arXiv subjects

A. Di Prisco

Publications and source records attributed to A. Di Prisco.

At least 19 recordsLinked to original sources

arXiv2026

The imprints of the instantaneous appearance of a conformal Killing vector field on the evolution of self-gravitating fluid spheres

We study the influence of the instantaneous appearance of a conformal Killing vector (CKV) in self-gravitating fluid spheres during their evolution. For doing that we introduce a tensor variable whose time dependence allows the existence of a CKV for a given value of the time-like coordinate. We consider adiabatic and dissipative fluids. The analysis of different relevant physical variables in this process provides a smoking gun signature from the emergence of CKV at some point of the evolution. Prospective applications of these results, as well as open questions and pending issues related to this problem, are discussed.

gr-qc↗

arXiv2025

Complexity hierarchies in Euclidean stars

We establish a hierarchy of Euclidean stars according to their degree of complexity, as measured by the complexity factor and the complexity of the pattern of evolution. We consider both, nondissipative and dissipative systems. Solutions are ranged from the simplest one, in order of increasing complexity. Some specific models are found and analyzed in detail.

gr-qc↗

arXiv2025

Axially symmetric ghost stars

We present static axially symmetric fluid distributions not producing gravitational field outside their boundaries (i.e. fluid sources which match smoothly on the boundary surface to Minkowski space-time). These solutions provide further examples of ghost stars. A specific model is fully described, and its physical and geometrical properties are analyzed in detail. This includes the multipole moment structure of the source and its complexity factors, both of which vanish for our solution.

gr-qc↗

arXiv2025

The birth of a ghost star

We present a model of an evolving spherically symmetric dissipative self-gravitating fluid distribution which tends asymptotically to a ghost star, meaning that the end state of such a system corresponds to a static fluid distribution with vanishing total mass, and energy-density distribution which is negative in some regions of the fluid. The model is inspired in a solution representing a fluid evolving quasi-homologously and with vanishing complexity factor. However in order to satisfy the asymptotic behavior mentioned above, the starting solution has to be modified, as a consequence of which the resulting model only satisfies the two previously mentioned conditions, asymptotically. Additionally a condition on the variation of the infinitesimal proper radial distance between two neighboring points per unit of proper time is imposed, which implies the presence of a cavity surrounding the center. Putting together all these conditions we are able to obtain an analytical model depicting the emergence of a ghost star. Some potential observational consequences of this phenomenon are briefly discussed at the last section.

gr-qc↗

arXiv2024

Evolution of self-gravitating fluid spheres involving ghost stars

Exact solutions are presented which describe, either the evolution of fluid distributions corresponding to a ghost star (vanishing total mass), or describing the evolution of fluid distributions which attain the ghost star status at some point of their lives. The first two solutions correspond to the former case, they admit a conformal Killing vector (CKV) and describe the adiabatic evolution of a ghost star. Other two solutions corresponding to the latter case are found, which describe evolving fluid spheres absorbing energy from the outside, leading to a vanishing total mass at some point of their evolution. In this case the fluid is assumed to be expansion-free. In all four solutions the condition of vanishing complexity factor was imposed. The physical implications of the results, are discussed

gr-qc↗

arXiv2024

Ghost stars in general relativity

We explore an idea put forward many years ago by Zeldovich and Novikov concerning the existence of compact objects endowed with arbitrarily small mass. The energy-density of such objects, which we call ``Ghost stars'', is negative in some regions of the fluid distribution, producing a vanishing total mass. Thus, the interior is matched on the boundary surface to Minkowski space-time. Some exact analytical solutions are exhibited and their properties are analyzed. Observational data that could confirm or dismiss the existence of this kind of stellar object is commented.

