Universal Horizon Research Articles

Spherically symmetric static black holes in Einstein-aether theory

In this paper, we systematically study spherically symmetric static spacetimes in the framework of Einstein-aether theory, and pay particular attention to the existence of black holes (BHs). In the present studies we first clarify several subtle issues. In particular, we find that, out of the five non-trivial field equations, only three are independent, so the problem is well-posed, as now generically there are only three unknown functions, {$F(r), B(r), A(r)$, where $F$ and $B$ are metric coefficients, and $A$ describes the aether field.} In addition, the two second-order differential equations for $A$ and $F$ are independent of $B$, and once they are found, $B$ is given simply by an algebraic expression of $F,\; A$ and their derivatives. To simplify the problem further, we explore the symmetry of field redefinitions, and work first with the redefined metric and aether field, and then obtain the physical ones by the inverse transformations. These clarifications significantly simplify the computational labor, which is important, as the problem is highly involved mathematically. In fact, it is exactly because of these, we find various numerical BH solutions with an accuracy that is at least two orders higher than previous ones. More important, these BH solutions are the only ones that satisfy the self-consistent conditions and meantime are consistent with all the observational constraints obtained so far. The locations of universal horizons are also identified, together with several other observationally interesting quantities, such as the innermost stable circular orbits (ISCO), the ISCO frequency, and the maximum redshift $z_{max}$ of a photon emitted by a source orbiting the ISCO. All of these quantities are found to be quite close to their relativistic limits.

Entropies and the first laws of black hole thermodynamics in Einstein-aether-Maxwell theory

Using the solution phase space method, we investigate the thermodynamics of black holes in Einstein-aether-Maxwell theory, for which the traditional Wald method (covariant phase space method) fails. We show the first laws of thermodynamics and definitive entropy expressions at both Killing and universal horizons for some examples of exact black hole solutions, including three-dimensional static charged quasi-BTZ black hole, two four-dimensional static charged black holes and three-dimensional rotating solution. At Killing horizons the entropies are exactly one quarter of the horizon area, but at universal horizons of three-dimensional black holes, the entropies have a corrected term in addition to the one proportional to the horizon area.

Matter accretion onto Einstein-aether black holes via well-known fluids * * Pakistan for its financial support under the grant No: 9290/Balochistan/NRPU/R&D/HEC/2017

We study matter accretion onto Einstein-aether black holes by adopting the Hamiltonian approach. The general solution of accretion is discussed using the isothermal equation of state. Different types of fluids are considered, including ultra-relativistic, ultra-stiff, sub-relativistic, and radiation fluids, and the accretion process onto Einstein-aether black holes is analyzed. The behavior of the fluid flow and the existence of critical points is investigated for Einstein-aether black holes. We further discuss the general expression and behavior of polytropic fluid onto Einstein-aether black holes. The most important feature of this work is the investigation of the mass accretion rate of the above-mentioned fluids and the comparison of our findings with the Schwarzschild black hole, which generates particular signatures. Moreover, the maximum mass accretion rate occurs near the Killing and universal horizons, and the minimum accretion rate lies between them.

