Einstein Equations Research Articles

Stability of charged anisotropic thin shell gravastars admitting conformal motion in [formula omitted] gravity

This paper investigates the interesting features of gravastar configuration under charged anisotropic fluid distribution considering geometry-matter coupling theory. In the context of f(R,T) gravity, we derive exact solutions of the Einstein field equations using the linear form of the function f(R,T)=R+2γT, where R is the Ricci scalar and T is the trace of the energy–momentum tensor with conformal motion. We consider three different regions of gravastar, the interior region having a definite radius r, the intermediate layer of matter with the thickness ϵ, and the outer region with the radius r+ϵ. For the viability of our developed solution, we examine the metric potentials, energy density, and pressure components inside the thin shell for different values of theory parameter γ, and charge Q. Using the well-known matching conditions, we interpret the dynamical properties of gravastar such as surface charge density, surface pressure, proper length, equation of state parameter, entropy, and evolution of shell energy under the allowed range of parameters. Moreover, we analyze the presence of realistic matter inside the thin shell through energy conditions. The dynamical stability of the gravastar is checked through the equilibrium profile and speed of sound parameter, which shows the stable configuration of the gravastar in our theoretical studies.

Realistic compact objects in the f(R, T) gravity in the background of polytropic and barotropic gas models

The f(R, T) gravity in the background of the polytropic and barotropic fluid has been investigated in this work. We have selected the TOV equation to determine the internal spacetime of a spherically symmetric galactic object. With the use of the Einstein equation, we have selected KB-spacetime to calculate the mass, compactness, and surface redshift of a spherically symmetric body. Explicit conditions for model parameters have been constructed for the boundary conditions of the interior and exterior spacetime, and the Schwarzschild solution has been employed in the modified f(R, T) gravity theory to evaluate different matching criteria. An increasing pattern in compactness with respect to the different radii is evident in the graphical representation of the compactness evolution for each of the individual star models. After selecting a non-vacuum field equation for higher order curvature, we reformulated f(R, T) for R and T. As a result, the tangential pressure, radial pressure, and matter density have all been calculated. According to the study, as the radius goes to infinity, the tangential and radial pressures display asymptotic flatness and converge to zero. Polytropic and barotropic gas EoS have been adopted since the star model confronts the presence of an isotropic fluid backdrop. It has been noted that in a polytropic background, density and pressure increase with distance from the star’s core, but in a barotropic background, the pressure exhibits an ascending pattern as a function of radius.

A comparative study on the maximum mass and radius of anisotropic compact stars from Heintzmann geometry and the TOV approach

In this paper, a class of anisotropic compact stars is analyzed in Heintzmann geometry. The Einstein field equations (EFEs) have been solved to obtain the stellar model in presence of pressure anisotropy. We have considered the [Formula: see text] metric component as proposed by Heintzmann, and by solving the EFEs, the [Formula: see text] metric component is evaluated in the presence of pressure anisotropy. It is noted that for an isotropic star ([Formula: see text]), the maximum mass lies within the range 1.87–3.04[Formula: see text] for radii ranging between 8–13[Formula: see text]km. For anisotropic compact stars, the maximum mass increases with [Formula: see text] and lies within the range 1.99–3.23 [Formula: see text] for anisotropy parameter [Formula: see text]. The physical viability of the model is examined by applying our model to study the properties of a few known compact objects. All the stability conditions are fulfilled in the proposed model. It is also interesting to note that the maximum mass calculated from our model from geometrical consideration and solving the TOV equation is approximately equal, and the radii predicted from the present model comply with the estimated radius from observations of recently observed pulsars and lighter compact objects of GW events such as GW 190814 and GW 170817.

Dynamical de Sitter black holes in a quasi-stationary expansion

We revisit and improve the analytic study (Gregory et al 2018 Class. Quantum. Grav. 35 155008) of spherically symmetric but dynamical black holes in Einstein’s gravity coupled to a real scalar field. We introduce a series expansion in a small parameter ε that implements slow time dependence. At the leading order (LO), the generic solution is a quasi-stationary Schwarzschild–de Sitter (SdS) metric, i.e. one where time-dependence enters only through the mass and cosmological constant parameters of SdS. The two coupled ODEs describing the LO time dependence are solved up to quadrature for an arbitrary scalar potential. Higher order corrections can be consistently computed, as we show by explicitly solving the Einstein equations at the next to LO as well. We comment on how the quasi-stationary expansion we introduce here is equivalent to the non-relativistic 1/c expansion.

