Self-gravitating Objects Research Articles

Smallest remnants of early matter domination

The evolution of the universe prior to Big Bang Nucleosynthesis could have gone through a phase of early matter domination which enhanced the growth of small-scale dark matter structure. If this period was long enough, self-gravitating objects formed prior to reheating. We study the evolution of these dense early halos through reheating. At the end of early matter domination, the early halos undergo rapid expansion and eventually eject their matter. We find that this process washes out structure on scales much larger than naively expected from the size of the original halos. We compute the density profiles of the early halo remnants and use them to construct late-time power spectra that include these non-linear effects.We evolve the resulting power spectrum to estimate the properties of microhalos that would form after matter-radiation equality.Surprisingly, cosmologies with a short period of early matter domination lead to an earlier onset of microhalo formation compared to those with a long period. In either case, dark matter structure formation begins much earlier than in the standard cosmology, with most dark matter bound in microhalos in the late universe.

Study of gravastars admitting conformal motion in [formula omitted] gravity

In this paper, we explore analytic solutions of self-gravitating celestial objects known as gravastars in energy–momentum squared gravity. A gravastar is referred as an alternative model to a black hole which is exhibited by three different regions, i.e., the intermediate shell, interior, and exterior regions. We examine these regions for a particular model of this gravity with conformal motion. The field equations are developed with perfect fluid for a spherically symmetric spacetime. The mathematical formulation for these domains is expressed by using a barotropic equation of state. We use Israel matching conditions for the smooth joining of outer and inner spacetimes. We analyze various physical features of gravastars along with the thickness of the shell, i.e., equation of state parameter, proper length, entropy and energy. These characteristics represent physically consistent non-singular solutions.

Complexity analysis of dynamical cylinder in massive Brans–Dicke gravity

In this paper, a complexity factor is devised for a non-static cylindrical system in the framework of massive Brans–Dicke theory. The definition of complexity is developed by taking into account the essential physical characteristics (such as anisotropy and inhomogeneity.) of the system. In order to determine the complexity factor of the self-gravitating object, we acquire structure scalars from the orthogonal splitting of the Riemann tensor. Moreover, we discuss two patterns of evolution and choose the homologous mode as the simplest pattern under the influence of massive scalar field. We derive solutions in the absence as well as the presence of heat dissipation for a specific form of the scalar field. The factors that induce complexity in an initially complexity-free system are also examined. It is concluded that the massive scalar field as well as heat dissipation contribute to the complexity of the celestial system. Thus, a dynamical cylinder is more complex as compared to its static counterpart.

Analysis of strange quark stars in massive Brans–Dicke gravity

In this paper, we explore the behavior and anisotropic structure of quark stellar models in the framework of massive Brans–Dicke gravity. The system of field equations, representing a static sphere, is formulated by incorporating the MIT bag model. We use the Karmarkar condition for embedding class-one to formulate a relativistic model corresponding to a well-behaved radial metric function. The values of unknown parameters are determined through the matching of internal and external space–times at the hypersurface. The observed masses and radii of the strange star candidates (RXJ 1856-37, Her X-1 and PSR J1614-2230) specify the solution. Further, we evaluate the impact of the massive scalar field on state parameters and investigate the viability as well as stability of the self-gravitating objects. It is found that the obtained values of the bag constant (corresponding to each star) lie within the accepted range. Moreover, the anisotropic structure meets the necessary viability and stability criteria.

