot the radial profiles of the metric functions and the scalar field as well as the mass-radius diagram of the star employing a set of four different realistic equations of state (EoS). Our findings show a high degree of compatibility with the observational constraints of the GW170817 event and indicate a maximum mass $2.37M_{\odot}$ obtained with the BSk21 EoS for a coupling parameter $ ξ=0.25 $. We extend our analysis to include slow rotation, and determine the relation between the star's moment of inertia and its mass. We also show that the universality relation of the two forms of the normalized moment of inertia continue to hold in scalar torsion theory with non minimal coupling.

gr-qc↗
arXiv2024

Cotton gravity is not predictive

It is well-known that the theory of Cotton gravity proposed by Harada is trivially solved by all isotropic and homogeneous cosmologies. We show that this under-determination is more general. More precisely, the degree of arbitrariness in the solutions increases with the degree of symmetry. We give two simple examples. The first is that of static spherically symmetric solutions, which depend on an arbitrary function of the radial coordinate. The second is that of anisotropic cosmologies, which depend on an arbitrary function of time.

gr-qc↗
arXiv2024

Spherical symmetric solutions of conformal Killing gravity: black holes, wormholes, and sourceless cosmologies

The most general set of static and spherically symmetric solutions for conformal Killing gravity coupled to Maxwell fields is presented in closed form. These solutions, depending on six parameters, include non-asymptotically flat black holes or naked singularities, non-asymptotically flat traversable wormholes, and (possibly singularity-free) closed universes. We also consider the inverse problem, showing that the most general energy-momentum tensor generating a given static spherically symmetric metric depends on three parameters. Sourceless time-dependent isotropic solutions are also given. These solutions depending on the curvature, the cosmological constant, and a new integration constant $α$, present a rich variety, including singularity-free eternal cosmologies and universes evolving symmetrically from a big bang to a big crunch within a finite lapse of time.

gr-qc↗
arXiv2024

Alleviating the $H_0$ and $σ_8$ tensions in the interacting cubic covariant Galileon model

The interaction between dark matter and dark energy has become a focal point in contemporary cosmological research, particularly in addressing current cosmological tensions. This study explores the cubic Galileon model's interaction with dark matter, where the interaction potential in the dark sector is proportional to the dark energy density of the Galileon field. By employing dimensionless variables, we transform the field equations into an autonomous dynamical system. We calculate the critical points of the corresponding autonomous systems and demonstrate the existence of a stable de Sitter epoch. Our investigation proceeds in two phases. First, we conduct a detailed analysis of the exact interacting cubic Galileon (ICG) model, derived from the precise solution of the equations of motion. Second, we explore an approximate tracker solution, labeled TICG, assuming a small coupling parameter between dark matter and dark energy. We evaluate the evolution of these models using data from two experiments, aiming to resolve the tensions surrounding $H_0$ and $S_8$. The analysis of the TICG model indicates a preference for a phantom regime and provides a negative coupling parameter in the dark sector at a $68\%$ confidence level. This model also shows that the current tensions regarding $H_0$ and $S_8$ are alleviated. Conversely, the ICG model, despite its preference for the phantom regime, is plagued by an excess in today's matter density and a higher expansion rate, easing only the $H_0$ tension.

gr-qc↗
arXiv2024

Farewell to Cotton gravity

We reformulate our proof of the under-determination of Cotton gravity in terms of the Codazzi parametrization.

