Anisotropic stars as ultracompact objects in General Relativity

Anisotropic stresses are ubiquitous in nature, but their modeling in General Relativity is poorly understood and frame dependent. We introduce the first study on the dynamical properties of anisotropic self-gravitating fluids in a covariant framework. Our description is particularly useful in the context of tests of the black hole paradigm, wherein ultracompact objects are used as black hole mimickers but otherwise lack a proper theoretical framework. We show that: (i) anisotropic stars can be as compact and as massive as black holes, even for very small anisotropy parameters; (ii) the nonlinear dynamics of the 1+1 system is in good agreement with linearized calculations, and shows that configurations below the maximum mass are nonlinearly stable; (iii) strongly anisotropic stars have vanishing tidal Love numbers in the black-hole limit; (iv) their formation will usually be accompanied by gravitational-wave echoes at late times.Comment: 7+2 pages, 6 figures; v2: include extra material (general covariant framework for anisotropic fluids in General Relativity without symmetries and code validation); to appear in PR

Testing strong-field gravity with tidal Love numbers

The tidal Love numbers (TLNs) encode the deformability of a self-gravitating object immersed in a tidal environment and depend significantly both on the object's internal structure and on the dynamics of the gravitational field. An intriguing result in classical general relativity is the vanishing of the TLNs of black holes. We extend this result in three ways, aiming at testing the nature of compact objects: (i) we compute the TLNs of exotic compact objects, including different families of boson stars, gravastars, wormholes, and other toy models for quantum corrections at the horizon scale. In the black-hole limit, we find a universal logarithmic dependence of the TLNs on the location of the surface; (ii) we compute the TLNs of black holes beyond vacuum general relativity, including Einstein-Maxwell, Brans-Dicke and Chern-Simons gravity; (iii) We assess the ability of present and future gravitational-wave detectors to measure the TLNs of these objects, including the first analysis of TLNs with LISA. Both LIGO, ET and LISA can impose interesting constraints on boson stars, while LISA is able to probe even extremely compact objects. We argue that the TLNs provide a smoking gun of new physics at the horizon scale, and that future gravitational-wave measurements of the TLNs in a binary inspiral provide a novel way to test black holes and general relativity in the strong-field regime.Comment: 18 pages + appendices; 9 figures. v2: references updated and legend of Fig.7 corrected; v3: clarifications and improvements in the discussion; v4: minor changes to match the PRD version (selected as Editors' Suggestion

