arXiv subjects

Ramon Torres

Publications and source records attributed to Ramon Torres.

11 recordsLinked to original sources

arXiv2026

An analogue first law for general closed marginally trapped surfaces

We formulate an analogue transverse first law for general closed marginally trapped surfaces in arbitrary spacetimes. The construction is intrinsically quasi-local and is attached directly to an individual marginally trapped surface, rather than to a preferred horizon worldtube. Taking the Hawking energy as the internal energy and an invariant effective surface gravity associated with the marginally trapped surface as the quantity controlling the thermal term, we derive a balance law in which the variation of energy splits into a generalized heat contribution and a total work contribution. In this way, the resulting law provides a codimension-two, transverse counterpart to existing horizon-based formulations of black-hole thermodynamics. We show that the formalism reproduces the expected results for round spheres in spherically symmetric spacetimes. We then examine semiclassical equilibrium and evaporating regimes, and extend the analysis to non-spherically symmetric marginally trapped surfaces in Kerr. These examples indicate that the framework remains applicable in situations where a horizon-based treatment is either nonunique or technically cumbersome, and suggest that closed marginally trapped surfaces provide a natural arena for a genuinely quasi-local thermodynamics of black holes.

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arXiv2026

Nonsingular Rotating Black Holes in the Dark-Energy Dominated Universe

Motivated by quantum-gravity scenarios that replace the classical black hole singularity with a regular core, and by the possibility that the dark-energy sector may be scale dependent, we construct a broad class of nonsingular rotating black-hole spacetimes embedded in an improved de Sitter--like background with either constant or running $\Lambda$. Because the Newman--Janis algorithm is generically incompatible with a cosmological-constant fluid, we instead propose a generalized Kerr--Schild construction on a (possibly scale-dependent $\Lambda$) de Sitter seed, yielding a Carter-type metric characterized by a mass function and a $\Lambda$ function. Our construction provides a direct map from static, spherically symmetric regular models to their rotating counterparts. We derive sharp regularity conditions at the ring and we identify a minimal-order subclass. We analyze chronology and show that, for non-negative mass function and $\Lambda$ above a certain negative limit, the spacetimes are stably causal. For minimal-order geometries with non-negative mass, we prove that the weak energy condition must be violated. Finally, we illustrate the framework with an asymptotic-safety--inspired model and discuss horizon structure, surface gravities, and conformal diagrams. These results provide a controlled, observationally oriented arena to confront regular rotating black holes in dark-energy backgrounds with the rapidly improving gravitational-wave and horizon-scale imaging data.

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arXiv2024

Observational and theoretical aspects of Superspinars

This article delves into the observational signatures and theoretical underpinnings of rotating astrophysical objects, with a particular focus on superspinars -exotic objects characterized by the absence of event horizons due to their high angular momentum. While solutions within General Relativity (Kerr superspinars) predict such objects, their classical forms harbor naked singularities, violate causality, and exhibit problematic repulsive gravitational effects. These characteristics render classical superspinars theoretically objectionable, leading to the consideration of them as physically implausible. On the other hand, the incompatibility between General Relativity and Quantum Mechanics suggests the exploration of alternative models, particularly those in which Quantum Gravity dominates the core and prevents the formation of scalar curvature singularities. This work demonstrates that superspinars without scalar curvature singularities can avoid all the complications associated with Kerr superspinars. Moreover, from a phenomenological standpoint, it is shown that the silhouettes of these superspinars could be markedly distinct from those of black holes and classical Kerr superspinars. To substantiate these differences, we perform a comprehensive analysis of inner null geodesics and investigate the structure of the Planckian region within superspinars without scalar curvature singularities. Our study reveals that only these superspinars provide the potential for distant observers to directly observe the extremely high curvature regions within their interiors.

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arXiv2023

The Interiors of Singularity-Free Rotating Black Holes

General Relativity provides us with some solutions for rotating black holes. However, there are some problems associated with them: the appearance of singularities, the possibility of violations of the cosmic censorship conjecture, the existence of regions where the mass acts repulsively and the violation of causality. Many authors consider that these problems reveal the existence of certain limits in the applicability of General Relativity. For instance, it is believed that the same existence of singularities in the classical black hole solutions is a weakness of the theory and that a full Quantum Gravity Theory would provide us with singularity-free black hole models. In this paper, the generic properties of the interiors of singularity-free rotating black holes are analyzed. Remarkably, it is shown that they are devoid of any of the aforementioned problems of the classical solutions.

