Narrow Results

This study employs $f(R,T)$ gravity, where $R$ is the Ricci scalar, and $T$ is the trace of energy–momentum tensor, to analyze charged spherically symmetric fluid distributions governed by a polytropic equation of state, thereby exploring the impact of direct coupling between matter fields and the curvature of spacetime on stellar configurations. We present two families of relativistic polytropes for both anisotropic and isotropic matter sources in the presence of electromagnetic field and derive the structural equations (mass conservation and modified Tolman–Oppenheimer–Volkoff equations). The numerical method is utilized to solve structural equations by varying the values of arbitrary parameters and polytropic constants. Using energy constraints, the physical feasibility of polytropes is examined. In addition to this, some other stellar properties such as compactness, redshift and adiabatic index are found to be well-behaved as they fulfill the required criteria. Also, the speed of sound parameter fulfills the stability criterion of the stellar body. For both isotropic and anisotropic stellar configurations, the effects of charge and the curvature-matter coupling parameter are important since they highly affect how compact these structures are.

Quark matter is expected to exist in the interior of compact stellar objects as neutron stars or even the more exotic strange stars, based on the Bodmer–Witten conjecture. Bare strange quark stars and (normal) strange quark-matter stars, those possessing a baryon (electron-supported) crust, are hypothesized as good candidates to explain the properties of a set of peculiar stellar sources such as the enigmatic X-ray source RX J1856.5-3754, some pulsars such as PSR B1828-11 and PSR B1642-03, and the anomalous X-ray pulsars and soft γ-ray repeaters. In the MIT bag model, quarks are treated as a degenerate Fermi gas confined to a region of space having a vacuum energy density B_bag (the Bag constant). In this note, we modify the MIT bag model by including the electromagnetic interaction. We also show that this version of the MIT model implies the anisotropy of the bag pressure due to the presence of the magnetic field. The equations of state of the degenerate quarks gases are studied in the presence of ultra strong magnetic fields. The behavior of a system made up of quarks having (or not) anomalous magnetic moment is reviewed. A structural instability is found, which is related to the anisotropic nature of the pressures in this highly magnetized matter. The conditions for the collapse of this system are obtained and compared to a previous model of neutron stars that is built on a neutron gas having anomalous magnetic moment.

We review the recent literature on modified theories of gravity in the Palatini approach. After discussing the motivations that lead to consider alternatives to Einstein's theory and to treat the metric and the connection as independent objects, we review several topics that have been recently studied within this framework. In particular, we provide an in-depth analysis of the cosmic speed-up problem, laboratory and solar system tests, the structure of stellar objects, the Cauchy problem, and bouncing cosmologies. We also discuss the importance of going beyond the f(R) models to capture other phenomenological aspects related with dark matter/energy and quantum gravity.

This paper represents the second step towards understanding stellar structure using pure geometric tools. It is an attempt to get a theoretical expression for a mass–radius relation. The stellar model used has been obtained as an analytic solution of the field equations of a pure geometric field theory. The method suggested to get this relation is very simple. It depends mainly on a set of differential equations implying the vanishing of all components of a geometric material-energy tensor on a boundary of stellar configuration. The theoretical relation obtained is a linear one $(R=\beta M)$ with one free parameter $(\beta ).$ Comparison with observation, using a sample of lower main-sequence stars, members of binary systems, is given. For the primary members $(M<2M_{\odot })$, we get $\beta =1.16$. It is worthy of mention that the model obtained is a simple one. Rotation, magnetic field, etc. are not considered in the present treatment. So, the model is far from being complete. It is just a step to show that pure geometric consideration objects can be used to treat problems of stellar structure.

A few questions related to white dwarfs’ physics is posed. It seems that the modified gravity framework can be a good starting point to provide alternative explanations to cooling processes, their age determination, and Chandrasekhar mass limits. Moreover, we have also obtained the Chandrasekhar limit coming from Palatini $f({ℛ})$ gravity provided by a simple Lane–Emden model.