АКСІОЛОГІЧНЕ ОСМИСЛЕННЯ ОСВІТНЬОГО ПРОЦЕСУ

The article deals with the research of axiological nature of the theory and practice of modern education. The problem is relevant in that education can be the key to solving the global problems of the modern world. The more popular the knowledge of people, their experience, abilities, skills, their professional and personal qualities in social development, the more important education is. Improvement and modernization of national education systems is becoming the leading direction of the sustainable development strategy of many states of the modern world. Consequently, the philosophical understanding of the educational process, in particular the definition of the values of education is relevant.The authors substantiate that, given the diversity of educational values, they can be divided into two main groups: the values of maintaining the existing order of things and the values of its transformation.In various worldview systems, the problem of the values of education is resolved in different ways. The article gives the basic ideas of representatives of such systems: classical realism, essentialism, perinalism, analytical philosophy, experimentalism, existentialism, neo-Freudianism.Analyzing the views of researchers, the authors of the article attribute free ideological self-determination to the cardinal values of education as the main mission, which provides the conditions for the free self-determination of each person in the worldview world to accept their own values in the form of life goals, main motives and interests, aspirations, needs, principles, etc.It is substantiated that the axiological guidelines of education should be associated primarily with the orientation of education towards the interests of the individual – a free, creative person, capable of self-realization. The mission of education is to find an individual approach to a person, to contribute to the realization of his personal potential, internal, unconscious, even irrational needs (otherwise this culturally unrealized potential can become destructive for both the individual and society). Thus, education should be aimed at the formation of fundamental foundations that will allow a person to solve worldview problems, make a moral, legal or ideological choice, navigate in the modern sociocultural situation.The authors emphasize that in a modern dynamic society, value-semantic variability and the influence of the accelerated development of education on the formation of social innovations are necessary. The dynamically changing conceptual field of modern educational practices, the desire and attempts to humanize education are favorable factors for the creation and implementation of the concept of modern education. The humanization of education is aimed at creating a sustainable and flexible value attitude to the world. The universal value horizon, the idea of universal values are not reflected in modern cultural practices. On the one hand, culture becomes value-oriented, its phenomena become subject to assessment, its values are structured and organized, on the other hand, obvious value pluralism, poly-paradigmalism, relativism and simplification of values as such.Thus, the analysis of theoretical studies on the value aspects of education, and the study of the possibilities that arise as a result of the humanization of education, allow us to state: only as a result of changes in the content of education in the context of its humanization it is possible to talk about the transition to understanding a specialist as a harmoniously developed personality, which is the ultimate purpose of education. The primary task is to eliminate the discrepancy between the generally accepted and declared values, on the one hand, and their implementation, primarily in the minds of participants in the educational process, on the other.

László Tengelyi’s Phenomenological Metaphysics of the World

In my paper, I reconstruct the basic features of the metaphysical conception of the world in László Tengelyi. I outline that he understands the world and its existence as a fact. In opposition to traditional ontology, Tengelyi believes that the fact of the world is of necessary. However, its necessity is not of logical a priori nature but conditioned by the concordance of experience. One ought to point out that the Tengelyi’s phenomenological metaphysics of the world is limited by the realm of experience. Following Husserl, he treats the world as the universal horizon of the experience. This is the horizon in which a thing can appear as limitless system of gradations. I analyze also the Tengelyi’s concept ‘project of world’ and explore two ‘projects’ of such a kind pointed in his metaphysics, namely (1) metontological transcendentalism and (2) naturalistic autarchism. I pay a special attention to the Tengelyi’s concept of the infinitude as a basic characteristic of the world. He considers it as the openness to the generation of the absolute new qualities of the things in the world.

FOR THE EDUCATION OF THE COSMOPOLITAN. THE CASE OF TAGORE AND THE GLOBALIZED SELF

The phenomenon of globalization creates a variety of tensions and dilemmas. The present paper attempts to approximate this phenomenon by tracing the different trends arising from the corresponding descriptive, evaluative and regulatory judgments. In addition, the various philosophers' views on the dialogue on cosmopolitanism and patriotism are outlined, and Tagore's reflections and positions on cosmopolitanism and ecumenical education are highlighted, with emphasis on the development of critical thinking and constructive dialogue. The debate on globalization raises concerns and dilemmas in educational theory as well, with the consequent urgency of the need for rapid adaptation to the challenging environment. The conclusion drawn from the presentation of the aforementioned positions is the need for the implementation of educational policies that will be based on education for empathy, solidarity and intercultural respect. Revisiting the commitment to culture within a universal horizon based on acceptance of diversity requires the existence of strategic planning that contains actions and choices that make it flexible and efficient.

