Narrow Results

Compact stars in the Kaluza–Klein space-time are investigated, with multiple additional compactified spatial dimensions (d). Within the extended phenomenological model, a static, spherically symmetric solution is considered, with the equation of state provided by a zero temperature, interacting multi-dimensional Fermi gas. The maximal masses of compact stars are calculated for different model parameters. We investigated the effect of the existence of multiple extra compactified dimensions within the Kaluza–Klein compact star structure. We found that the number of extra dimensions plays a similar role, and to a similar order, as the excitation number: increasing their number, d, reduces the maximal mass by a few percent.

A new compact stars nonsingular model is presented with the generalized Bardeen–Hayward mass function. Generalized Bardeen–Hayward described the regular black hole, however, due to its regularity or nonsingular nature we were inspired to construct an anisotropic compact stars model. Along with the ansatz mass function, we coupled it with a linear equation of state (EoS) to find the solutions of field equations. Indeed, the new solutions are physically viable in all physical ground. The energy conditions and Tolman–Oppenheimer–Volkoff (TOV)-equation are well satisfied signifying that the mass distribution is physically possible and at equilibrium. Also, the static stability criterion, the causality condition and Abreu’s stability condition hold good and therefore configurations are physically static stable. The same condition is further supported by the condition that the adiabatic index, which is greater than the Newtonian limit, i.e. ${\mathrm{\Gamma }}_r(r)>4/3$. It is also noticed that the bag constant $B_g$ is proportional to the surface density in our model.

This paper is related to the dynamics of stellar structure under the influence of modified gravity (in particular $f(T)$ gravity, where T is associated with a torsion scalar). We contemplate non-static plane geometry equipped with fluid dissipation in the form of diffusion limit. The suitable variables to handle the analysis are explored. To comprehend the dynamics of the system, we acquire the dynamical equations with the aid of Bianchi identities. We calculate the Taub mass for our structure and develop the relations of mass in the form of collapsing velocity, derivative of proper radial, and time. The scalars to comprehend the evolution and structure are explored for non-static plane geometry. These scalars have a novel existence for this structure. We find that the homogeneous expansion and homologous ways of evolution interrelate with each other. We perform the investigation for two cases that is dissipative and non-dissipative. The nature of the fluid in the dissipative case is shear and geodesic. We calculate the variety of solutions in this case. The analysis is concluded by analyzing the stability of the vanishing $Y_{\mathrm{\text{TF}}}$ constraint.

Unitarity of evolution in gravitational collapses implies existence of macroscopic stable horizonless objects. With such objects in mind, we study the effects of anisotropy of pressures on the stability of stars. We consider stars in four or higher dimensions and also stars in M theory made up of (intersecting) branes. Taking the stars to be static, spherically symmetric and the equations of state to be linear, we study “singular solutions” and the asymptotic perturbations around them. Oscillatory perturbations are likely to imply instability. We find that nonoscillatory perturbations, which may imply stability, are possible if an appropriate amount of anisotropy is present. This result suggests that it may be possible to have stable horizonless objects in four or any higher dimensions, and that anisotropic pressures may play a crucial role in ensuring their stability.

We present an exact solution that could describe compact star composed of color-flavor locked (CFL) phase. Einstein’s field equations were solved through CFL equation of state (EoS) along with a specific form of $g_{rr}$ metric potential. Further, to explore a generalized solution we have also included pressure anisotropy. The solution is then analyzed by varying the color superconducting gap $\delta$ and its effects on the physical parameters. The stability of the solution through various criteria is also analyzed. To show the physical validity of the obtained solution we have generated the $M-R$ curve and fitted three well-known compact stars. This work shows that the anisotropy of the pressure at the interior increases with the color superconducting gap leading to decrease in adiabatic index closer to the critical limit. Further, the fluctuating range of mass due to the density perturbation is larger for lower color superconducting gap leading to more stable configuration.

The physically viable models of compact stars like SAX (J1808.4-3658) can be obtained using Vaidya–Tikekar ansatz prescribing spheroidal geometry for their interior space–time. We discuss here the suitability of an alternative ansatz in this context. The models of superdense star are proposed using a general three parameter family of solutions of relativistic field equations obtained adopting the alternative ansatz. The setup is shown to admit physically viable models of superdense stars and strange matter stars such as Her. X-1.

Astrophysical bounds on the cosmological constant are examined for spherically symmetric bodies. Similar limits emerge from the hydrostatical and gravitational equilibrium and the validity of the Newtonian limit. The methods in use seem to be disjoint from the basic principles, however they have the same implication regarding the upper bounds. Therefore we will compare different inequalities and comment on the possible relationship between them. These inequalities are of relevance for the so-called coincidence problem and for the bound of the cosmological constant which comes surprisingly close to the "experimental" value.

