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Franco Fiorini

Publications and source records attributed to Franco Fiorini.

At least 19 recordsLinked to original sources
arXiv2026

Bounded Electromagnetic Power Flux at Naked Kasner Singularities

In some occasions, it would appear that classical electromagnetic fields could be able to coexist with strong curvature, naked singularities, in the sense that the electromagnetic power flux would be bounded in any open neighborhood of them. We illustrate this rare behavior by considering the naked singularity present in Kasner spacetimes. There, using the Plebanski--Tamm map between Maxwell theory in curved spacetime and the associated description corresponding to an effective anisotropic medium in flat space, we found not only some regular electrostatic field configurations, but also electromagnetic waves whose amplitudes lead to a well defined, bounded power flux as they approach the singularity. Sometimes, in these unusual situations, the singularity resembles a mirror, perfectly reflecting its environs; in other occasions, the existence of a non-null, finite power flux as the waves travel to or from the singular plane, offers a very simple explanation as to how the singularity might absorb or create radiation fields.

gr-qc↗
arXiv2026

Can Light Cross a Singularity? Exact Solutions from Analogue Gravity

Using a simple spacetime hosting a strong curvature naked singularity, we employ an analogue gravity model to study electromagnetic fields in this background. We find exact solutions to the full set of electrostatic and electrodynamic equations that remain regular even in the presence of the singularity. Moreover, certain solutions sustain a regular and bounded power flux across the singularity, suggesting that electromagnetic energy may be transmitted through it.

gr-qc↗
arXiv2025

Shedding Light on Naked Singularities

Electromagnetic waves propagating in the background provided by a spacetime hosting a strong curvature, naked singularity, are fully studied. The analysis is performed not only in the realm of geometrical optics -- which, not surprisingly, proves to be inadequate in the strong field regime characterizing the vicinity of the singularity -- but also in the physical one in which the field amplitudes must necessarily be incorporated into the description. In addition to the expected divergent outcome with regard to the field amplitudes and power flux as the waves approach the singularity, we found a number of regular (bounded) solutions which seem to coexist with the unbridled effects of the spacetime curvature. In some of them, the singularity operates as a perfect mirror reflecting the surrounding fields. Strikingly, other solutions exhibit a perfectly well behaved, bounded power flux as they propagate towards the singularity, suggesting thus the possibility of having electromagnetic energy transference through it.

gr-qc↗
arXiv2024

Exact results in a punctured neighborhood of a strong curvature singularity

By constructing a model of spacetime having a strong curvature singularity in which causal geodesics are complete, but more generic causal curves are not, we explicitly show that some electrostatic field configurations on that background are bounded on every open punctured neighborhood of the curvature singularity. Our calculations are performed using the analog gravity description provided by Plebanski and Tamm, according to which the characterization of the electromagnetic field on a generic curved background is equivalent to solving Maxwell's equations in flat space with a matter content verifying certain nontrivial constitutive relations. The regularity of the electric field as it approaches what could be considered the worst conceivable physical condition, opens the door to further investigation into the possibility of propagating signals capable of crossing a spacetime singularity.

gr-qc↗
arXiv2024

Teleparallel Jackiw-Teitelboim gravity

We introduce a new class of two dimensional gravity models using ideas motivated by the Teleparallel Equivalent of General Relativity. This leads to a rather natural formulation of a theory that has close links with Jackiw-Teitelboim gravity. After introducing the theory and discussing its vacuum solutions, we present the Hamiltonian analysis. This implies the presence of a single dynamical degree of freedom, which is in sharp contrast to General Relativity, where there are no degrees of freedom in two spacetime dimensions. Our approach can be extended to various other lower-dimensional gravity theories and thus could be of wider interest.

gr-qc↗
arXiv2023

Reference Frames and Black Hole Thermodynamics

In the context of the absolute parallelism formulation of General Relativity, and because of the fact that the scalar curvature can be written in purely torsional terms, it was known for a long time that a surface term based solely on the torsion tensor appears in the action. It was subsequently suggested that this term might play the role of the Gibbons-Hawking-York boundary term which, in turn, is associated to the free energy in the path integral approach, and then, to the black hole entropy by standard thermodynamic arguments. We show that the identification of the two boundary terms is rather incomplete, and that it strongly depends on the choice of the tetrad (frame) field used to reproduce a given metric. By considering variations of the tetrad field not necessarily subjected to Dirichlet-like conditions on the boundary surface, we find a class of frames adapted to the Schwarzschild spacetime in which the Gibbons-Hawking-York/torsion link is actually established, and conducing to the right black hole entropy without the need of any background subtraction. Remarkably, these frames are also responsible for the correct value of the gravitational energy as computed from the teleparallel energy-momentum pseudo-current.