Neutron star sensitivities in Hořava gravity after GW170817

Ho\v{r}ava gravity breaks boost invariance in the gravitational sector by introducing a preferred time foliation. The dynamics of this preferred slicing is governed, in the low-energy limit suitable for most astrophysical applications, by three dimensionless parameters $\alpha$, $\beta$ and $\lambda$. The first two of these parameters are tightly bound by solar system and gravitational wave propagation experiments, but $\lambda$ remains relatively unconstrained ($0\leq\lambda\lesssim 0.01-0.1$). We restrict here to the parameter space region defined by $\alpha=\beta=0$ (with $\lambda$ kept generic), which in a previous paper we showed to be the only one where black hole solutions are non-pathological at the universal horizon, and we focus on possible violations of the strong equivalence principle in systems involving neutron stars. We compute neutron star 'sensitivities', which parametrize violations of the strong equivalence principle at the leading post-Newtonian order, and find that they vanish identically, like in the black hole case, for $\alpha=\beta=0$ and generic $\lambda\neq0$. This implies that no violations of the strong equivalence principle (neither in the conservative sector nor in gravitational wave fluxes) can occur at the leading post-Newtonian order in binaries of compact objects, and that data from binary pulsars and gravitational interferometers are unlikely to further constrain $\lambda$.

A Great Lesson of Spirituality and Humanism. Pope John Paul II at the Headquarters of UNESCO in Paris (2 June 1980)

The speech given by St. Pope John Paul II at the UNESCO headquarters in Paris on June 2, 1980 has a special place among his numerous addresses concerning culture. Referring to the cultural changes which took place in the last three centuries the pope stood up for culture, emphasising that, in an individual dimension, culture represents the constitutive element of life of every man and, correspondingly, in a social dimension, it provides a support for self-determination and for the freedom of nations. Formulating such a message, the pope referred to Polish experiences, in order to highlight the significance and the primacy of the person and to point out the subjective meaning of a nation and the ethical dimension of culture. In the papal message one can also find a call for cultural opposition against contemporary subjectivism and other tendencies heading towards the disinheritance of man and to the shutting of the door of the universal horizon of hope and love, which is based on manifold human experiences from the past.

The Meaning of the Common World in Perioperative Nursing Care; A Hermeneutic Study

The aim of this study is to bring forth the meaning of the common world as it appears in perioperative nursing care. We employed the epistemological standpoints of preunderstanding, the hermeneutic spiral and fusion of horizons grounded in Gadamer’s hermeneutic philosophy as well as Eriksson’s Theory of Caritative Caring based on the ontology of caring science, where caritas is the basic motive and ethos of caring. Four hermeneutic spiral activities were performed, consisting of a mimetic presentation bearing the ontological depth of the common world, its distinctive features, the universal and lasting and finally, the truth inherent in the common world. The inherent truth of the common world is the prevalence of harmony, wholeness and the idea of love, mercy and reverence for human dignity. The common world brings ethics to existence, achieved by the word of honour, which in its true being makes visible the universal and ontological horizons of a common reality. The common world is the creation of a hermeneutic movement inside each suffering human being, where the boundless life-giving time represents the inhabited movement of time, like coming home.

Thermodynamical study on universal horizons in higher D -dimensional spacetime and aether waves

We investigate thermodynamic behaviors of the $D$-dimensional gravity coupled to a dynamical unit timelike vector, the aether, present two kinds of exact charged solutions and study the linearized wave spectrum of this theory. There is an universal horizon behind the Killing horizon in these aether black holes. It is interesting that there exist $D$-dimensional Schwarzschild, Reissner-Nordstr\"{o}m and charged Schwarzschild black holes but now with an universal horizon inside. We find that in the uncharged case for both kinds, or charged case but the charge $\bar Q\ll \bar r_0/2$ for the first kind, one can construct a Smarr formula and the (slightly modified) first law of black hole mechanics at the universal horizons for the aether black holes. An entropy can be associated with the universal horizon and a temperature can be defined there. For the second kind aether black hole and for the first kind one in the extremal higher dimensions, the work term involving charge $V_{UH}\delta Q$ disappeared. For aether wave, our results show that the spin-1 and spin-2 modes are the same as those in 4-dimensional spacetime, and only the spin-0 one is different and dependant on the dimension number $n$.