Particle dynamics in spherically symmetric electro-vacuum instantons

In this paper, we study the geodesic motion in spherically symmetric electro-vacuum Euclidean solutions of the Einstein equation. There are two kinds of such solutions: the Euclidean Reissner–Nordström (ERN) metrics, and the Bertotti–Robinson-like (BR) metrics, the latter having constant Kretschmann scalar. First, we derive the motion equations for the ERN spacetime and we generalize the results of Battista–Esposito, showing that all orbits in as ERN spacetime are unbounded if and only if it has an event horizon. We also obtain the Weierstrass form of the polar radial motion, providing an efficient tool for numerical computations. We then study the angular deflection of orbits in the Euclidean Schwarzschild spacetime which, in contrast to the Lorentzian background, can be either positive or negative. We observe the presence of a null and a maximal deflection rings for particles with velocity at infinity v>1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$v>1$$\\end{document} and we give approximate values for their size when v≳1.\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$v > rsim 1.$$\\end{document} For BR spacetimes, we obtain analytic solutions for the radial motion in proper length, involving (hyperbolic) trigonometric functions and we deduce that orbits either exponentially go to the singularity or are periodic. Finally, we apply the previous results and use algorithms related to Weierstrass’ elliptic functions to produce a Python code to plot orbits of the spacetimes ERN and BR, and draw “shadows” of the first ones, as it was already done before for classical black holes.

A generalized static spherically symmetric anisotropic compact stellar model

We report here a new exact solution of the Einstein field equations that describes a static spherically symmetric, anisotropic compact stellar fluid sphere. A generalized metric form corresponding to the grr\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$g_{rr}$$\\end{document} component and a specific anisotropic profile generates the model. The new class of generalized solutions reduces to Finch-Skea, Vaidya Tikekar and Schwarzschild interior solutions for specific cho-ices of the model parameters. The model parameters have been determined from the continuity of the interior and exterior metrics at the surface of the star and setting the radial pressure at the boundary to zero. Recent observed measurements from some pulsars as sources have been utilized to validate our model.

Self-gravity and Bekenstein-Hawking entropy

We study the effect of self-gravity on entropy by directly solving the 4D semi-classical Einstein equation. In particular, we focus on whether the Bekenstein-Hawking formula holds when self-gravity is extremely strong. As an example, we consider a simple spherically symmetric static configuration consisting of many quanta and construct a self-consistent non-perturbative solution for ħ in which the entropy exactly follows the area law for many local degrees of freedom of any kind. This can be a candidate for black holes in quantum theory. It represents a compact dense configuration with near-Planckian curvatures, and the interior typically behaves like a local thermal state due to particle creation inside. Here, the information content is stored in the interior bulk, and the self-gravity plays an essential role in changing the entropy from the volume law to the area law. We finally discuss implications to black holes in quantum gravity and a speculative view of entropy as a gravitational charge.

Microscopic state of BHs and an exact one body method for binary dynamics in general relativity

In gravitational collapses, the horizon and singularity’s realisation in the finite future of the proper time used co-moving observer happens in the future of infinitely far away future of the normal time used outside probe. To the latter the horizon and singularity defined in the singularity theorem are physical realities only in the sense of uncertainty principle and ensemble interpretation. We provide two exact time dependent solution families to the Einstein equation and show that they form a pair of complementary description for the microscopic state of black holes by showing that the Bekenstein–Hawking entropy formula follows properly from their canonical wave function’s degeneracy. We also develop an eXact One Body method for general relativity two-body dynamics whose conservative part calls no post newtonian approximation as input and applies to the full three stages of black hole binary merger events. By this method, we analytically calculate the gravitational wave forms following from such merger processes. In the case black holes carry exact and apriori horizon and singularity our wave forms agree with those following from conventional effective one body method but exhibit more consistent late time behaviour. In the case black holes carry only asymptotic horizon and extended inner structure thus experiencing banana shape deformation as the merger occurs, our wave forms exhibit all features especially the late time quasi-normal mode type oscillation seen in real observations.

