Dark Energy Stars Research Articles

Stellar features of strange dark energy stars

Dark energy stars are theoretical objects that could potentially provide an explanation for the observable properties of black holes, including their event horizons and emission of the Hawking radiation. Because of this, they are considered to be an important area of research in modern theoretical physics and could help to answer fundamental questions about the Universe and the nature of dark energy. So, identifying and characterizing dark energy stars are a theoretically active and significant field in literature. In this context, for a theoretical model describing a new static symmetric singularity-free anisotropic dark energy star in view of the Rastall theory of gravity, which is an alternative to the general theory of relativity and is consistent with a wide range of astronomical observations, we focus our awareness in the present study on the modified field equations as well as the hydrostatic equilibrium expression and then discuss the corresponding solutions. It is shown that this new-type dark energy star meets all the necessary astronomical requirements, including stability and feasibility can potentially represent a compact cosmic structure.

Analysis of the boundary condition and equation of state in radiating stars

A model of a radiating star undergoing gravitational collapse is constructed by systematically studying both the boundary condition and equation of state for the first time. The resulting system of partial differential equations is analysed using Lie symmetries. The group analysis produces new classes of exact solutions with acceleration, expansion and shear in the presence of the cosmological constant. Known exact solutions are recovered as special cases of our solutions. In particular, we regain the Euclidean star model which is the relativistic version of the Newtonian star. The range of the equation of state parameters allows the solution to be interpreted as barotropic matter stars or dark energy stars. A particular case shows that the model is physically reasonable and the energy conditions are satisfied.

Dark Energy Star: Physical Constraints on the Bounds

AbstractIn the present article, we explore a new solution to the Einstein‐Maxwell field equations to describe spherically symmetric, charged and anisotropic dark energy stars. The interior of such stars containing two non‐interacting fluids like dark and normal matter with a relation that the dark energy density is proportional to the energy density of a isotropic matter. We have used modified chaplygin gas equation to define the dark energy part. Also we have defined the interior space‐time through Krori‐Barua (KB) space‐time and matched it with the exterior Reissner‐Nordström space‐time. The well‐behaveness of the physical attributes have been discussed by plotting graphs for three specific compact objects with respect to our solutions. We conclude that all the physical attributes satisfy the required conditions for viability as well as stability and hence our model can be used to characterize dark energy stars for a set of parametric values belong to the present study.

Radial pulsations, moment of inertia and tidal deformability of dark energy stars

We construct dark energy stars with Chaplygin-type equation of state (EoS) in the presence of anisotropic pressure within the framework of Einstein gravity. From the classification established by Iyer et al. (Class Quantum Grav 2:219, 1985), we discuss the possible existence of isotropic dark energy stars as compact objects. However, there is the possibility of constructing ultra-compact stars for sufficiently large anisotropies. We investigate the stellar stability against radial oscillations, and we also determine the moment of inertia and tidal deformability of these stars. We find that the usual static criterion for radial stability dM/drho _c >0 still holds for dark energy stars since the squared frequency of the fundamental pulsation mode vanishes at the critical central density corresponding to the maximum-mass configuration. The dependence of the tidal Love number on the anisotropy parameter alpha is also examined. We show that the surface gravitational redshift, moment of inertia and dimensionless tidal deformability undergo significant changes due to anisotropic pressure, primarily in the high-mass region. Furthermore, in light of the detection of gravitational waves GW190814, we explore the possibility of describing the secondary component of such event as a stable dark energy star in the presence of anisotropy.

Dark energy star in gravity's rainbow

The concept of dark energy can be a candidate for preventing the gravitational collapse of compact objects to singularities. According to the usefulness of gravity's rainbow in UV completion of general relativity (by providing a new description of spacetime), it can be an excellent option to study the behavior of compact objects near phase transition regions. In this work, we obtain a modified Tolman-Openheimer-Volkof (TOV) equation for anisotropic dark energy as a fluid by solving the field equations in gravity's rainbow. Next, to compare the results with general relativity, we use a generalized Tolman-Matese-Whitman mass function to determine the physical quantities such as energy density, radial pressure, transverse pressure, gravity profile, and anisotropy factor of the dark energy star. We evaluate the junction condition and investigate the dynamical stability of dark energy star thin shell in gravity's rainbow. We also study the energy conditions for the interior region of this star. We show that the coefficients of gravity's rainbow can significantly affect this non-singular compact object and modify the model near the phase transition region.