Bundestheologie in der Johannesoffenbarung – eine Spurensuche

Abstract The article is looking for traits of the concept of God’s covenant with Israel within the Book of Revelation. It persues the question how far elements of this theological concept were adopted and to which extent they have been altered by John. In analyzing the use of διαθήκη in Rev 11:16 as well as of the formula of the covenant in Rev 21:3 and 21:7 the idea of “covenant” presents itself to be just one among several other motives taken up by John from the Old Testament without stressing it. John is putting the idea of God’s covenant in a new and emphasised universal horizon extending it to all humankind.

Charged Einstein-æther black holes in n-dimensional spacetime

In this work, we investigate the [Formula: see text]-dimensional charged static black hole solutions in the Einstein-æther theory. By taking the metric parameter [Formula: see text] to be [Formula: see text], and [Formula: see text], we obtain the spherical, planar, and hyperbolic spacetimes, respectively. Three choices of the cosmological constant, [Formula: see text], [Formula: see text] and [Formula: see text], are investigated, which correspond to asymptotically de Sitter, flat and anti-de Sitter spacetimes. The obtained results show the existence of the universal horizon in higher dimensional cases which may trap any particle with arbitrarily large velocity. We analyze the horizon and the surface gravity of four- and five-dimensional black holes, and the relations between the above quantities and the electrical charge. It is shown that when the aether coefficient [Formula: see text] or the charge [Formula: see text] increases, the outer Killing horizon shrinks and approaches the universal horizon. Furthermore, the surface gravity decreases and approaches zero in the limit [Formula: see text] or [Formula: see text], where [Formula: see text] is the extreme charge. The main features of the horizon and surface gravity are found to be similar to those in [Formula: see text] case, but subtle differences are also observed.

Constraints on Hořava gravity from binary black hole observations

Ho\v{r}ava gravity breaks Lorentz symmetry by introducing a preferred spacetime foliation, which is defined by a timelike dynamical scalar field, the khronon. The presence of this preferred foliation makes black hole solutions more complicated than in General Relativity, with the appearance of multiple distinct event horizons: a matter horizon for light/matter fields, a spin-0 horizon for the scalar excitations of the khronon, a spin-2 horizon for tensorial gravitational waves, and even, at least in spherical symmetry, a universal horizon for instantaneously propagating modes appearing in the ultraviolet. We study how black hole solutions in Ho\v{r}ava gravity change when the black hole is allowed to move with low velocity relative to the preferred foliation. These slowly moving solutions are a crucial ingredient to compute black hole "sensitivities" and predict gravitational wave emission (and in particular dipolar radiation) from the inspiral of binary black hole systems. We find that for generic values of the theory's three dimensionless coupling constants, slowly moving black holes present curvature singularities at the universal horizon. Singularities at the spin-0 horizon also arise unless one waives the requirement of asymptotic flatness at spatial infinity. Nevertheless, we have verified that at least in a one-dimensional subset of the (three-dimensional) parameter space of the theory's coupling constants, slowly moving black holes are regular everywhere, even though they coincide with the general relativistic ones (thus implying in particular the absence of dipolar gravitational radiation). Remarkably, this subset of the parameter space essentially coincides with the one selected by the recent constraints from GW170817 and by solar system tests.

Gravitational collapse and formation of universal horizons in Einstein-æther theory

We numerically study the gravitational collapse of a massless scalar field with spherical symmetry in Einstein-aether theory, and show that apparent, spin-0 and dynamical universal horizons (dUHs) can be all formed. The spacetime and the aether field are well-behaved and regular, including regions nearby these horizons (but away from the center of spherical symmetry). The spacetime outside the apparent and spin-0 horizons settles down to a static configuration, and some of such resulting static black holes were already found numerically in the literature. On the other hand, the proper distance of the outermost dUH from the apparent (or spin-0) horizon keeps increasing on $\mbox{\ae}$ther-orthogonal time slices. This indicates that the outermost dUH is evolving into the causal boundary, even for excitations with large speeds of propagation.

Smarr integral formula of D-dimensional stationary spacetimes in Einstein-æther–Maxwell theory

Using the Wald formalism, we investigate the thermodynamics of charged black holes in D-dimensional stationary spacetimes with or without rotations in Einstein-æther–Maxwell theory. In particular, assuming the existence of a scaling symmetry of the action, we obtain the Smarr integral formula, which can be applied to both Killing and universal horizons. When restricted to 4-dimensional spherically symmetric spacetimes, previous results obtained by a different method are re-derived.

