arXiv subjects

Everton M. C. Abreu

Publications and source records attributed to Everton M. C. Abreu.

At least 19 recordsLinked to original sources

arXiv2026

Shift vector sign reversal in the Alcubierre warp drive spacetime geometry and nonlinear Burgers-type dynamics

This work investigates a sign reversal of the shift vector in the Alcubierre Warp Drive geometry and its effect on the nonlinear reduced structure of the Einstein equations. Previous analyses showed that Burgers-type equations can arise in warp drive spacetimes, leading to vacuum solutions under suitable assumptions on the sign of the shift vector and on matter-source reductions. Here, we analyze the original Alcubierre shift-vector sector and show that, under an appropriate mathematical ansatz, the $\{22\}$ and $\{33\}$ components of the Einstein equations can be formally decomposed into viscous Burgers-type and heat-type equations. The resulting heat-type structure and the constant analogous to a diffusivity coefficient constitute new formal features of the reduced shift-vector dynamics. Since these terms are introduced through an ansatz and are not generated by a specific energy-momentum tensor source, they should not be interpreted as physical diffusivity without an additional matter model. The vacuum reductions of the Einstein equations for the warp-drive geometry come with an important caveat: the shift vector must depend only on time and on one spatial coordinate, namely $β(t,x)$. Consequently, the Alcubierre regulating function no longer retains its original spherical dependence on $r_s(t)$, and the resulting solutions should be interpreted as lower-dimensional shift-sector reductions rather than complete spherically symmetric warp bubble configurations.

gr-qc↗

arXiv2026

Thermostatistical analysis and negative heat capacities of Yukawa and Lee-Wick potentials in noncommutative phase spaces

In recent years, physical models based on noncommutative algebras have attracted considerable interest, as they provide a natural framework to incorporate a fundamental scale, often associated with semiclassical aspects of quantum gravity. Noncommutative geometry modifies the underlying phase-space structure, potentially leading to new insights into unresolved problems in theoretical physics. In this work, we adopt a semiclassical approach to perform a thermostatistical analysis of well-established interaction models, namely the Yukawa and Lee--Wick potentials, within a noncommutative phase space. We investigate how phase-space deformations affect the density of states, partition function, mean energy, and heat capacity, considering both microcanonical and canonical ensembles within the Boltzmann--Gibbs framework. Our results show that the introduction of the noncommutative parameter $Θ$ induces nontrivial modifications in thermodynamic quantities, including qualitative changes in the heat capacity. In particular, regions with negative heat capacity may emerge, which we interpret as signatures of the limitations of the semiclassical and perturbative treatment rather than definitive physical effects. The analysis is carried out under the assumption of weak noncommutativity and $|βV(r)| \ll 1$, which constrains the regime of validity of the results. Within this domain, our findings highlight the role of phase-space geometry in shaping thermodynamic behavior.

hep-th↗

arXiv2026

Joule-Thomson expansion for quantum corrected AdS-Reissner-Nordström black holes in Kiselev spacetime with Barrow fractal entropy

Recently, Barrow proposed an extension of the Bekenstein-Hawking black hole entropy to include the effects of fractal geometry through a parameter $Δ$. Since then, several interesting issues related to this extended entropy have been explored in the literature. In this work, we investigate the effects of the fractal parameter $Δ$ on the inversion temperature connected to the Joule-Thomson expansion that can be obtained from the thermodynamics of AdS-Reissner-Nordström black holes in Kiselev spacetime. The description of such physical systems involves numerical solutions concerning the fractal parameter $Δ$. The results are shown by temperature-pressure curves for multiple values of the set of parameters that define the black hole thermodynamics. In conclusion of our analysis, we also show isenthalpic curves corresponding to fixed-mass black hole processes.

hep-th↗

arXiv2025

Matching the Alcubierre and Minkowski spacetimes

This work analyzes the Darmois junction conditions matching an interior Alcubierre warp drive spacetime to an exterior Minkowski geometry. The joining hypersurface requires that the shift vector of the warp drive spacetime must satisfy the solution of a particular inviscid Burgers equation, namely, the gauge where the shift vector is not a function of the $y$ and $z$ spacetime coordinates. Such a gauge connects the warp drive metric to shock waves via a Burgers-type equation, which was previously found to be an Einstein equations vacuum solution for the warp drive geometry. It is also shown that not all Ricci and Riemann tensors components are zero at the joining hypersurface, but for that to happen they depend on the shift vector solution of the inviscid Burgers equation at the joining wall. This means that the warp drive geometry is not globally flat.