Dark energy stars in Tolman–Kuchowicz spacetime in the context of Einstein gravity

Dark energy is the component in the present Universe with the greatest abundance, and it is responsible for the accelerating expansion of the Universe. As a result, dark energy is likely to interact with any compact astrophysical object [Muhammad F.A.R. Sakti and Anto Sulaksono, {\it Phys. Rev. D} {\bf 103}, 084042 (2021)]. In present paper, we propose a model for a dark energy star made up of dark and ordinary matter in which the density of dark energy is proportional to the density of isotropic perfect fluid matter. In the context of general relativity, the model is derived in the curved Tolman-Kuchowicz spacetime geometry [Tolman, Phys Rev 55:364, (1939); Kuchowicz, Acta Phys Pol 33:541, (1968)]. Here, we look at how dark energy affects stellar mass, compactness, and equilibrium etc. The physical parameters of the model e.g., pressure, density, mass function, surface redshift etc. are investigated, and the stability of stellar configuration is studied in detail. The model has interesting properties because it meets all energy criteria and is free from central singularities. The maximum allowable mass has been obtained from our model with the help of $M-R$ diagram. We analyse many physical properties of the model and checked that it meets all regularity constraints, is stable, and therefore physically realistic.

Slowly rotating dark energy stars

We study isotropic and slowly-rotating stars made of dark energy adopting the extended Chaplygin equation-of-state. We compute the moment of inertia as a function of the mass of the stars, both for rotating and non-rotating objects. The solution for the non-diagonal metric component as a function of the radial coordinate for three different star masses is shown as well. We find that (i) the moment of inertia increases with the mass of the star, (ii) in the case of non-rotating objects the moment of inertia grows faster, and (iii) the curve corresponding to rotation lies below the one corresponding to non-rotating stars.

A Relativistic Compact Stellar Model of Anisotropic Quark Matter Mixed with Dark Energy

The possibility of strange stars mixed with dark energy to be one of the candidates for dark energy stars is the main issue of the present study. Our investigation shows that quark matter atcs as dark energy after a certain yet unknown critical condition inside the quark stars. Our proposed model reveals that strange stars mixed with dark energy feature a physically acceptable stable model and mimic characteristics of dark energy stars. The plausible connections are shown through the mass-radius relation as well as the entropy and temperature. We particularly note that a two-fluid distribution is a major reason for the anisotropic nature of the spherical stellar system.

Type-II Weyl Semimetal versus Gravastar

The boundary between the type I and type II Weyl semimetals serves as the event horizon for the "relativistic" fermions. The interior of the black hole is represented by the type II Weyl semimetal, where the Fermi surface is formed. The process of the filling of the Fermi surface by electrons results in the relaxation inside the horizon. This leads to the Hawking radiation and to the reconstruction of the interior vacuum state. After the Fermi surface is fully occupied, the interior region reaches the equilibrium state, for which the Hawking radiation is absent. If this scenario is applicable to the real black hole, then the final state of the black hole will be the dark energy star with the event horizon. Inside the event horizon one would have de Sitter space time, which is separated from the event horizon by the shell of the Planck length width. Both the de Sitter part and the shell are made of the vacuum fields without matter. This is distinct from the gravastar, in which the matter shell is outside the "horizon", and which we call the type I gravastar. But this is similar to the other type of the vacuum black hole, where the shell is inside the event horizon, and which we call the type II gravastar. We suggest to study the vacuum structure of the type II gravastar using the $q$-theory, where the vacuum variable is the 4-form field introduced for the phenomenological description of the quantum vacuum.