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arXiv2017

Non-Singular Black Holes, the Cosmological Constant and Asymptotic Safety

Quantum gravitational effects in black hole spacetimes with a cosmological constant $\Lambda$ are considered. The effective quantum spacetimes for the black holes are constructed by taking into account the renormalization group improvement of classical solutions obtained in the framework of Unimodular Gravity (a theory which is identical to General Relativity at a classical level). This allows us to avoid the usual divergences associated with the presence of a running $\Lambda$. The horizons and causal structure of the improved black holes are discussed taking into account the current observational bounds for the cosmological constant. It is shown that the resulting effective quantum black hole spacetimes are always devoid of singularities.

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arXiv2016

On regular rotating black holes

Different proposals for regular rotating black hole spacetimes have appeared recently in the literature. However, a rigorous analysis and proof of the regularity of this kind of spacetimes is still lacking. In this note we analyze rotating Kerr-like black hole spacetimes and find the necessary and sufficient conditions for the regularity of all their second order scalar invariants polynomial in the Riemann tensor. We also show that the regularity is linked to a violation of the weak energy conditions around the core of the rotating black hole.

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arXiv2015

On the quantum corrected gravitational collapse

Based on a previously found general class of quantum improved exact solutions composed of non-interacting (dust) particles, we model the gravitational collapse of stars. As the modeled star collapses a closed apparent 3-horizon is generated due to the consideration of quantum effects. The effect of the subsequent emission of Hawking radiation related to this horizon is taken into consideration. Our computations lead us to argue that a total evaporation could be reached. The inferred global picture of the spacetime corresponding to gravitational collapse is devoid of both event horizons and shell-focusing singularities. As a consequence, there is no information paradox and no need of firewalls.

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arXiv2013

On the interior of (Quantum) Black Holes

Different approaches to quantum gravity conclude that black holes may possess an inner horizon, in addition to the (quantum corrected) outer `Schwarzschild' horizon. In this paper we assume the existence of this inner horizon and explain the physical process that might lead to the tunneling of particles through it. It is shown that the tunneling would produce a flux of particles with a spectrum that deviates from the pure thermal one. Under the appropriate approximation the extremely high temperature of this horizon is calculated for an improved quantum black hole. It is argued that the flux of particles tunneled through the horizons affects the dynamics of the black hole interior leading to an endogenous instability.

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arXiv2012

On the causal characterization of singularities in spherically symmetric spacetimes

The causal character of the zero-areal-radius (R=0) singularity in spherically symmetric spacetimes is studied. By using the techniques of the qualitative behaviour of dynamic systems, we are able to present the most comprehensive scheme so far to try to find out their causal characterization, taking into account and analyzing, the possible limitations of the approach. We show that, with this approach, the knowledge of the scalar invariant $m\equiv R(1-g^{μν}\partial_μR\partial_νR)/2$ suffices to characterize the singularity. We apply our results to the study of the outcome of Black Hole evaporation and show different possibilities. In this way, we find that a persistent naked singularity could develop in the final stages of the evaporation and we show its distinctive features. Likewise, we study the options for the generation of naked singularities in the collapse of an object (such as a star) as a means of violating the cosmic censorship conjecture.

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arXiv2012

On the invariant causal characterization of singularities in spherically symmetric spacetimes

The causal character of singularities is often studied in relation to the existence of naked singularities and the subsequent possible violation of the cosmic censorship conjecture. Generally one constructs a model in the framework of General Relativity described in some specific coordinates and finds an ad hoc procedure to analyze the character of the singularity. In this article we show that the causal character of the zero-areal-radius (R=0) singularity in spherically symmetric models is related with some specific invariants. In this way, if some assumptions are satisfied, one can ascertain the causal character of the singularity algorithmically through the computation of these invariants and, therefore, independently of the coordinates used in the model.

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arXiv2002

Spherically symmetric models for charged radiating stars and voids: Theoretical approach

We study the matching of a general spherically symmetric spacetime with a Vaidya-Reissner-Nordstrom solution. To that end, we study the properties of spherically symmetric electromagnetic fields and develop the proper gravitational and electromagnetic junction conditions. We prove that generic spacetimes can be matched to a Vaidya-Reissner-Nordstrom solution or one of its specializations, and that these matchings have clear physical interpretations. Furthermore, the non-spacelike nature of the matching hypersurface is proved under very general hypotheses. We obtain the fundamental result that any spherically symmetric body, be it in evolution or not, has un upper limit for the total net electric charge that carries.

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