Dynamics of spherical collapse in energy–momentum squared gravity

This paper investigates the dynamics of spherical collapse in the framework of energy–momentum squared gravity. This theory overcomes the big-bang singularity and provides viable cosmological consequences in the early time universe. We proceed our work by considering the nonstatic spherically symmetric space–time in the interior and static spherically symmetric metric in the exterior regions of the star. The Darmois junction conditions between interior and exterior geometries are derived. We construct dynamical equations through the Misner–Sharp technique to analyze the impact of matter variables and dark source terms on the collapsing phenomenon. A correlation among dark source terms, Weyl scalar and matter variables is also established. Due to the presence of multivariate function and its derivatives, space–time is no longer considered to be conformally flat. To obtain conformally flat space–time, we have considered a particular model of this gravity which yields conformally flat space–time and homogeneity of the energy density through the entire system. We conclude that positive dark source terms as well as negative pressure gradient provide the anti-gravitational behavior leading to the stability of self-gravitating objects and hence prevent the collapsing process.

Structure scalars and their evolution for massive objects in f(R) gravity

In this manuscript, the Riemann tensor is split orthogonally to get five scalar functions known as structure scalars which have significance to gain insight into the composition and structure of spherically symmetric self-gravitating objects. Certain stellar equations are then evaluated to gather information about physical characteristics of such astrophysical objects. These stellar equations are further written in terms of acquired structure scalars so that the basic properties such as pressure anisotropy and energy density inhomogeneity of the fluid under consideration can be explored. Also, we have explored few static spherically symmetric solutions to show significance of structure scalars in the background of f(R) gravity.

Thermodynamic Constraints on the Non-Baryonic Dark Matter Gas Composing Galactic Halos

To explain rotation curves of spiral galaxies through Newtonian orbital models, massive halos of non-baryonic dark matter (NBDM) are commonly invoked. The postulated properties are that NBDM interacts gravitationally with baryonic matter, yet negligibly interacts with photons. Since halos are large, low-density gaseous bodies, their postulated attributes can be tested against classical thermodynamics and the kinetic theory of gas. Macroscopic models are appropriate because these make few assumptions. NBDM–NBDM collisions must be elastic to avoid the generation of light, but this does not permit halo gas temperature to evolve. If no such collisions exist, then the impossible limit of absolute zero would be attainable since the other available energy source, radiation, does not provide energy to NBDM. The alternative possibility, an undefined temperature, is also inconsistent with basic thermodynamic principles. However, a definable temperature could be attained via collisions with baryons in the intergalactic medium since these deliver kinetic energy to NBDM. In this case, light would be produced since some proportion of baryon collisions are inelastic, thereby rendering the halo detectable. Collisions with baryons are unavoidable, even if NBDM particles are essentially point masses. Note that <0.0001 × the size of a proton is needed to avoid scattering with γ-rays, the shortest wavelength used to study halos. If only elastic collisions exist, NBDM gas would collapse to a tiny, dense volume (zero volume for point masses) during a disturbance—e.g., cosmic rays. NBDM gas should occupy central galactic regions, not halos, since self-gravitating objects are density stratified. In summary, properties of NBDM halos as postulated would result in violations of thermodynamic laws and in a universe unlike that observed.

Hydrodynamic modeling of self-gravitating astrophysical objects on tetrahedral meshes

The paper proposes a new numerical method for solving the equations of gravitational hydrodynamics on a tetrahedral mesh. The proposed numerical method is focused on modeling the evolution of astrophysical objects of spherical shape, which is appropriate for gravitational collapse and star formation, and also for supernova explosion. The construction of tetrahedral grids is carried out in three stages. At the first stage, a geodesic grid methodology is used to construct a triangular grid on the surface of the sphere, which encompasses the computational domain. At the second stage, the resulting triangular mesh is serialized from the surface of the sphere to its center, and at the third stage, the obtained prisms are divided into tetrahedra. This approach allows us to simulate spherical objects without singularities that occur when using spherical or cylindrical coordinates. The paper describes numerical methods for solving the equations of hydrodynamics and the Poisson equation. Numerical examples are given that verify the developed numerical methods.