Creation Field Cosmological Model with Variable Cosmological Term (Λ) in Bianchi Type III Space-Time

The paper is devoted to the study of cosmological models with time-varying cosmological term (Λ) in the presence of creation field in the framework of Bianchi type III space-time. To obtain deterministic model of the universe, we have assumed Λ∝1/R3, where R is the scale factor, for steady state cosmology and creation field, and shear (σ) is proportion to expansion (θ) which leads to B=Cn, where B and C are the metric potentials to explain small anisotropic behaviour of the universe and its isotropy. To obtain the results in terms of cosmic time t, we have assumed n=2. The model satisfies conservation equations, and creation field increases with time. The present model is free from singularity, has particle horizon, and provides a natural explanation for inflationary scenario and isotropization. Creation field and Einstein field equations are derived using principle of least action and Lagrangian formulation of variable cosmological term. For illustrative purposes, evolutionary behaviour of some cosmological parameters are shown graphically. The other physical aspects like accelerating behaviour of the model are also discussed. Thus, the model represents not only expanding universe but also accelerating which matches with the results of present-day observations.

Analyzing warp drive spacetimes with Warp Factory

The field of warp research has been dominated by analytical methods to investigate potential solutions. However, these approaches often favor simple metric forms that facilitate analysis but ultimately limit the range of exploration of novel solutions. So far the proposed solutions have been unphysical, requiring energy condition violations and large energy requirements. To overcome the analytical limitations in warp research, we introduce Warp Factory: a numerical toolkit designed for modeling warp drive spacetimes. By leveraging numerical analysis, Warp Factory enables the examination of general warp drive geometries by evaluating the Einstein field equations and computing energy conditions. Furthermore, this comprehensive toolkit provides the determination of metric scalars and insightful visualizations in both 2D and 3D, offering a deeper understanding of metrics and their corresponding stress–energy tensors. The paper delves into the methodology employed by Warp Factory in evaluating the physicality of warp drive spacetimes and highlights its application in assessing commonly modeled warp drive metrics. By leveraging the capabilities of Warp Factory, we aim to further warp drive research and hopefully bring us closer to realizing physically achievable warp drives.

Graviton-photon oscillations in an expanding universe

Through the Gertsenshtein effect, the presence of a large external B-field may allow photons and gravitons to mix in a way that resembles neutrino oscillations and is even more similar to axion-photon mixing. Assuming a background B-field (or any field that behaves like one), we examine the Gertsenshtein mechanism in the context of FLRW expanding universe models, as well as de Sitter space. The conformal invariance of Maxwell’s equations and the conformal noninvariance of the Einstein equations preclude the operation of the Gertsenshtein effect at all scales. In general we find for the matter- and radiation-dominated cases, graviton-oscillations are possible only at late conformal times or when the wavelengths are much shorter than the horizon (κη≫1), but that the time-dependent oscillations eventually damp out in any case. The presence of charged particles additionally damps out the oscillations. For the de Sitter universe, we find that oscillations are possible only at early conformal times (η≪H−1) and for wavelengths short compared to the Hubble radius, but eventually freeze in when wavelengths become longer than the Hubble radius. In principle a Gertsenshtein-like mechanism might influence the balance of particle species in an inflationary phase before freezing in; however, we find that in all our models the mixing length is larger than the Hubble radius. We discuss several possible remedies to this situation.

Gaussian black holes in brane-world model

We present regular black hole solutions in the framework of Brane-world gravity sourced by a Gaussian matter distribution. The black hole metric shares all the common features of regular black holes in the modified General Relativity (GR) with some exciting features. Considering the energy momentum tensor for an isotropic fluid on the brane, the modified Einstein field equation results with an effective energy momentum tensor that describes an anisotropic fluid determined by brane world parameters. Although the effective radial pressure and energy density satisfy the vacuum energy condition, the effective transverse pressure behaves differently. Gaussian black hole (GBH) solutions are obtained from a Gaussian matter distribution. In the paper, a new class of GBH solutions are obtained in the brane-world gravity with effective normal matter in addition to exotic matter distribution. In the brane world gravity, the mass of a GBH depends on the brane tension. The mass of a GBH formed in the brane world is greater than that at low energy (i.e., GR). We study the trajectories of the massive and the massless particles that can be trapped around a GBH for a set of model parameters. The radii of the photon spheres around the GBH and the condition for the stability of the trajectories of the photon spheres are determined. The properties of the GBHs are studied in detail, including their possible observable features.

