arXiv subjects

Juliano C. S. Neves

Publications and source records attributed to Juliano C. S. Neves.

At least 19 recordsLinked to original sources

arXiv2026

Geometrization 3.0: the black hole shadow

There have been three geometrizations in history. The first one is historically due to the Pythagorean school and Plato, the second one comes from Galileo, Kepler, Descartes and Newton, and the third geometrization of nature begins with Einstein's general relativity. Here the term geometrization of nature means the conception according to which nature (with its different meanings) is largely described by using geometry. In this article, I focus on the third geometrization, in which the black hole shadow phenomenon relates shape to dynamics. As a consequence, spacetime symmetry could play the role of the formal cause in black hole physics. Spacetime symmetry as formal cause of spacetime could be an interesting point in the kinematics-dynamics debate in the theory of relativity.

physics.hist-ph↗

arXiv2025

Black holes and wormholes in Deser-Woodard gravity

The Deser-Woodard gravity is a modified theory of gravity in which nonlocality plays a central role. In this context, nonlocality is a consequence of the inverse of the d'Alembertian operator $\square^{-1}$ in the effective action. Here, exact black hole and wormhole solutions are built in the revised Deser-Woodard gravity following a recent approach, where a special tetrad frame simplifies the complicated field equations of the theory. Using the Schwarzschild metric and the Reissner-Nordström metric as initial seed solutions, the developed algorithm generates new traversable wormholes, singular black holes and a regular black hole as solutions of the vacuum field equations of the modified theory. Also, the auxiliary fields, which are responsible for the nonlocality, are computed. However, even for a regular black hole solution, in which spacetime does not contain a curvature singularity, the corresponding auxiliary fields diverge at the event horizon. Regarding observational results, the shadow angular radius is computed for the new solutions. In particular, the deviation of the Schwarzschild black hole in the Deser-Woodard gravity casts a larger shadow than the corresponding black hole in general relativity.

gr-qc↗

arXiv2025

Wormholes from beyond

In a brane-world context in which our universe would be a four-dimensional brane embedded into a five-dimensional spacetime or bulk, wormhole geometries are induced on branes. In this article, the Morris-Thorne wormhole and the Molina-Neves wormhole are obtained on the brane using the Nakas-Kanti approach, which starts from a regular five-dimensional spacetime to obtain known black hole and wormhole solutions on the four-dimensional brane. From the bulk perspective, these wormholes are five-dimensional solutions supported by an exotic fluid, but from the brane perspective, such objects are wormholes not supported by any fields or particles that live on the four-dimensional spacetime. Thus, the cause of these wormholes is the bulk influence on the brane.

gr-qc↗

arXiv2025

Stars and quark stars in bumblebee gravity

In this work, the interior spacetime of stars is built in a Lorentz symmetry breaking model called bumblebee gravity. Firstly, we calculated the modified Tolman-Oppenheimer-Volkoff equation in this context of modified gravity. Then we show that the bumblebee field, responsible for the symmetry breaking, increases the star mass-radius relation when it assumes its vacuum expectation value. When compared to the general relativity mass-radius relation, a Lorentz symmetry breaking context, like the bumblebee gravity, could provide more massive stars, surpassing the $2.5 M_{\odot}$ limit as the interior of the star is described by quark matter with the MIT bag model. Also, we investigate the stability of the solution with the MIT bag equation of state in this context of modified gravity.

gr-qc↗

arXiv2024

Special theory of regularity

The theory of regularity is a philosophical perspective in which laws of nature are just descriptions, that is to say, laws of nature do not govern the world. Moreover, according to the theory of regularity, the number of laws of nature might be infinite, thus any attempt towards the theory of everything is doomed. Here I propose a special or restricted theory of regularity. The main difference as to the well-known version of that theory is both the range of validity and the scale of the laws of nature. Laws of nature ought to be considered just inside the observable universe and within certain energy and length scales. Even so I apply the theory of regularity to the multiverse scenario. As a consequence, the special theory of regularity supports only two types of multiverses by comparison with our world: those ones with a different sequence of unique events and different laws of nature and those ones with the same sequence of unique events and the same laws of nature instanced by the unique events. The latter case is some sort of eternal recurrence or a parallel eternal recurrence.

physics.hist-ph↗

arXiv2023

Shadows of black holes at cosmological distances in the co-varying physical couplings framework