gr-qc↗

arXiv2024

Cracking and complexity of self-gravitating dissipative compact objects

The concept of cracking refers to the tendency of a fluid distribution to "split'', once it abandons the equilibrium. In this manuscript we develop a general formalism to describe the occurrence of cracking within a dissipative fluid distribution, in comoving coordinates. The role of dissipative processes in the occurrence of cracking is brought out. Next, we relate the occurrence of cracking with the concept of complexity for self-gravitating objects defined in [1-3]. More specifically we relate the occurrence of cracking with the condition of the vanishing of the scalar function intended to measure the complexity of the fluid distribution (the complexity factor). We also relate the occurrence of cracking with the specific mode of leaving the equilibrium. Thus, we prove that leaving the equilibrium in either, the homologous (H), or the quasi--homologous regime (QH), prevents the occurrence of cracking. Also, it is shown that imposing the condition of vanishing complexity factor alone, (independently of the mode of leaving the equilibrium) prevents the occurrence of cracking in the non-dissipative geodesic case, and in the non-dissipative isotropic case. These results bring out further the relevance of the complexity factor and its related definition of complexity, in the study of self-gravitating systems.

gr-qc↗

arXiv2024

Irreversibility and gravitational radiation: A proof of Bondi's conjecture

It is shown that the evolution of an axially and reflection symmetric fluid distribution, satisfying the Tolman condition for thermal equilibrium, is not accompanied by the emission of gravitational radiation. This result, which was conjectured by Bondi many years ago, expresses the irreversibility associated to the emission of gravitational waves. The observational consequences emerging from this result are commented. The resulting models are not only non--dissipative and vorticity free, but also shear--free and geodesic, furthermore all their complexity factors vanish.

gr-qc↗

arXiv2023

Quasi--hyperbolically symmetric $γ$-metric

We carry out a systematic study on the motion of test particles in the region inner to the naked singularity of a quasi--hyperbolically symmetric $γ$-metric. The geodesic equations are written and analyzed in detail. The obtained results are contrasted with the corresponding results obtained for the axially symmetric $γ$-metric, and the hyperbolically symmetric black hole. As in this latter case, it is found that test particles experience a repulsive force within the horizon (naked singularity), which prevents them to reach the center. However in the present case this behavior is affected by the parameter $γ$ which measures the departure from the hyperbolical symmetry. These results are obtained for radially moving particles as well as for particles moving in the $θ-r$ subspace. Possible relevance of these results in the explanation of extragalactic jets, is brought out.

gr-qc↗

arXiv2023

Expansion-free dissipative fluid spheres: Analytical models

We search exact analytical solutions of spherically symmetric dissipative fluid distributions satisfying the vanishing expansion condition (vanishing expansion scalar $Θ$). To do so we shall impose additional restrictions allowing the integration of the field equations. A detailed analysis of the obtained solutions, their prospective applications to astrophysical scenarios, as well as alternative approaches to obtain new solutions, are discussed.

gr-qc↗

arXiv2022

Non-static fluid spheres admitting a conformal Killing vector: Exact solutions

We carry on a general study on non--static spherically symmetric fluids admitting a conformal Killing vector (CKV). Several families of exact analytical solutions are found for different choices of the CKV, in both, the dissipative and the adiabatic regime. To specify the solutions, besides the fulfillment of the junction conditions on the boundary of the fluid distribution, different conditions are imposed, such as vanishing complexity factor and quasi--homologous evolution. A detailed analysis of the obtained solutions, its prospective applications to astrophysical scenarios, as well as alternative approaches to obtain new solutions, are discussed.

gr-qc↗

arXiv2021

Hyperbolically symmetric versions of Lemaitre-Tolman-Bondi spacetimes

We study fluid distributions endowed with hyperbolical symmetry, which share many common features with Lemaitre-Tolman-Bondi (LTB) solutions (e.g. they are geodesic, shearing, non--conformally flat and the energy density is inhomogeneous). As such they may be considered as hyperbolically symmetric versions of LTB, with spherical symmetry replaced by hyperbolical symmetry. We start by considering pure dust models, and afterwards we extend our analysis to dissipative models with anisotropic pressure. In the former case the complexity factor is necessarily non-vanishing, whereas in the latter cases models with vanishing complexity factor are found. The remarkable fact is that all solutions satisfying the vanishing complexity condition are necessarily non-dissipative and satisfy the stiff equation of state.