The objective of this paper is to find out the suitability of an ansatz similar to that suggested by Vaidya–Tikekar, but prescribing paraboloidal geometry for the 3-space of the interior space–time of a relativistic spherical star in describing a family of physically viable models of superdense stars like Her X-1, SAX, and X-ray brust.

We study for the first time how a new class of stars could impact an ensemble of pulsars with known masses and spin-periods. These new compact objects are strange stars admixed with condensed dark matter (DM). In this exploratory theoretical work, our goal is to determine how the basic parameters of pulsars are modified for such a new class of compact objects. In particular, we consider three different scenarios that correspond to a DM mass fraction of $5\%$, $11\%$ and $25\%$. Within each scenario with fixed parameters, we predict theoretically other properties of the pulsars, such as the radius, the compactness, the moment of inertia, as well as the angular momentum. Our numerical results are summarized in tables and also shown graphically for better visualization, where a comparison between the different scenarios can be made.

Asymmetric Dark Stars, i.e. compact objects formed from the collapse of asymmetric dark matter could potentially produce a detectable photon flux if dark matter particles self-interact via dark photons that kinetically mix with ordinary photons. The morphology of the emitted spectrum is significantly different and therefore distinguishable from a typical black-body one. Given the above and the fact that asymmetric dark stars can have masses outside the range of neutron stars, the detection of such a spectrum can be considered as a smoking gun signature for the existence of these exotic stars.

The motion and acceleration of an electrically charged and magnetized particle around a cylindrical black hole in the presence of an external asymptotically uniform magnetic field parallel to the $z$-axis are investigated. We look at circular orbits around a central object and study the dependence of the most internal stable circular orbits (ISCO) on the so-called magnetic coupling parameters, which are responsible for the interaction between the external magnetic field and magnetized and charged particles. It is shown that the ISCO radius decreases with increasing magnetized interaction parameter. Therefore, we also studied collisions of magnetized particles around a cylindrical black hole immersed in an external magnetic field, and showed that the magnetic field can act as a particle accelerator near nonrotating cylindrical black holes.

In this paper, we study dynamics of massless (photon and neutrino-like) particles in the spacetime of regular Bardeen black hole (BH) in novel four-dimensional Einstein–Gauss–Bonnet (4D EGB) theory. First, we have analyzed the hor properties of the spacetime of the Bardeen BH with respect to the Gauss–Bonnet (GB) coupling parameter $\alpha$. Our detailed analysis has shown that the GB coupling parameter is limited by maximum and minimum values at, $\alpha \in (-0.15÷1)$ and the extreme value of magnetic charge of the BH depends on the parameter $\alpha$. We have explored motion of neutrino-like particles and photons around the Bardeen BH. We have also explored the possibility of mimicking cases of the spin of Kerr BH and charge of Reissner–Nordström BH by the Bardeen charge, providing the same values of impact parameter for photons corresponding to the BH shadow size. Finally, we have investigated spherically infalling accretion and shadow of the Bardeen BHs in 4D EGB theory.

We study the impact of nonlocal modifications of General Relativity on stellar structure. In particular, assuming an analytic distortion function and specific equations of state, we made use of remnant stars to put qualitative constraints on a parameter not directly restricted by solar system tests. Using current data sets available for white dwarfs and strange quark stars candidates, we find that the most stringent bounds come from the objects displaying the highest core densities, namely strange quark stars. Specifically, the constraints obtained from this class of stars are three to four orders of magnitude tighter than those obtained using white dwarfs.

We investigate the behavior of expansion free collapsing fluids, as studied by L. Herrera, A. Di Prisco and J. Ospino [Symmetry 15 (2023) 754], in the framework of $f(R)$ gravity, which represents a modification of Einstein’s general relativity by establishing a function of the Ricci scalar $R$ in the gravitational action. We explore dynamical equations from Bianchi identities that demonstrate the motion and evolution of physical systems under the influence of gravitational fields. We match the inner and outer geometries of spacetime on the hypersurface to develop junction conditions by using the Misner–Sharp formalism. This allows us to identify the connection between mass functions for the inner and outer space as well as the relationship for heat flux $q$ and radial pressure ${\mathbb{P}}_r$. We also investigate analytical solutions of dissipative fluid distribution that fulfill the vanishing expansion condition together with the vanishing complexity factor constraint. For this, we introduce new constraints that permit the integration of the complex system in $f(R)$ gravity. Next, we extract a set of differential equations that explain the dynamical structure of the dissipative spheres both in geodesic and non-geodesic fluids. Furthermore, we explore the physical characteristics of the obtained solutions, such as heat flux, energy density, shear stress, fluid’s temperature along with tangential and radial pressure, to assess their viability in describing real astrophysical systems.