Dark energy stars with a phantom field

Dark energy is the constituent with an enormous abundance in the present Universe, responsible for the Universe's accelerated expansion. Therefore, it is plausible that dark energy may interact within any compact astrophysical object. The author in [S. S. Yazadjiev, Phys. Rev. D 83, 127501 (2011)] constructs an exact star solution consisting of ordinary matter and a phantom field from a constant density star (CDS) known as the Schwarzschild interior solution. The star denotes a dark energy star (DES). The author claims that the phantom field represents dark energy within the star. So far, the role of the phantom field as dark energy in a DES is not systematically studied yet. Related to this issue, we analyze the energy condition of a DES. We expect that DESs shall violate the strong energy condition (SEC) for a particular condition. We discover that a SEC is fully violated only when the compactness reaches the Buchdahl limit. Furthermore, we also investigate the causal conditions and stabilities due to the convective motion and gravitational cracking. We also find that those conditions are violated. These results indicate that DES is not physically stable. However, we may consider DES as an ultracompact object of which we can calculate the gravitational wave echo time and echo frequency and compare them to those of CDS. We find that the contribution of the phantom field delays the gravitational wave echoes. The effective potential of the perturbed DES is also studied. The potential also enjoys a potential well like CDS but with a deeper well. We also investigate the possibility that DES could form a gravastar when $C=1$. It is found that a gravastar produced from DES possesses no singularity with a dS-like phase as the interior. These results could open more opportunities for the observational study of dark energy in the near future, mostly from the compact astrophysical objects.

Modelling of dark energy stars with tolman IV gravitational potential

The present study explores the properties of dark energy star with high density under the dark energy equation of state (EOS) (pr=ωρ) with anisotropic fluid distribution, where ω describes the dark energy parameter. The relativistic dark-energy stellar structures are studied by adopting Tolman IV-type gravitational potential. In this relation we examine many physical properties through graphical plots and mathematical calculation, such as energy condition, compactness, mass-radius ratio, anisotropic nature and surface redshift. It is observed that the strong energy condition is violated whereas all other energy conditions are satisfied. We have also fitted radius corresponding to observed mass value for observed neutron stars, namely, PSRJ1416−2230,VelaX−1,4U1608−52,HerX−1 and PSRJ1903+327, which are compatible with the predicted values.

Analytical model of dark energy stars

In this paper, we study the structure and stability of compact astrophysical objects which are ruled by the dark energy equation of state (EoS). The existence of dark energy is important for explaining the current accelerated expansion of the universe. Exact solutions to Einstein field equations (EFE) have been found by considering particularized metric potential, Finch and Skea ansatz. 1 The obtained solutions are relevant to the explanation of compact fluid sphere. Further, we have observed at the junction interface that the interior solution is matched with the Schwarzschild’s exterior vacuum solution. Based on that, we have noticed the obtained solutions are well in agreement with the observed maximum mass bound of [Formula: see text], namely, PSR J1416-2230, Vela X-1, 4U 1608-52, Her X-1 and PSR J1903+327, whose predictable masses and radii are not compatible with the standard neutron star models. Also, the stability of the stellar configuration has been discussed briefly, by considering the energy conditions, surface redshift, compactness, mass-radius relation in terms of the state parameter [Formula: see text]. Finally, we demonstrate that the features so obtained are physically acceptable and consistent with the observed/reported data.[Formula: see text] Thus, the present dark energy equation of state appears talented regarding the presence of several exotic astrophysical matters.

Lorentz violation with an invariant minimum speed as foundation of the Gravitational Bose Einstein Condensate of a Dark Energy Star

We aim to search for the connection between the spacetime with an invariant minimum speed so-called Symmetrical Special Relativity (SSR) with Lorentz violation and the Gravitational Bose Einstein Condensate (GBEC) as the central core of a star of gravitational vacuum (gravastar), where one normally introduces a cosmological constant for representing an anti-gravity. This usual model of gravastar with an equation of state (EOS) for vacuum energy inside the core will be generalized for many modes of vacuum (dark energy star) in order to circumvent the embarrassment generated by the horizon singularity as the final stage of a gravitational collapse. In the place of the problem of a singularity of an event horizon, we introduce a phase transition between gravity and anti-gravity before reaching the Schwarzschild (divergent) radius $R_S$ for a given coexistence radius $R_{coexistence}$ slightly larger than $R_S$ and slightly smaller than the core radius $R_{core}$ of GBEC, where the metric of the repulsive sector (core of GBEC) would diverge for $r=R_{core}$, so that for such a given radius of phase coexistence $R_S<R_{coexistence} <R_{core}$, both divergences at $R_S$ of Schwarzschild metric and at $R_{core}$ of the repulsive core are eliminated, thus preventing the formation of the event horizon. So the causal structure of SSR helps us to elucidate such puzzle of singularity of event horizon by also providing a quantum interpretation for GBEC and thus by explaining the origin of a strong anisotropy due to the minimum speed that leads to the phase transition gravity/anti-gravity during the collapse of the star. Furthermore, due to the absence of an event horizon of black hole (BH) where any signal cannot propagate, the new collapsed structure presents a signal propagation in its region of coexistence of phases where the coexistence metric does not diverge.