From Phonons to Dark Photons. The Role of Quantum Vacuum BEC in Dark Matter Research

Exploring the quantum vacuum as a dynamic Bose-Einstein Condensate (BEC), this study draws a parallel between phonons and photons to reveal how dark photons can be transformed into visible light. Through phonon-photon interactions, akin to sonoluminescence, we demonstrate the quantum vacuum's ability to illuminate dark matter phenomena. By adapting Einstein's Field Equations, we present a model showcasing spacetime's responsiveness to both visible and dark matter. The process of quantum vacuum cavitation underscores phonons' crucial role in bridging dark and visible realms, providing novel insights into the universe's dark sector.

Infinite Quantum Twisting at the Cauchy Horizon of Rotating Black Holes.

We present a numerical calculation of the expectation value of the quantum angular-momentum current flux density for a scalar field in the Unruh state near the inner horizon of a Kerr-de Sitter black hole. Our results indicate that this flux diverges as V_{-}^{-1} in a suitable Kruskal coordinate such that V_{-}=0 at the inner horizon. Depending on the parameter values of the scalar field and black hole that we consider, and depending on the polar angle (latitude), this flux can have different signs. In the near extremal cases considered, the angle average of the expectation value of the quantum angular momentum current flux is of the opposite sign as the angular momentum of the background itself, suggesting that, in the cases considered, quantum effects tend to decrease the total angular momentum of the spheres away from the extremal value. We also numerically calculate the energy flux component, which provides the leading order divergence of the quantum stress energy tensor, dominant over the classical stress energy tensor, at the inner horizon. Taking our expectation value of the quantum stress tensor as the source in the semiclassical Einstein equation, our analysis suggests that the spheres approaching the inner horizon can undergo an infinite twisting due to quantum effects along latitudes separating regions of infinite expansion and contraction.

Viscosity of Suspensions of Strongly Bonded Spherical Particles of Nickel-Manganese-Cobalt Mixed Oxides (NMC) in Molten Poly(Ethylene Carbonate) for Batteries.

Ionically conductive polymers highly filled with active materials, such as metal oxides are increasingly studied for their potential use in all solid-state batteries. They offer the desirable processing ease of polymers for mass production despite interfacial issues that remain to be solved. In this study, it is shown that spherical particles of transition metal oxides can be introduced in co-polymers of alkene carbonate and ethylene oxide at loading close to the maximum packing fraction, without imparting the processability in the melt of the material. In particular, the viscosity does not show any yield stress and the increase of viscosity shows that the intrinsic viscosity of the filler does not match with the usual 2.5 value in the limit of the Einstein's equation. Conversely, rheological data show that the value is rather close to unity consistently with theoretical arguments that predicted that this scaling factor should be unity when particle rotation is precluded. In the present case, this behavior is attributed to strong bonding between polymer and filler that is proved by electronic microscopy and by dynamical mechanical spectroscopy showing a relaxation due to bound polymer.

How Schwarzschild could have discovered and fixed the problem with his metric

Karl Schwarzschild's landmark paper revealed the first ever exact solution to Einstein's gravitational field equation. This was a major scientific achievement. However, his solution for the metric of a spherically symmetric spacetime is different than the one found in textbooks today—because Schwarzschild assumed, like in Newtonian gravity, that only one singularity could exist. Given the newness of Einstein's general theory of relativity and the existing paradigm of Newtonian gravity, his assumption was natural. While other authors have previously pointed out this shortcoming and corrected it, the contribution of this paper is to show how Schwarzschild might have discovered it himself. A simple geometric proof indicates how Schwarzschild, with a different assignment of a single constant of integration, would have arrived at the metric found in textbooks today.

The Synge G-Method: cosmology, wormholes, firewalls, geometry

Unphysical equations of state result from the unrestricted use of the Synge G-trick of running the Einstein field equations backwards; in particular often this results in ρ+p<0 which implies negative inertial mass density, which does not occur in reality. This is the basis of some unphysical spacetime models including phantom energy in cosmology and traversable wormholes. The slogan ‘ER = EPR’ appears to have no basis in physics and is merely the result of vague and unbridled speculation. Wormholes (the ‘ER’ of the slogan) are a mathematical curiosity of general relativity that have little to no application to a description of our Universe. In contrast quantum correlations (the ‘EPR’ of the slogan) are a fundamental property of quantum mechanics that follows from the principle of superposition and is true regardless of the properties of gravity. The speculative line of thought that led to ‘ER = EPR’ is part of a current vogue for anti-geometrical thinking that runs counter to (and threatens to erase) the great geometrical insights of the global structure program of general relativity.