Revisiting the cuscuton as a Lorentz-violating gravity theory

The cuscuton is a scalar field with infinite speed of propagation. It was introduced in the context of cosmology but it has also been claimed to resemble Horava gravity in a certain limit. Here we revisit the cuscuton theory as a Lorentz-violating gravity theory. We clarify its relation with Horava gravity and Einstein-aether theory, analyze its causal structure, and consider its initial value formulation. Finally, we discuss to which extent the cuscuton theory can be used as a proxy for Horava gravity in the context of gravitational collapse and formation of universal horizons.

In Exile From Exile: The Ethics of Uprootedness and Marginal Cosmopolitanism in the Poetry of Tomás Segovia

The rhetoric of nomadism used by Tomas Segovia (Spain, 1927-Mexico, 2011) renders his exilic poetry suggestively paradoxical: the poetic “I” aims to become exiled from exile. Segovia saw himself as marginal and without a homeland long before the exile brought about by the Spanish Civil War. He arrived in Mexico in 1940, and from 1976 until his death, lived between Spain and Mexico. He is considered a prominent representative of the Spanish Republican exile on both sides of the Atlantic, a characterization that the writer rejected. In light of the sociohistorical circumstances that defined the Spanish Republican exile in Mexico, this essay interrogates the ethico-political sense in which the condition of exile can be read in Segovia’s poetry. Thus it critiques the sometimes reductive contextual reading favored within political institutions. Drawing on Kwame Anthony Appiah’s definition of cosmopolitan ethics as an ethics that addresses the challenge of harmonizing universality and particularity, I read Segovia’s poetry as pointing toward a universal horizon that derives from a difference that Segovia could not entirely shed.

First law of black holes with a universal horizon

In Lorentz violating theories of gravitation with a preferred foliation a notion of black hole is still possible, despite the presence of infinitely fast propagating modes. Such event horizons are known as universal horizons. Their discovery poses the question of whether they satisfy mechanical laws, analogous to the ones of Killing horizons in Lorentz symmetric theories, and whether they admit a thermodynamical interpretation. In this paper we study the viability of the first law for several exact universal horizon solutions previously derived in the literature. Our results show that a simple mechanical and thermodynamical interpretation is problematic in these cases, and call for a more systematic study of rotating universal horizons.

Black hole horizons at the extremal limit in Lorentz-violating gravity

Lorentz-violating gravity theories with a preferred foliation can have instantaneous propagation. Nonetheless, it has been shown that black holes can still exist in such theories and the relevant notion of an event horizon has been dubbed `universal horizon'. In stationary spacetimes the universal horizon has to reside in a region of spacetime where the Killing vector associated with stationarity is spacelike. This raises the question of what happens to the universal horizon in the extremal limit, where no such region exists anymore. We use a decoupling limit approximation to study this problem. Our results suggest that at the extremal limit, the extremal Killing horizon appears to play the role of a degenerate universal horizon, despite being a null and not a spacelike surface, and hence not a leaf of the preferred foliation.

Universal horizons and Hawking radiation in nonprojectable 2d Hořava gravity coupled to a nonrelativistic scalar field

In this paper, we study the non-projectable 2d Ho\v{r}ava gravity coupled with a non-relativistic scalar field, where the coupling is in general non-minimal and of the form $f(\phi)R$, where $f(\phi)$ is an arbitrary function of the scalar field $\phi$, and $R$ denotes the 2d Ricci scalar. In particular, we first investigate the Hamiltonian structure, and show that there are two-first and two-second class constraints, similar to the pure gravity case, but now the local degree of freedom is one, due to the presence of the scalar field. Then, we present various exact stationary solutions of this coupled system, and find that some of them represent black holes but now with universal horizons as their boundaries. At these horizons, the Hawking radiations are thermal with temperatures proportional to their surface gravities, which normally depend on the non-linear dispersion relations of the particles radiated, similar to the (3+1)-dimensional case.