Formation of dark energy stars

Dark energy stars are finite size astrophysical objects with an interior equation of state typical of dark energy. Examples are self-gravitating false vacuum bubbles, vacuum non-singular black holes, and gravastars. We present a time-dependent solution of Einstein's field equations that describes the collapse of a spherical system from an initial state of positive pressure to a final state with a dark energy core. Our solution has no singularities, no event horizons, and does not violate the weak or null energy conditions.

New Models of Dark Energy Stars with Charge Distributions

In this paper, we have obtained a relativistic and spherically symmetric stellar configuration that describes an anisotropic fluid with a charge distribution that represents a potential model for a dark energy star and we specify particular forms in the gravitational potential and the electric field intensity which allows solve the Einstein-Maxwell field equations. The reason for proposing this model originates from the evidence that recent observational findings suggest that the universe has an accelerated cosmic expansion and the model of dark energy star is one of the most reasonable explanations of this phenomena. The field equations are integrated analytical and new stellar configurations are obtained are analyzed. For each these solutions we found that the radial pressure, the anisotropy factor, energy density, metric coefficients, mass function, charge density are regular and well behaved in the stellar interior. With the new solutions can be developed models of dark energy stars physically acceptable where the causality condition is not satisfied or the strong energy condition is violated. This model has a great application in the study of the fundamental theories of physics and cosmology. Several independent observations indicate that the greater part of the total energy density of the universe is in the form of dark energy and the rest in the form of nonbaryonic cold dark matter particles, but which have never been detected.

Magnetized anisotropic stars

We extend a known solution generating technique for isotropic fluids in order to construct more general models of anisotropic stars with poloidal magnetic fields. In particular, we discuss the magnetized versions of some well-known exact solutions describing anisotropic stars and dark energy stars and we describe some of their properties.

Anisotropic Geodesic Fluid in Non-Comoving Spherical Coordinates

We start with a recently introduced spherically symmetric geodesic fluid model (arXiv: 1601.07030) whose energy-momentum tensor in the comoving frame is dust-like with nontrivial energy flux. In the non-comoving energy frame (vanishing energy flux) the same EMT contains besides dust only radial pressure. We present Einstein's equations together with the matter equations in static spherically symmetric coordinates. These equations are self-contained (four equations for four unknowns). We solve them analytically except for a resulting nonlinear ordinary differential equation (ODE) for the gravitational potential. This ODE can be rewritten as a Lienard differential equation which, however, may be transformed into a rational Abel differential equation of the first kind. Finally we list some open mathematical problems and outline possible physical applications (galactic halos, dark energy stars) and related open problems.

Non-static spherically symmetric exact solution of the Einstein-Maxwell field equations

We present a class of exact spherically symmetric and non-static solutions of Einstein-Maxwell’s field equations. We have assumed isotropic pressure distribution and have taken ansatz on two of the gravitational potentials. The solutions admit negative pressure. We show that the solutions satisfy physical boundary conditions associated with the Einstein-Maxwell exact solutions. Therefore, these solutions can model physical systems such as moving dark energy stars.

Solution of the Einstein–Maxwell equations with anisotropic negative pressure as a potential model of a dark energy star

We have obtained a new class of solutions for the Einstein–Maxwell field equations for static spherically symmetric space–times by considering the negative anisotropic pressures, which represents a potential model of a dark energy star. We take the equation of state pr = −ρ, where pr is the radial pressure and ρ is the density. We have also checked that for these solutions metric coefficients, mass density, radial pressure, transverse pressure, electric field, and current density are well defined for suitable values of the parameters involved in the solution. These exact solutions can be used to develop models of dark energy stellar interiors satisfying all physical constraints except for the causality condition, which cannot be satisfied for the equation of state considered here, and which is arguably not an applicable physical constraint for dark matter–energy.

The dark energy star and stability analysis

We propose a new model of dark energy star consisting of five zones, namely, the solid core of constant energy density, the thin shell between core and interior, an inhomogeneous interior region with anisotropic pressures, a thin shell, and the exterior vacuum region. We discuss various physical properties. The model satisfies all the physical requirements. The stability condition under a small linear perturbation is also discussed.