The purpose of this paper is to examine the anisotropic matter distribution concerning one of the well-known $f(R,T)$ theories of gravity, where $R$ and $T$ represent the Ricci scalar and trace of the energy–momentum tensor, respectively. We consider anisotropic matter distribution in spherically symmetric spacetime for our desired work. We investigate the physical behavior of pressure components, energy density, anisotropic function, equation of state parameters, and energy conditions. Furthermore, we analyze the stability of compact stars by investigating the causality condition, adiabatic index, and generalized Tolman–Oppenheimer–Volkoff equation. Some fundamental features of compact stars have been investigated, like compactness factor, mass function, and surface redshift. Our calculated solutions for Bardeen stellar structures are consistent and stable in the background of the $f(R,T)$ theory of gravity.

In several classes of modified gravity theories, extra degrees of freedom are not completely screened in the interiors of stellar and substellar objects. In such theories, the hydrostatic equilibrium condition inside these objects is altered. Moreover, the interior structures of these objects might have a small pressure anisotropy induced by several physical phenomena, including rotation and magnetic fields. All these effects, both individually and collectively, induce changes in predicted stellar observables. Such changes have an impact on different phases of the stellar life cycle, starting from its birth to its death, covering almost all the branches of the Hertzsprung–Russell diagram. The aim of this work is to systematically review the current literature on the topic. We discuss the main results and constraints obtained on a class of modified gravity theories.

In this study, our main focus is to investigate the mass–radius relation and several important properties of massive neutron stars to realize the nature, behavior and evolution of these kinds of compact objects at present time. Also, we want to understand the equation-of-state of the core nuclear matter precisely with their stable equilibrium configuration. We have chosen a few massive binary pulsars and investigated on maximum attainable mass and lowest possible radius of them. We have considered Einstein–Hilbert action as $f(R,T)=R+\alpha T$, where $R$ is the Ricci scalar and $T$, the trace of energy–momentum tensor with $\alpha$ as the coupling parameter and also used modified TOV equations. The interior space-time of the spherical neutron star is matched to the exterior Schwarzschild line element at the surface of the star. The $M$–$R$ curve can predict the maximum achievable mass is about $3.51M_{\odot }$ with lowest possible radius of around $10.5$ km for the massive compact stars under stable equilibrium. We have obtained a clear picture of structural evolution of massive neutron stars through accelerating space-time and can put some constraints on several quantities related to them. It is depicted from our present investigation that all the derived outcomes are compatible with physically adopted regimes which reveal the viability of our current model in the context of $f(R,T)$ modified gravity.

Traditional stellar structure models of non-rotating neutron stars work under the assumption that these stars are perfect spheres. This assumption of perfect spherical symmetry is not correct if the matter inside neutron stars is described by an anisotropic model for the equation of state. Certain classes of neutron stars such as Magnetars and neutron stars which contain color-superconducting quark matter cores are expected to be deformed making them oblong spheroids. In this work, we investigate the stellar structure of these deformed neutron stars by deriving stellar structure equations in the framework of general relativity. Using a non-isotropic equation of state model, we solve these structure equations numerically in two dimensions. We calculate stellar properties such as masses and radii along with pressure profiles and investigate changes from standard spherical models.

We investigate the influence of the surface electrical charge in the static equilibrium configuration of white dwarfs, this is possible by solving numerically the hydrostatic equilibrium equation for the charged case. We consider that the fluid in the star is described by a fully degenerate electron gas and that the electric charge is distributed close to the surface of the white dwarf. We found that super-Chandrasekhar mass white dwarfs are found for a large surface electrical charge.

Any infrared modification of gravity, which explains the current cosmic acceleration, should not spoil the successes of general relativity in solar-system observations. In many theories of modified gravity, it is ensured by the Vainshtein mechanism that works near dense sources. Recently, it has been found that the Vainshtein mechanism can be broken inside a dense source, although not outside, in a general class of scalar-tensor theories. In this paper, I will discuss its impact on the density profile of a star, modeling simply it as a polytropic sphere. I will also show the existence of a universal upper bound on the amplitude of this type of modification, independently of the details of the equation of state.