Further evidence for the non-existence of a unified hoop conjecture

The hoop conjecture, introduced by Thorne almost five decades ago, asserts that black holes are characterized by the mass-to-circumference relation 4pi {mathcal {M}}/{mathcal {C}}ge 1, whereas horizonless compact objects are characterized by the opposite inequality 4pi {mathcal {M}}/{mathcal {C}}<1 (here {mathcal {C}} is the circumference of the smallest ring that can engulf the self-gravitating compact object in all azimuthal directions). It has recently been proved that a necessary condition for the validity of this conjecture in horizonless spacetimes of spatially regular charged compact objects is that the mass {mathcal {M}} be interpreted as the mass contained within the engulfing sphere (and not as the asymptotically measured total ADM mass). In the present paper we raise the following physically intriguing question: is it possible to formulate a unified version of the hoop conjecture which is valid for both black holes and horizonless compact objects? In order to address this important question, we analyze the behavior of the mass-to-circumference ratio of Kerr–Newman black holes. We explicitly prove that if the mass {mathcal {M}} in the hoop relation is interpreted as the quasilocal Einstein–Landau–Lifshitz–Papapetrou and Weinberg mass contained within the black-hole horizon, then these charged and spinning black holes are characterized by the sub-critical mass-to-circumference ratio 4pi {mathcal {M}}/{mathcal {C}}<1. Our results provide evidence for the non-existence of a unified version of the hoop conjecture which is valid for both black-hole spacetimes and spatially regular horizonless compact objects.

Axisymmetric deformations of neutron stars and gravitational-wave astronomy

Einstein's theory of general relativity predicts that the only stationary configuration of an isolated black hole is the Kerr spacetime, which has a unique multipolar structure and a spherical shape when non-spinning. This is in striking contrast to the case of other self-gravitating objects, which instead can in principle have arbitrary deformations even in the static case. Here we develop a general perturbative framework to construct stationary stars with small axisymmetric deformations, and study explicitly compact stars with an intrinsic quadrupole moment. The latter can be sustained, for instance, by crust stresses or strong magnetic fields. While our framework is general, we focus on quadrupolar deformations of neutron stars induced by an anisotropic crust, which continuously connect to spherical neutron stars in the isotropic limit. Deformed neutron stars might provide a more accurate description for stellar remnants formed in supernovae and in binary mergers, and can be used to improve constraints on the neutron-star equation of state through gravitational-wave detections and through the observation of low-mass X-ray binaries. We argue that, if the (dimensionless) intrinsic quadrupole moment is of a few percent or higher, the effect of the deformation is stronger than that of tidal interactions in coalescing neutron-star binaries, and might also significantly affect the electromagnetic signal from accreting neutron stars.

Complexity analysis of cylindrically symmetric self-gravitating dynamical system in f(R,T) theory of gravity

In this article, we have studied a cylindrically symmetric self-gravitating dynamical object via complexity factor which is obtained through orthogonal splitting of Reimann tensor in $f(R,T)$ theory of gravity. Our study is based on the definition of complexity for dynamical sources, proposed by Herrera \cite{12b}. We actually want to analyze the behavior of complexity factor for cylindrically symmetric dynamical source in modified theory. For this, we define the scalar functions through orthogonal splitting of Reimann tensor in $f(R,T)$ gravity and work out structure scalars for cylindrical geometry. We evaluated the complexity of the structure and also analyzed the complexity of the evolutionary patterns of the system under consideration. In order to present simplest mode of evolution, we explored homologous condition and homogeneous expansion condition in $f(R,T)$ gravity and discussed dynamics and kinematics in the background of a generic viable non-minimally coupled $f(R,T)=\alpha_1 R^m T^n +\alpha_2 T(1+\alpha_3 T^p R^q)$ model. In order to make a comprehensive analysis, we considered three different cases (representing both minimal and non-minimal coupling) of the model under consideration and found that complexity of a system is increased in the presence of higher order curvature terms, even in the simplest modes of evolution. However, higher order trace terms affects the complexity of the system but they are not crucial for simplest modes of evolution in the case of minimal coupling. The stability of vanishing of complexity factor has also been discussed.