The co-varying physical couplings (CPC) framework states that physical parameters like the speed of light in vacuum $c$, the Newtonian constant $G$, and the cosmological constant $Λ$ could indeed vary with the spacetime coordinates $x^μ$. Here, we assume a temporal variation, that is, $c(t),G(t)$ and $Λ(t)$. We show that the McVittie spacetime, a black hole in an expanding universe, is a solution of the CPC framework providing naturally an important parameter of the model. Then, we calculate the shadow angular radius of this black hole at cosmological distances. A black hole shadow in the CPC context could be either larger or smaller than the same shadow in the standard cosmology. It depends on how the set $\{ c,G,Λ\}$ varies with time or with the cosmic expansion.

gr-qc↗

arXiv2023

Viable wormhole solution in Bopp-Podolsky electrodynamics

Following a recent approach in which the gravitational field equations in curved spacetimes were presented in the Bopp--Podolsky electrodynamics, we obtained an approximate and spherically symmetric wormhole solution in this context. The calculations were carried out up to the linear approximation in both the spacetime geometry and the radial electric field. The solution presents a new parameter that comes from the Lagrangian of the model. Such a parameter was constrained by using the shadow radius of Sagittarius A*, recently revealed by the Event Horizon Telescope Collaboration. Remarkably, the wormhole presented here is viable when its shadow is compared to the Sagittarius A* shadow.

gr-qc↗

arXiv2023

Kasner cosmology in bumblebee gravity

Kasner cosmology is a vacuum and anisotropically expanding spacetime in the general relativity context. In this work, such a cosmological model is studied in another context, the bumblebee model, where the Lorentz symmetry is spontaneously broken. By using the bumblebee context it is possible to justify the anisotropic feature of the Kasner cosmology. Thus, the origin of the anisotropy in this cosmological model could be in the Lorentz symmetry breaking. Lastly, an application in the pre-inflationary cosmology is suggested.

gr-qc↗

arXiv2022

Bianchi type I cosmology with a Kalb-Ramond background field

An effect of the Lorentz symmetry breaking is pointed out in the cosmological context. Using a Bianchi I geometry coupled to the Kalb-Ramond field, a consequence of the Lorentz symmetry violation is indicated by a different rate of expansion in a given spatial direction. This article focuses on the coupling constant $ξ_1$, which generates, from the Kalb-Ramond field, all three coefficients that give rise to the Lorentz violation in the gravity sector of the minimal Standard Model Extension. The coupling constant $ξ_1$ increases the rate of expansion of the universe in a given direction during a dark energy era. As a consequence, a range of validity of that coupling constant is also obtained.

gr-qc↗

arXiv2021

Bumblebee field as a source of cosmological anisotropies

In this work, a bumblebee field is adopted in order to generate cosmological anisotropies. For that purpose, we assume a Bianchi I cosmology, as the background geometry, and a bumblebee field coupled to it. Bumblebee models are examples of a mechanism for the Lorentz symmetry violation by assuming a nonzero vacuum expectation value for the bumblebee field. When coupled to the Bianchi I geometry, which is not in agreement with a cosmological principle, the bumblebee field plays the role of a source of anisotropies and produces a preferred axis. Thus, a fraction of the cosmic anisotropies would come from the Lorentz symmetry violation. In the last part of the article, we try to assume an upper bound on the bumblebee field using the quadrupole and octopole moments of the cosmic microwave background radiation.

gr-qc↗

arXiv2021

Five-dimensional regular black holes in a brane world

Following a recent approach, complete and analytic solutions (brane and bulk) of regular black holes are shown in a brane context. The metrics are regular both on the four-dimensional brane and in the five-dimensional bulk. Like many brane world scenarios, the bulk spacetime is asymptotically anti-de Sitter. On the other hand, a de Sitter core on the brane avoids the singularity inside the event horizon, providing then well-known regular black holes on the brane. From the bulk perspective, the regular black holes are five-dimensional objects, with the event horizon extending to the extra dimension, but the de Sitter core is entirely on the four-dimensional brane.

hep-th↗

arXiv2021

Black holes with a cosmological constant in bumblebee gravity

In this work, we present black hole solutions with a cosmological constant in bumblebee gravity, which provides a mechanism for the Lorentz symmetry violation by assuming a nonzero vacuum expectation value for the bumblebee field. From the gravitational point of view, such solutions are spherically symmetric black holes with an effective cosmological constant and are supported by an anisotropic energy-momentum tensor, conceived of as the manifestation of the bumblebee field in the spacetime geometry. Then we calculate the shadow angular radius for the proposed black hole solution with a positive effective cosmological constant. In particular, our results are the very first relation between the bumblebee field and the shadow angular size.