Francisco Suárez. Último medieval, primeiro moderno: a ideia exemplar

Francisco Suárez (1548-1617) representa, en el contexto de la Segunda Escolástica, una búsqueda incesante de la modernidad para tratar de diseñar una nueva filosofía que responda a las exigencias de la ciencia moderna y también a las necesidades de un nuevo mundo político y epistemológico. Esta determinación, así como el interés por el estudio de Aristóteles y de Santo Tomás Aquino, se convertirá en una referencia a los autores de su tiempo e influirán en la formación de filósofos modernos. Francisco Suárez dedica en “Disputationes metaphysicae”, su obra más emblemática, un capítulo entero a la "causa ejemplar" estudiada de una manera lineal, más en relación con el mundo natural. El núcleo de su pensamiento se encuentra en la idea de causalidad o libre albedrío, que es el principio de la inteligibilidad, pues comprender la causa significa comprender la estructura interna de cualquier sustancia. La causa ejemplar no se considera como un tipo de causa fundamental, pero sí como el aspecto principal de una cadena de causas. La noción de un orden causal en el mundo se remonta a Dios como primera causa. Dios, eterna e inmutable sustancia, identificado como la suprema bondad y en la inteligibilidad, que mueve todo lo demás, que por lo tanto constituye la causa primera, el principio activo del mundo.Francisco Suárez (1548-1617) represents, in the context of the Second Scholastic, an incessant search of modernity to try and redesign a new philosophy that would fulfill the demands of modern science and also the request of a new political and epistemological world. This determination, as well as the interest in studying Aristotle and St. Tomas Aquinas, would become a reference to the authors of their time and would education the formation of modern philosophers. Francisco Suárez dedicates a whole chapter to the ‘exemplary cause’ of this “Disputationes Metaphysicae”, his most emblematic work, where he studies in a non linear way, but in one associated to the natural world. The core of his way of thinking lies on the idea of causality or free will, which is the principle of intelligibility, since understanding a cause means comprehending the internal structure of any substance. The exemplary cause is not considered as a sort of fundamental cause, but as the primary aspect of a chain of causes. The notion of a causal order in the world goes back to God as first cause. God, eternal and immutable substance, identified as the supreme goodness and intelligibility, moves everything else, therefore constituting the primary cause, the world’s active principle.Francisco Suárez (1548-1617) representa, no contexto da Segunda Escolástica, a procura incessante da modernidade ao tentar desenhar uma nova filosofia que respondesse às exigências da ciência moderna e às solicitações de um novo mundo político e epistemológico. Esta determinação e o interesse em estudar Aristóteles e S. Tomás de Aquino contribuíram para que fosse uma referência para os autores do seu tempo e influenciasse a formação dos filósofos da modernidade. Francisco Suárez dedica em “Disputationes Metaphysicae, a sua obra mais emblemática”, um capítulo intenso à «causa exemplar», estudando-a não de uma forma linear, mas associada ao mundo natural. A matriz do seu pensamento reside na ideia de causalidade ou acção livre, que é o princípio de inteligibilidade, pois compreender a causa significa compreender a organização interna de uma substância qualquer. A causa exemplar não é considerada como um tipo de causal fundamental, mas como o primeiro aspecto da cadeia causal. A noção de uma ordem causal do mundo é remetida a Deus como primeira causa. Deus, substância eterna e imóvel, identificada com o inteligível e bem supremo, move como causa final todas as outras coisas constituindo, por conseguinte, a causa primeira, o princípio activo do mundo

Ultralight boson cloud depletion in binary systems

Ultralight scalars can extract rotational energy from astrophysical black holes through superradiant instabilities, forming macroscopic boson clouds. This process is most efficient when the Compton wavelength of the boson is comparable to the size of the black hole horizon, i.e. when the "gravitational fine structure constant" $\alpha\equiv G \mu M/\hbar c\sim 1$. If the black hole/cloud system is in a binary, tidal perturbations from the companion can produce resonant transitions between the energy levels of the cloud, depleting it by an amount that depends on the nature of the transition and on the parameters of the binary. Previous cloud depletion estimates considered binaries in circular orbit and made the approximation $\alpha\ll 1$. Here we use black hole perturbation theory to compute instability rates and decay widths for generic values of $\alpha$, and we show that this leads to much larger cloud depletion estimates when $\alpha \gtrsim 0.1$. We also study eccentric binary orbits. We show that in this case resonances can occur at all harmonics of the orbital frequency, significantly extending the range of frequencies where cloud depletion may be observable with gravitational wave interferometers.Comment: 12 pages, 6 figures. v2: references added, matches published versio

Simulation of a milk run material transportation system in the semiconductors industry

This paper deals with a project that consisted in the implementation of a Milk Run Lot Transportation System in Qimonda Porto Test Area, done by a multidisciplinary team formed by Qimonda Porto's workers, and the development of the corresponding simulation model. The first part of the study concerns an industrial engineering assessment of the test area, which identified sources of waste and improvement possibilities, and the implementation process of a Milk Run system in this area. Secondly, the results of the system implementation are discussed, and the construction of a simulation model in Arena® is presented. The purpose of the simulation exercise is to test different system configurations that may allow the improvement of the real-world system. Finally, some information about the simulation results and further steps to be taken regarding the improvement of the system is presented. The target of the project, framed in a Lean approach, was to reduce waste, namely transportation waste, thus optimizing the utilization of the test area human resources.info:eu-repo/semantics/publishedVersio