Fuzzy dark matter and the dark dimension

We propose a new dark matter contender within the context of the so-called “dark dimension”, an innovative 5-dimensional construct that has a compact space with characteristic length-scale in the micron range. The new dark matter candidate is the radion, a bulk scalar field whose quintessence-like potential drives an inflationary phase described by a 5-dimensional de Sitter (or approximate) solution of Einstein equations. We show that the radion could be ultralight and thereby serve as a fuzzy dark matter candidate. We advocate a simple cosmological production mechanism bringing into play unstable Kaluza–Klein graviton towers which are fueled by the decay of the inflaton.

Physical physiognomies of hybrid Karmarkar stars

Our current study intends to investigate the peculiar features of a Hybrid compact star that is both static and anisotropic, wherein the core consists of strange quark matter, while regular baryonic matter is distributed in its crust. The relationship between the pressure of strange quark matter and density within the stellar interior is described using the simplest form of the phenomenological MIT Bag model equation of state: Pq=13(ρq−4Bg). On the other hand, the radial pressure and matter density resulting from baryonic matter are correlated through a straightforward linear equation of state: Pr=mρ. Apart from that, we also incorporate the Karmarkar condition, which establishes a connection between the two components of the metric potentials, specifically grr and gtt. For one metric potential, we presumptively choose a specific model i.e. eλ=1+a2r2(1+br2)4, and the Karmarkar condition leads to us the second potential. Moreover, the Einstein field equations are solved by using Karmarkar technique, and additional identification of the unknown constants is carried out using some physical constraints. We employ both graphical and analytical techniques to assess the physical plausibility of our suggested framework. More specifically, we focused on seven compact stars as candidates for strange quark stars in this investigation. For this purpose, the existing observational data of the compact objects were compared to theoretical mass–radius estimates. Furthermore, a series of physical tests has been conducted to substantiate the physical feasibility and stability of our proposed model. These experiments encompass both analytical and graphical assessments and involve examining various aspects, such as the dynamic balance of applied forces, compactness factor, energy conditions, surface redshift, and other relevant parameters. In conclusion, our comprehensive analysis reveals that the present system fulfills all the essential physical requirements necessary for a realistic representation of the hybrid compact star. Moreover, the model demonstrates suitability for conducting in-depth investigations pertaining to strange quark stars. The adherence to physical prerequisites and the potential for further exploration make our current model a promising candidate for studying and understanding the intriguing properties of such astrophysical objects.

ОТНОСИТЕЛЬНОЕ ЗАПАЗДЫВАНИЕ СИГНАЛОВ В ПРОСТРАНСТВЕ-ВРЕМЕНИ ВРАЩАЮЩИХСЯ УСКОРЯЮЩИХСЯ ЧЕРНЫХ ДЫР

The paper studies a potentially observable effect – relative time delay– on the background of the rotating accelerating black holes, called the C-metric. The C-metric proposed by Griffiths and Podolsky is an exact solution to Einstein's equations. The metric is characterized by mass, angular momentum and acceleration parameter, and is reduced to Kerr's solution when the acceleration parameter is zero. Relative time delay occurs if two light beams come from a variable source located behind a rotating gravitational lens. Passing on opposite sides of the lens, the rays arrive at the observer with a time difference. The time difference arises due to the frame dragging effect. Black holes described by the C-metric have the angular momentum which is necessary to occur the relative time delay. To calculate the relative delay of signals, the Dymnikova method was used. The xpression for the relative delay was expanded into a series to determine the influence of the acceleration parameter of the C-metric. It was found that the relative time delay in the leading order for the C-metric does not depend on the mass of black holes and the acceleration parameter, but depends only on the angular momentum. In the corrections, the relative time delay for the C-metric differs from the corrections to the delay in the Kerr solution by the influence of the free parameter of the C-metric. To estimate the influence of the acceleration parameter on the relative time delay, numerical delay values were obtained for a range of accelerations. Numerical values were calculated by substituting observed data from realistic pulsar–black hole configurations into the expression for the relative signal delay. The obtained numerical estimates show that the magnitude of the relative time delay in the first order is measurable with the capabilities of modern technology. Considering that all previously studied solutions of objects using relative delay had the influ- ence of mass, the numerical values of the delay in the C-metric are significantly different. Therefore, black holes described by the C-metric can be distinguished from previously studied black hole solutions using potential observations of relative time delay. However, the corrections containing the influence of the acceleration parameter are too small to take their influence into account.