gr-qc↗
arXiv2019

Growth of Matter Perturbations in the Bi-Galileons Field Model

We study a dark energy cubic bi-Galileons field model based on truncation of the recently proposed generalized covariant multi-Galileons model. We investigate the cosmological dynamic of the model by the theory of dynamical systems through the analysis of the properties of the fixed points in each cosmological epoch. We show the existence of two tracker solutions, one of which is that of the cubic single Galileon model and the other solution is the signature of the second Galileon field. Exploiting the competition between the two Galileon fields, we find a dark energy solution that avoids the approach to the tracker solution with dark energy equation of state $w_{DE}=-2$ during the matter epoch which is disfavored by the observational data. We study also the growth rate of matter perturbations. Using recent $fσ_{8}$ redshift space distortion (RSD) and model-independent observational Hubble (OHD) data sets, we place observational constraints on the coupling constant and cosmological parameters of the bi-Galileons model through Monte Carlo numerical method based on the Metropolis-Hastings algorithm. We find that the amplitude of growth matter fluctuations is consistent with the Planck15 data and ease the tension between early and later clustering, and fits better the data from the DES survey over the data from KiDS-450 survey. We also find that the best fit value for the Hubble constant is compatible with new measurements of Cepheid-supernovae distance scale. Finally, we perform a model selection through the Bayes factor and found that the bi-Galileons model is disfavored in comparison to the $Λ$CDM model, but slightly preferred to $w$CDM model.

astro-ph.CO↗
arXiv2018

Hairy black hole solutions in a three-dimensional Galileon model

We investigate stationary rotationally symmetric solutions of a particular truncation of Horndeski theory in three dimensions, including a non-minimal scalar kinetic coupling to the curvature. After discussing the special case of a vanishing scalar charge, which includes most of the previously known solutions, we reduce the general case to an effective mechanical model in a three-dimensional target space. We analyze the possible near-horizon behaviors, and conclude that black hole solutions with degenerate horizons and constant curvature asymptotics may exist if the minimal and non-minimal scalar coupling constants have the same sign. In a special case, we find a new analytic rotating black hole solution with scalar hair and degenerate horizon. This is geodesically and causally complete, and asymptotic to the extreme BTZ metric. We also briefly discuss soliton solutions in another special case.

gr-qc↗
arXiv2018

Cosmology of the interacting Cubic Galileon

We consider the cosmological dynamics of the cubic Galileon field model interacting with dark matter, where the interaction between the cosmic dark sectors is proportional to Hubble parameter and dark energy density. The background field equations are converted into an autonomous dynamical system of first order equations using suitable dimensionless variables. The fixed points are determined and their stability analyzed by means of the eigenvalue method. The analysis of the stability of the fixed points along that of the avoidance of ghosts and Laplacian instabilities implies negative coupling parameter. We find an attractor fixed point in the de Sitter phase, and construct an approximated tracker solution, which to a good approximation, mimics the background cosmic evolution obtained by solving numerically the autonomous system of equations. In addition, we perform the statefinder and Om diagnostics and show that for some appropriate values of the coupling constant and initial conditions, the trajectories in the statefinder parameter phase space evolves towards a stable state corresponding to Λ CDM model. The comparison with the observed distance modulus and Hubble parameter data sets clearly indicates that the cosmological dynamics of the interacting cubic model is compatible with the existence of an interaction between the dark sectors.

gr-qc↗
arXiv2010

Hawking radiation and thermodynamics of dynamical black holes in phantom dominated universe

The thermodynamic properties of dark energy-dominated universe in the presence of a black hole are investigated in the general case of a varying equation-of-state-parameter $w(a)$. We show that all the thermodynamics quantities are regular at the phantom divide crossing, and particularly the temperature and the entropy of the dark fluid are always positive definite. We also study the accretion process of a phantom fluid by black holes and the conditions required for the validity of the generalized second law of thermodynamics. As a results we obtain a strictly negative chemical potential and an equation-of-state parameter $w<-5/3.$

gr-qc↗
arXiv2010

Entropy of cosmological black holes and generalized second law in phantom energy-dominated universe