Scale invariant elastic stars in General Relativity

We present a model of relativistic elastic stars featuring scale invariance. This implies a linear mass-radius relation and the absence of a maximum mass. The most compact spherically symmetric configuration that is radially stable and satisfies all energy and causality conditions has a slightly smaller radius than the Schwarzschild light ring radius. To the best of our knowledge, this is the first material compact object with such remarkable properties in General Relativity, which makes it a unique candidate for a black-hole mimicker.Comment: 5 pages, 2 figure

Compactness bounds in general relativity

A foundational theorem due to Buchdahl states that, within General Relativity (GR), the maximum compactness $\mathcal{C}\equiv GM/(Rc^2)$ of a static, spherically symmetric, perfect fluid object of mass $M$ and radius $R$ is $\mathcal{C}=4/9$. As a corollary, there exists a compactness gap between perfect fluid stars and black holes (where $\mathcal{C}=1/2$). Here we generalize Buchdahl's result by introducing the most general equation of state for elastic matter with constant longitudinal wave speeds and apply it to compute the maximum compactness of regular, self-gravitating objects in GR. We show that: (i) the maximum compactness grows monotonically with the longitudinal wave speed; (ii) elastic matter can exceed Buchdahl's bound and reach the black hole compactness $\mathcal{C}=1/2$ continuously; (iii) however, imposing subluminal wave propagation lowers the maximum compactness bound to $\mathcal{C}\approx0.462$, which we conjecture to be the maximum compactness of \emph{any} static elastic object satisfying causality; (iv) imposing also radial stability further decreases the maximum compactness to $\mathcal{C}\approx 0.389$. Therefore, although anisotropies are often invoked as a mechanism for supporting horizonless ultracompact objects, we argue that the black hole compactness cannot be reached with physically reasonable matter within GR and that true black hole mimickers require either exotic matter or beyond-GR effects.publishe

Spherically symmetric elastic bodies in general relativity

The purpose of this review it to present a renewed perspective of the problem of self-gravitating elastic bodies under spherical symmetry. It is also a companion to the papers [Phys. Rev. D105, 044025 (2022)], [Phys. Rev. D106, L041502 (2022)], and [arXiv:2306.16584 [gr-qc]], where we introduced a new definition of spherically symmetric elastic bodies in general relativity, and applied it to investigate the existence and physical viability, including radial stability, of static self-gravitating elastic balls. We focus on elastic materials that generalize fluids with polytropic, linear, and affine equations of state, and discuss the symmetries of the energy density function, including homogeneity and the resulting scale invariance of the TOV equations. By introducing invariant characterizations of physical admissible initial data, we numerically construct mass-radius-compactness diagrams, and conjecture about the maximum compactness of stable physically admissible elastic balls.Comment: 130 pages, 18 figures; Invited review for the Special Issue of Class. Quantum Gravity on 'Focus on the Mathematics of Gravitation in the Non-Vacuum Regime

Compact elastic objects in general relativity

We introduce a rigorous and general framework to study systematically self-gravitating elastic materials within general relativity, and apply it to investigate the existence and viability, including radial stability, of spherically symmetric elastic stars. We present the mass-radius ($M-R$) diagram for various families of models, showing that elasticity contributes to increase the maximum mass and the compactness up to $\approx 22\%$, thus supporting compact stars with mass well above two solar masses. Some of these elastic stars can reach compactness as high as $GM/(c^2R)\approx 0.35$ while remaining stable under radial perturbations and satisfying all energy conditions and subluminal wave propagation, thus being physically realizable models of stars with a light ring. We provide numerical evidence that radial instability occurs for central densities larger than that corresponding to the maximum mass, as in the perfect-fluid case. Elasticity may be a key ingredient to build consistent models of exotic ultracompact objects and black-hole mimickers, and can also be relevant for a more accurate modelling of the interior of neutron stars.publishe