Gravastars in f(R,T,RμνTμν) gravity

This work is devoted to study the analytical and regular solutions of a particular self-gravitating object (i.e., gravastar) in a particular theory of gravity. We derive the corresponding field equations in the presence of effective energy momentum tensor associated with the perfect fluid configuration of a spherical system. We then describe the mathematical formulations of the three respective regions i.e., inner, shell and exterior of a gravastar separately. Additionally, the significance and physical characteristics along with the graphical representation of gravastars are discussed in detail. It is seen that under some specific constraints, $f(R,T,R_{\mu\nu}T^{\mu\nu})$ gravity is likely to host gravastars.

Adler–Finch–Skea Anisotropic Solution in f($$\mathcal{G}$$) Gravity

In this paper, we develop anisotropic solution for spherically symmetric self-gravitating stellar object by employing Karmarkar embedding constraint in the framework of f($$\mathcal{G}$$) gravity. For this purpose, we use the mass and radius of three compact star models, i.e., Her X-1, SAX J 1808.4-3658, and 4U 1820-30 as well as smooth matching of spherical interior and Schwarzschild exterior spacetime geometries. The physical consistency and stability of the derived model is examined for these star models. We conclude that the resulting anisotropic model is physically viable as well as stable as it satisfies all the necessary requirements of the compact stellar objects.

Study of static charged spherical structure in f(R, T, Q) gravity

We generalize the definition of complexity for self-gravitating object proposed by Herrera (Phys Rev D 97(4):044010, 2018), for the case of charged spherical matter distribution in f(R, T, Q) theory of gravity, where R and T represent the Ricci scalar and the trace of the energy–momentum tensor, respectively, and $$Q\equiv R_{\mu \nu }T^{\mu \nu }$$. We split the Riemann tensor orthogonally to determine the modified structure scalars, and one of these scalars has been found to be involved in the emergence of complexity of the system. We consider the vanishing of the complexity factor to study some mathematical models of field equations under the influence of dark source terms of f(R, T, Q) gravity. By applying the restriction, $$f(R,T,Q)=R$$, one can get all these results in general relativity.

Gravitational antiscreening in stellar interiors

A new class of relativistic stellar structure equations which include the effects of an energy-dependent Newton coupling is presented. Significant modifications in the mass-radius relation for neutron stars are possible only if the running of the Newton coupling due to quantum gravity occurs at low energies. A new Buchdahl limit is derived and its physical implications are discussed. In particular, sub-Planckian self-gravitating objects with arbitrarily small radii are possible.

Complexity analysis of dynamical spherically-symmetric dissipative self-gravitating objects in modified gravity

In this paper, we have worked on the concept of complexity factor for nonstatic spherically-symmetric self-gravitating source filled with anisotropic fluid distribution in [Formula: see text] gravity theory. The definition of complexity for dynamical sources, proposed by Herrera, is examined in the framework of [Formula: see text] gravity. We intended to analyze the behavior of complexity factor in modified theory. For this, we defined the scalar functions through orthogonal splitting of Reimann tensor in [Formula: see text] gravity and worked out the structure scalars. We considered the structure scalar [Formula: see text] as a complexity factor to evaluate the complexity of the structure of dynamical system and also to analyze the complexity of the evolutionary patterns of the system under consideration. We took into account the homologous condition and homogeneous expansion condition in order to present the simplest mode of evolution, and found that homologous evolution is the simplest one. We considered both dissipative and nondissipative cases and found that shearing behavior of the fluid is not the same in both cases, however it remained geodesic in both cases. In the end, we established the results for the vanishing of the complexity factor. It has been found that zero complexity condition is satisfied if the energy density inhomogeneity and pressure anisotropy of the fluid configuration cancel each other.