gr-qc↗
arXiv2023

Local symmetries in $f(T)$-like models: lessons from 2D

The comprehension of the intricate structure associated to the local symmetries encoded in the tetrad field, as well as its physical meaning, is perhaps the most important unsolved problem within $f(T)$ gravity. This is inextricably connected to the number, nature and potential impact that the additional degree/s of freedom might have within these --and other closely related--models of gravity in which the local Lorentz invariance is broken at some level. Here we review and further explain some recent results which make use of the more placid scenery provided by 2D-torsional models of gravity, where the local symmetries adapted to a given geometry can be fully characterized.

gr-qc↗
arXiv2022

Chronology protection in $f(T)$ gravity: the case of Gott's pair of moving cosmic strings

As a consequence of the spacetime structure, defined by the tetrad field instead of the metric tensor alone, $f(T)$ gravity seems to harbor its own chronology protection agency. When Gott's pair of moving cosmic strings is considered, it is shown that the requirement of having a global parallelization -- i.e. a global smooth field of tetrads -- drastically restrict the form of the tetrads on the junction surface between the two strings. The junction conditions on the tetrad field are satisfied only if the corresponding boosts needed to put the strings in motion are null on the matching surface. This seems to throw overboard Gott's construction from the outset without the need of analyzing the divergence of the expectation value of the energy momentum tensor on the Cauchy horizon, evading in this way bothering quarrels concerning the choice of vacuum.

gr-qc↗
arXiv2022

Role of the remnant symmetries in gravitational theories based on absolute parallelism: a 2D standpoint

By using simplified 2D gravitational, non-local Lorentz invariant actions constructed upon the torsion tensor, we discuss the physical meaning of the remnant symmetries associated with the near-horizon (Milne) geometry experienced by a radial observer in Schwarzschild spacetime. We then fully characterize the remnant symmetries corresponding to this near-horizon 2D geometry by solving the motion equations adapted to 2D Milne space. This symmetries, which represent special or privileged \emph{diads}, acquire the form of uniformly accelerated (Rindler) observers whose constant acceleration is proportional to the black hole mass $M$.

gr-qc↗
arXiv2021

Exact results on analogue gravity in optical Plebanski-Tamm media

Exact results concerning ray-tracing methods in Plebanski-Tamm media are derived. In particular, Hamilton equations describing the propagation of quasi-plane wave electromagnetic fields in the geometrical optics regime are explicitly written down in terms of the 3-metric representing the properties of the optical analogue, anisotropic medium. We exemplify our results by obtaining the trajectories of light in the resulting analogue medium recreating Gödel's universe.

gr-qc↗
arXiv2021

Matching tetrads in f(T) gravity

The procedure underlying the matching of 1-form (tetrad) fields in theories possessing absolute parallelism -- f(T) gravity being within this category -- is addressed and exemplified. We show that the remnant symmetries of the intervening spaces play a central role in the process, because the knowledge of the remnant group of local Lorentz transformations enables one to perform rotations and/or boosts in order to $\mathcal{C}^{1}$-match the corresponding tetrads on the junction surface. This automatically ensures the continuity of the Weitzenböck scalar there, even though this proves to be just a necessary condition in order to obtain a global parallelization of the spacetime.

gr-qc↗
arXiv2020

BTZ gems inside regular Born-Infeld black holes

The regular black hole solution arising as a spherically symmetric vacuum solution of Born-Infeld gravity possesses an asymptotic interior structure which is very well described by a four dimensional generalization of the non-rotating BTZ metric. According to this picture no singularity exists, and instead, infalling observers experience a constant curvature manifold as they travel towards future null infinity. This is characterized by the BTZ event horizon. The exterior structure of the black hole is also studied, and it is shown that it corresponds to the Schwarzschild solution provided the black hole mass is not too small. In this way, the regular black hole state can be seen as a spacetime which connects two constant curvature asymptotic spaces, namely, the flat Minkowski spacetime in the outside region, and the locally AdS constant negative curvature one characterizing the BTZ-like asymptotic interior.

