arXiv subjects

Youcef Kehal

Publications and source records attributed to Youcef Kehal.

3 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 $A(\phi)=\exp(\beta\phi^{2}/2)$. 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 ($\beta=0$). 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

Alleviating the $H_0$ and $\sigma_8$ 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↗

arXiv2023

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 $ \phi $ non-minimally coupled to the torsion scalar, and a potential of the form $ V(\phi)=-\mu^2\phi^2/2 +\lambda \phi^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 $ \xi=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↗