gr-qc↗

arXiv2021

Dynamics of hyperbolically symmetric fluids

We study the general properties of dissipative fluid distributions endowed with hyperbolical symmetry. Their physical properties are analyzed in detail. It is shown that the energy density is necessarily negative and the fluid distribution cannot fill the region close to the center of symmetry. Such a region may be represented by a vacuum cavity around the center. By assuming a causal transport equation some interesting thermodynamical properties of these fluids are found. Several exact analytical solutions which evolve in the quasi-homologous regime and satisfy the vanishing complexity factor condition, are exhibited

gr-qc↗

arXiv2021

Hyperbolically symmetric static fluids: A general study

We carry on a comprehensive study on static fluid distributions endowed with hyperbolical symmetry. Their physical properties are analyzed in detail. The energy density appears to be necessarily negative, which suggests that any possible application of this kind of fluids requires extreme physical conditions where quantum effects are expected to play an important role. Also, it is found that the fluid distribution cannot fill the region close to the center of symmetry. Such a region may be represented by a vacuum cavity around the center. A suitable definition of mass function, as well as the Tolman mass are explicitly calculated. While the former is positive defined, the latter is negative in most cases, revealing the repulsive nature of gravitational interaction. A general approach to obtain exact solutions is presented and some exact analytical solutions are exhibited.

gr-qc↗

arXiv2020

Quasi-homologous evolution of self-gravitating systems with vanishing complexity factor

We investigate the evolution of self-gravitating either dissipative or non--dissipative systems satisfying the condition of minimal complexity, and whose areal radius velocity is proportional to the areal radius (quasi-homologous condition). Several exact analytical models are found under the above mentioned conditions. Some of the presented models describe the evolution of spherically symmetric dissipative fluid distributions whose center is surrounded by a cavity. Some of them satisfy the Darmois conditions whereas others present shells and must satisfy the Israel condition on either one or both boundary surfaces. Prospective applications of some of these models to astrophysical scenarios are discussed.

gr-qc↗

arXiv2020

Geodesics of the hyperbolically symmetric black hole

We carry out a systematic study on the motion of test particles in the region inner to the horizon of a hyperbolically symmetric black hole. The geodesic equations are written and analyzed in detail. The obtained results are contrasted with the corresponding results obtained for the spherically symmetric case. It is found that test particles experience a repulsive force within the horizon, which prevents them to reach the center. These results are obtained for radially moving particles as well as for particles moving in the $θ-R$ subspace. To complement our study we calculate the precession of a gyroscope moving along a circular path (non--geodesic) within the horizon. We obtain that the precession of the gyroscope is retrograde in the rotating frame, unlike the precession close to the horizon ($R=2m+ε$) in the Schwarzschild spacetime, which is forward.

gr-qc↗

arXiv2019

Complexity of the Bondi metric

A recently introduced concept of complexity for relativistic fluids is extended to the vacuum solutions represented by the Bondi metric. A complexity hierarchy is established, ranging from the Minkowski spacetime (the simplest one) to gravitationally radiating systems (the more complex). Particularly interesting is the possibility to differentiate between natural non--radiative (NNRS) and non--natural non--radiative (NNNRS) systems, the latter appearing to be simpler than the former. The relationship between vorticity and the degree of complexity is stressed.

gr-qc↗

arXiv2019

Complexity factors for axially symmetric static sources

A previously found definition of complexity for spherically symmetric fluid distributions [1], is extended to axially symmetric static sources. In this case there are three different complexity factors, defined in terms of three structure scalars obtained from the orthogonal splitting of the Riemann tensor. All these three factors vanish, for what we consider the simplest fluid distribution, i.e a fluid spheroid with isotropic pressure and homogeneous energy density. However, as in the spherically symmetric case, they can also vanish for a variety of configurations, provided the energy density inhomogeneity terms cancel the pressure anisotropic ones in the expressions for the complexity factors. Some exact analytical solutions of this type are found and analyzed. At the light of the obtained results, some conclusions about the correlation (the lack of it) between symmetry and complexity, are put forward.

gr-qc↗