Adopting the thin-layer improved brick-wall method, we investigate the thermodynamics of a black hole embedded in a spatially flat Friedmann-Robertson-Walker universe. We calculate the temperature and the entropy at every apparent horizon for arbitrary solution of the scale factor. We show that the temperature and entropy display a non-trivial behavior as functions of time. In the case of black holes immersed in universe driven by phantom energy, we show that for specific ranges of the equation-of-state parameter and apparent horizons the entropy is compatible with the D-bound conjecture, even the null, dominant and strong energy conditions are violated. In the case of accretion of phantom energy onto black hole with small Hawking-Hayward quasi-local mass, we obtain an equation-of-state parameter in the range $w\leq-5/3$, guaranteeing the validity of the generalized second law.

gr-qc↗
arXiv2009

The Casimir force in noncommutative Randall-Sundrum models

In this paper we study the effect of spacetime noncommutativity in the 5-dimensional Randall-Sundrum brane worlds on the Casimir force acting on a pair of parallel plates. We show that the presence of a noncommutative scale length affects the nature of the Casimir force for small plate separation. Using accurate experimental bounds for the Casimir force in parallel plate geometry, we find an upper bound for the noncommutative cutoff of the order of $10^{3}$ TeV, and that the size of the interbrane distance in RSI model is approximately given by $kR\lesssim20.5$ and $kR\lesssim18.4$ for $k=10^{19}$ GeV and $k=10^{16}$GeV, respectively.

hep-th↗
arXiv2007

Entropy of the Randall-Sundrum black brane world to all orders in the Planck length

We study the effects, to all orders in the Planck length from a generalized uncertainty principle (GUP), on the statistical entropy of massive scalar bulk fields in the Randall-Sundrum black brane world. We show that the Bekenstein-Hawking area law is not preserved, and contains small corrections terms proportional to the black hole inverse area.

gr-qc↗
arXiv2007

Coulomb potential in one dimension with minimal length: A path integral approach

We solve the path integral in momentum space for a particle in the field of the Coulomb potential in one dimension in the framework of quantum mechanics with the minimal length given by $(ΔX)_{0}=\hbar \sqrtβ$, where $β$ is a small positive parameter. From the spectral decomposition of the fixed energy transition amplitude we obtain the exact energy eigenvalues and momentum space eigenfunctions.

quant-ph↗
arXiv2007

Black holes thermodynamics to all orders in the Planck length in extra dimensions

We investigate the effects to all orders in the Planck length, from a generalized uncertainty principle, on the thermodynamic parameters of radiating Schwarzschild black holes in a scenario with large extra dimesions. We show that black holes in this framework are hotter, decay faster and are less classical objects. Particularly, we show that the final stage of the radiation process is a black hole remnant with zero entropy, zero heat capacity and non zero finite temperature. We finally compare our results with the ones obtained in the standard Hawking picture and with the generalized uncertainty principle to leading order in the Planck length.

gr-qc↗
arXiv2007

Effect of Minimal lengths on Electron Magnetism

We study the magnetic properties of electron in a constant magnetic field and confined by a isotropic two dimensional harmonic oscillator on a space where the coordinates and momenta operators obey generalized commutation relations leading to the appearance of a minimal length. Using the momentum space representation we determine exactly the energy eigenvalues and eigenfunctions. We prove that the usual degeneracy of Landau levels is removed by the presence of the minimal length in the limits of weak and strong magnetic field.The thermodynamical properties of the system, at high temperature, are also investigated showing a new magnetic behavior in terms of the minimal length.

quant-ph↗
arXiv2007

Quantum-corrected black hole thermodynamics to all orders in the Planck length

We investigate the effects to all orders in the Planck length from a generalized un- certainty principle (GUP) on black holes thermodynamics. We calculate the corrected Hawking temperature, entropy, and examine in details the Hawking evaporation process. As a result, the evaporation process is accelerated and the evaporation end-point is a zero entropy, zero heat capacity and finite non zero temperature black hole remnant (BHR). In particular we obtain a drastic reduction of the decay time, in comparison with the results obtained in the Hawking semi classical picture and with the GUP to leading order in the Planck length.

gr-qc↗