Love in extrema ratio

The tidal deformability of a self-gravitating object leaves an imprint on the gravitational-wave signal of an inspiral which is paramount to measure the internal structure of the binary components. We unveil here a surprisingly unnoticed effect: in the extreme mass-ratio limit the tidal Love number of the central object (i.e. the quadrupole moment induced by the tidal field of its companion) affects the gravitational waveform at the leading order in the mass ratio. This effect acts as a magnifying glass for the tidal deformability of supermassive objects but was so far neglected, probably because the tidal Love numbers of a black hole (the most natural candidate for a compact supermassive object) are identically zero. We argue that extreme mass-ratio inspirals detectable by the future laser interferometric space antenna (LISA) mission might place constraints on the tidal Love numbers of the central object which are roughly eight orders of magnitude more stringent than current ones on neutron stars, potentially probing all models of black hole mimickers proposed so far.

Consistency of the Infrared Variability of SGR A* over 22 yr

Abstract We report new infrared (IR) measurements of the supermassive black hole at the Galactic Center, Sgr A*, over a decade that was previously inaccessible at these wavelengths. This enables a variability study that addresses variability timescales that are 10 times longer than earlier published studies. Sgr A* was initially detected in the near-infrared (NIR) with adaptive optics observations in 2002. While earlier data exists in form of speckle imaging (1995–2005), Sgr A* was not detected in the initial analysis. Here, we improved our speckle holography analysis techniques. This has improved the sensitivity of the resulting speckle images by up to a factor of three. Sgr A* is now detectable in the majority of epochs covering 7 yr. The brightness of Sgr A* in the speckle data has an average observed K magnitude of 16.0, which corresponds to a dereddened flux density of 3.4 mJy. Furthermore, the flat power spectral density of Sgr A* between ∼80 days and 7 yr shows its uncorrelation in time beyond the proposed single power-law break of ∼245 minutes. We report that the brightness and its variability is consistent over 22 yr. This analysis is based on simulations using the Witzel et al. model to characterize IR variability from 2006 to 2016. Finally, we note that the 2001 periapse of the extended, dusty object G1 had no apparent effect on the NIR emission from accretion flow onto Sgr A*. The result is consistent with G1 being a self-gravitating object rather than a disrupting gas cloud.

Scalar field dark matter with a cosh potential, revisited

Dark matter models in which the constituent particle is an ultra-light boson have become part of the mainstream discussion in cosmology and astrophysics. At the classical level, the models are represented by the dynamics of a (real or complex) scalar field endowed with a potential that contains its self-interactions, and for this reason are generically known as scalar field dark matter models. Here, we revisit the properties of such a model with a cosh potential and compare it with other known examples in the literature. Within the cosmological context, the self-interaction in the potential induces a radiation-like behavior at early times of the scalar field density, which is followed by a proper matter-like behavior at the onset of rapid field oscillations around the minimum of the potential. The solutions are found by numerical means, and from them we obtain information about the cosmological observables up to the level of linear density perturbations. We also study the general properties of self-gravitating objects in the non-relativistic limit and determine the role in them of the self-interaction obtained from the cosh potential. An overall conclusion is that, for the range of values in its parameters allowed by different constraints, a cosh potential behaves almost indistinguishable from the simpler quadratic one, which also means that the two models suffer the same tight constraints from cosmological and astrophysical observations.

Testing the gravitational weak equivalence principle in the standard model extension with binary pulsars

The Standard-Model Extension provides a framework to systematically investigate possible violation of the Lorentz symmetry. Concerning gravity, the linearized version was extensively examined. We here cast the first set of experimental bounds on the nonlinear terms in the field equation from the anisotropic cubic curvature couplings. These terms introduce body-dependent accelerations for self-gravitating objects, thus violating the gravitational weak equivalence principle (GWEP). Novel phenomena, that are absent in the linearized gravity, remain experimentally unexplored. We constrain them with precise binary-orbit measurements from pulsar timing, wherein the high density and large compactness of neutron stars are crucial for the test. It is the first study that seeks GWEP-violating signals in a fully anisotropic framework with Lorentz violation.