A linear response relation between metric and fluid perturbations driven by a background internal noise source is used as a framework for addressing stochastic effects in order to establish a mesoscopic theory for dense matter relativistic stars. In this paper, nonradial polar perturbations are worked out, which are important from the point of view of detection in future. We present qualitative first results in this paper, numerical estimates have to await further progress in theoretical modeling. These perturbations carry a new generalized stochastic nature and are obtained as solutions of the classical Einstein–Langevin equation which has been recently proposed. The significance of these stochastic nonradial polar perturbations lies at probing the intermediate sub-hydro scales inside the dense fluid. This formalism extends towards a mesoscopic scale nonequilibrium/near-equilibrium statistical mechanics study for relativistic star interiors. The generalized stochastic noise originates as the remnant of collapse mechanism and dynamical effects at intermediate scales in isolated massive stars which drives these polar perturbations. More specifically, for cold dense matter which is our focus in this paper, it is either the interplay between the degeneracy pressure and the gravitational pressure, or the multiscale phenomena like turbulences giving rise to the seeds of stochastic effects in the gravitating body. Characterizing such stochastic effects can lead to an improved understanding of the nature of dense matter and help to probe multiple scales which are yet untouched.

In this paper, we will manifest how polytropic fluids produce effects on an alternate gravitational source, regardless of its features, for spherically symmetric and static spacetimes. For this purpose, we use $f(T,\mathcal{𝒯})$ modified gravity, which is characterized by the trace of an energy–momentum tensor as $\mathcal{𝒯}$ and torsion scalar $T$, Additionally, using the polytropic equation of state, we will speculate about the distribution of energy among fluids within an astrophysical object. We offer a thorough methodology for identifying generic relativistic polytropes accompanied by the minimal geometric decoupling technique. Ultimately, a few tangible attributes of polytropes along with perfect fluids are observed using the Tolman IV solution, including total pressure, energy interchange, and thermal characteristics.

The objective of this paper is to provide a piece of theoretical evidence for the existence of cylindrically symmetric, time-independent wormhole models surrounded by the halos of fuzzy dark matter. To do so, we model our system by assuming a cylindrical spacetime coupled with locally anisotropic matter content. We calculate the shape function involved in the construction of fuzzy dark matter wormholes, by solving simultaneous equations of motion and density powered by Einasto energy profile. The investigation of the validity of null energy conditions is performed with the aim of seeing stable wormhole structures. The role of the equation of state parameter in the theoretical structure of the shape function and the redshift function is studied which would lead to understanding the signals’ gravitational redshift and the spatial distribution of the fuzzy wormholes. A few characteristics of the wormholes like hydrostatic equilibrium, and active C-energy are also noticed in the less complex wormholes. It is inferred that the stability as well as the size of the fuzzy dark matter wormholes can be fine-tuned by modulating the index parameter involved in the energy source.

In this paper, we have obtained a new class of solutions of modified field equations (FEqs) in $f(R,T)$ gravity with barotropic equation of state (EoS) i.e. $p_r=\alpha \rho +\beta$. For this purpose, we assume spherically symmetric metric with anisotropic matter distribution and formulated the FEqs. We apply Durgapal transformation and choose suitable gravitational potential to solve the dynamical equations numerically. By varying parametric values $a$ and $b$ (appearing in gravitational potential), we generate special cases of solutions. We plot physical quantities like energy density, anisotropy parameter, radial and tangential pressures in all particular cases. It is concluded that all the plots hold physical acceptability criteria and show consistency which admits modified $f(R,T)$ gravity for compact star models.

In this paper, we investigate the charged static spherically symmetric models in $f(R)$ theory of gravity. We consider a linear equation of state (EoS) in the background of anisotropic matter configuration. We formulate the modified field equations and implement Durgapal transformation to examine the gravitational nature of compact stellar objects. For this purpose, we choose a specific gravitational potential and electric charge intensity to analytically solve the set of field equations. We generate three special cases of solutions for specific parametric values of $a,b$ appearing in the expression of gravitational potential. The evolution of physical observables, such as energy density, anisotropy parameter, radial and tangential pressures and electric field intensity, are presented in all cases. Via physical analysis, it is observed that the solution of charged compact spheres satisfies acceptability criteria, models are well behaved, and depict stability and consistency in accordance with $f(R)$ gravity for generated models.

Non-rotating strange quark stars made of isotropic matter in Lorentz-violating theories of gravity are studied. In particular, Hořava gravity and Einstein-æther theory are considered. For quark matter we adopt both linear and non-linear equations-of-state, corresponding to the MIT bag model and color flavor locked phase, respectively. The new, modified structure equations generalize the usual Tolman-Oppenheimer-Volkoff equations valid in Einstein’s General Relativity. A dimensionless parameter ν measures the deviation from the standard TOV equations, which are recovered in the appropriate limit. We compute some properties, such as masses, radii as well as the factor of compactness of the stars, and we show pictorially the impact of the parameter ν on the mass-to-radius relationships for several different equations-of-state. Other physical considerations, such as stability criteria, causality and energy conditions, are also considered, and they are all found to be fulfilled.