hep-th↗
arXiv2019

D=11 cosmologies with teleparallel structure

The presence of additional compact dimensions in cosmological models is studied in the context of modified teleparallel theories of gravity. We focus the analysis on eleven dimensional spacetimes, where the seven dimensional extra dimensions are compactified. In particular, and due to the importance that global vector fields play within the conceptual body of teleparallel modified gravity models, we consider the additional dimensions to be products of parallelizable spheres. The global vector fields characterizing the different topologies are obtained, as well as the equations of motion associated to them. Using global dynamical system techniques, we discuss some physical consequences arising because of the existence of the extra dimensions. In particular, the possibility of having an early inflationary epoch driven by the presence of extra dimensions without other matter sources is discussed.

gr-qc↗
arXiv2019

The regular black hole in four dimensional Born-Infeld gravity

In the context of Born-Infeld gravity theories we report the existence of a regular black hole interior representing a spherically symmetric vacuum solution of the theory. It reduces to the Schwarzschild interior metric in the weak field region. In particular, there is a new length scale which is related to the Born-Infeld parameter. This endows the spacetime with an inner (i.e. well inside the event horizon) asymptotic region which is unattainable for observers. The central curvature singularity is replaced by an infinitely long cosmic string with constant curvature invariants related to the Born-Infeld constant. The presence of this limiting curvature spacetime renders the black hole timelike and null geodesically complete, free from the classical Schwarzschild singularity. The transition between the usual black hole interior and this maximum curvature space is achieved without introducing any kind of matter content nor topological changes.

gr-qc↗
arXiv2017

A New Approach to Frequency Independent Radiating Systems: Conformal Edge Antennas

Conformal techniques, based on the covariance of Maxwell's electrodynamics under the full conformal group in four spacetime dimensions, are discussed in relation with the constant input impedance properties of frequency-independent antennas. In particular we show that by applying suitable conformal transformations to existing self-complementary planar structures like logarithmic spirals, the constant input impedance property is considerably improved, specially in the low frequency domain, where it usually fails due to the large wavelength involved. The effect of the conformal transformation is to bring infinity up to a finite distance and, in a way, to imprison the otherwise infinite structure in a compact region. This procedure enables to perform a more intelligent truncation with the aim to capture some of the properties left behind in the cutting, and to design realistic ultra width-band antennas with a more controlled value of the input impedance in the whole operation range. Due to the fact that the tools here developed find their roots in the differential geometry of curved manifolds, they are quite general and also suitable for dealing with 3D-antennas of arbitrary shape, curvature and size, as well as with other systems in which the effects of truncation play a crucial role in defining physical properties of them.

physics.class-ph↗
arXiv2016

An analysis of Born-Infeld determinantal gravity in Weitzenbock spacetime

The Born-Infeld theory of the gravitational field formulated in Weitzenbock spacetime is studied in detail. The action, constructed quadratically upon the torsion two-form, reduces to Einstein gravity in the low field limit where the Born-Infeld constant goes to infinity, and it is described by second order field equations for the vielbein field in $D$ spacetime dimensions. The equations of motion are derived, and a number of properties coming from them are discussed. In particular, we show that under fairly general circumstances, the equations of motion are those of Einstein's General Relativity plus an energy-momentum tensor of purely geometrical character. This tensor is obtained solely from the parallelization defining the spacetime structure, which is encoded in a set of D smooth, everywhere non-null, globally defined 1-forms. Spherical symmetry is studied as an example, and we comment on the emergence of the Schwarzschild geometry within this framework. Potential (regular) extensions of it are envisioned.

hep-th↗
arXiv2016

Primordial brusque bounce in Born-Infeld determinantal gravity

We study a particular exact solution to the Born-Infeld determinantal gravity consisting of a cosmological model which undergoes a brusque bounce. The latter consists of an event characterized by a non-null (but finite) value of the squared Hubble rate occurring at a minimum (non-null) scale factor. The energy density and pressure of the fluid covering the whole manifold are perfectly well behaved in such an event, but the curvature invariants turn out to be undefined there because of the undefined character of the time derivative of H. It is shown that the spacetime results geodesically complete and singularity free, and that it corresponds to a picture of an eternal Universe in which a (somewhat unconventional) bounce replaces the standard Big Bang singularity. This example tends to emphasize that, beyond Einstein's theory of General Relativity, and in the context of extended theories of gravity formulated by purely torsional means, the criterion of a singularity based on pathologies of scalars constructed upon the Riemann curvature tensor, becomes objectionable.

gr-qc↗