gr-qc↗

arXiv2020

Constraining the tidal charge of brane black holes using their shadows

A constraint on the tidal charge generated within a brane world is shown. Using the shadow of a rotating black hole in a brane context in order to describe the M87* parameters recently announced by the Event Horizon Telescope Collaboration, the deviation from circularity of the reported shadow produces an upper bound on the bulk's nonlocal effect, which is conceived of as a tidal charge in the four-dimensional brane induced by the five-dimensional bulk. Therefore, a deviation from circularity $\lesssim 10\%$ leads to an upper bound on the tidal charge $\lesssim 0.004M^2$.

gr-qc↗

arXiv2020

Accretion of perfect fluids onto a class of regular black holes

We consider the stationary spherical accretion process of perfect fluids onto a class of spherically symmetric regular black holes corresponding to quantum-corrected Schwarzschild spacetimes. We show that the accretion rates can differ from the Schwarzschild case, suggesting that the de Sitter core inside these regular black holes, which indeed precludes the central singularity, can act for some cases as a sort of antigravitational source, decreasing the fluid's radial infall velocity in the accretion process, and for others as a gravitational enhancer, increasing the fluid flow into the black hole horizon. Our analysis and results can be extended and also applied to the problem of black hole evaporation in cosmological scenarios with phantom fluids. In particular, we show that the mass of typical regular black holes can be used in order to constrain turnaround events in cyclic cosmologies.

gr-qc↗

arXiv2020

Upper bound on the GUP parameter using the black hole shadow

An upper bound on the parameter that provides a generalized uncertainty principle (GUP) is obtained from the black hole shadow. With the aid of a recent constraint between regular black holes and the GUP parameter, it is indicated a relation between this parameter and the deviation from circularity of the black hole shadow. In the case of the recent announcement of the M87* results from the Event Horizon Telescope collaboration, a deviation from circularity $\lesssim 10\%$ imposes a GUP parameter $β_0 <10^{90}$.

gr-qc↗

arXiv2019

Proposal for a degree of scientificity in cosmology

In spite of successful tests, the standard cosmological model, the $Λ$CDM model, possesses the most problematic concept: the initial singularity, also known as the big bang. In this paper---by adopting the Kantian difference between to think of an object and to cognize an object---it is proposed a degree of scientificity using fuzzy sets. Thus, the notion of initial singularity will not be conceived of as a scientific issue because it does not belong to the fuzzy set of what is known. Indeed, the problematic concept of singularity is some sort of what Kant called the noumenon, but science, on the other hand, is constructed in the phenomenon. By applying the fuzzy degree of scientificity in cosmological models, one concludes that cosmologies with a contraction phase before the current expansion phase are potentially more scientific than the standard model. At the end of this article, it is shown that Kant's first antinomy of pure reason indicates a limit to our cosmological models.

physics.hist-ph↗

arXiv2019

A fuzzy process of individuation

It is shown that an aspect of the process of individuation may be thought of as a fuzzy set. The process of individuation has been interpreted as a two-valued problem in the history of philosophy. In this work, I intend to show that such a process in its psychosocial aspect is better understood in terms of a fuzzy set, characterized by a continuum membership function. According to this perspective, species and their members present different degrees of individuation. Such degrees are measured from the membership function of the psychosocial process of individuation. Thus, a social analysis is suggested by using this approach in human societies.

physics.soc-ph↗

arXiv2019

A philosophical argument against time machines

Time machines are predictions of Einstein's theory of general relativity and provide a myriad of unsolved paradoxes. Convincing and general arguments against time machines and their paradoxes are missing in physics and philosophy so far. In this article, a philosophical argument against time machines is given. When thought of as a process, individuation refuses the idea of time machines, in particular travels into the past. With the aid of Nietzsche-Heraclitus' philosophy of becoming and Simondon's notion of process of individuation, I propose that time machines are modern fables, created by the man of $\textit{ressentiment}$. In the $\textit{amor fati}$ formula of Nietzsche, I suggest the antipode to time machines.

physics.hist-ph↗