gr-qc↗

arXiv2025

Revisiting the Immirzi parameter: Landauer's principle and alternative entropy frameworks in Loop Quantum Gravity

This paper investigates the implications from area quantization in Loop Quantum Gravity, particularly focusing on the application of the Landauer principle -- a fundamental thermodynamic concept establishing a connection between information theory and thermodynamics. By leveraging the Landauer principle in conjunction with the Bekenstein-Hawking entropy law, we derive the usual value for the Immirzi parameter precisely, $γ= \ln2/(π\sqrt{3})$, without using the typical procedure that involves the Boltzmann-Gibbs entropy. Furthermore, following an analogous procedure, we derive a modified expression for the Immirzi parameter aligned with Barrow's entropy formulation. Our analysis also yields a new expression for the Immirzi parameter consistent with a corresponding modified Kaniadakis entropy for black hole entropy further illustrating, along with Barrow's entropy, the applicability of Landauer's principle in alternative statistical contexts within black hole physics.

gr-qc↗

arXiv2025

From modified Tsallis-Renyi entropy to a MOND-like force law, Bekenstein bound, and Landauer principle for black holes

We examine black hole thermodynamics within the framework of modified Renyi entropy and explore its implications in Modified Newtonian Dynamics (MOND), an extension of Newton second law proposed to explain galaxy rotation curves without invoking dark matter. We conjecture that Tsallis entropy provides an exact description of Bekenstein Hawking entropy, from which the modified Renyi entropy is derived. Using this formulation, we show that a MOND like force law emerges naturally from entropic considerations. We also analyze the Bekenstein bound conjecture, which imposes an upper limit on the entropy of confined quantum systems, and verify its validity under the Renyi modified framework for typical values of the deformation parameter. Furthermore, by invoking the Landauer principle, we obtain an expression for the mass loss due to black hole evaporation. These results suggest that modified Renyi statistics, originating from Tsallis entropy, provides a coherent and promising approach to gravitational dynamics and information theoretic aspects of black hole physics.

gr-qc↗

arXiv2024

Surface gravity analysis in Gauss-Bonnet and Barrow black holes

We have different definitions of the surface gravity (SG) of a horizon since we can say we have distinct classifications of horizons. The SG has an underlying role in the laws of black hole (BH) thermodynamics, being constant in the event horizon. The SG also acts in the emission of Hawking radiation being connected to its temperature. Concerning this last issue, the quantum features that permeate Hawking radiation provide us a direct indication that a BH has its temperature directly connected to its area and that its entropy is proportional to the horizon area. In this work we analyzed some aspects of event horizons. Analyzing how the SG can be classically defined for stationary BHs together with the radial pressure computation. So, the SG, through the laws of BH mechanics is connected to the real thermodynamical temperature of a thermal spectrum. We discussed these subjects in two different BHs scenarios, the five dimensional Gauss-Bonnet one and the recently developed Barrow entropy construction. We discussed how the quantum fluctuations affect these both quantities.

gr-qc↗

arXiv2024

Barrow black hole variable parameter model connected to information theory

One of the greatest challenges of theoretical physics today is to unveil the quantum information theory concerning what happens when one bit of information enters the black hole (BH) horizon. The Landauer principle showed that a certain amount of energy is generated when one-bit of information is erased as it enters the event horizon system. In this paper we used the recently developed Barrow BH model to calculate the addition to the area of the event horizon of his toy model by using the Landauer concept. Besides we make this computation considering $Δ$ as a constant and a variable parameter. We formulate the Barrow parameter ($Δ$) as a function of the energy/mass, which is new in the Barrow BH literature. We will investigate the differences between the Bekenstein-Hawking entropy ($Δ=0$) and the fractal ($Δ=1$) cases concerning the addition in the area of the BH. The asymptotical analysis is also mentioned and we will see that it affects only the fractal case. All the results accomplished here are new concerning BHs in general and the Barrow model literature in particular.

gr-qc↗

arXiv2023

Gauge Symmetry of the Chiral Schwinger model from an improved Gauge Unfixing formalism

In this paper, the Hamiltonian structure of the bosonized chiral Schwinger model (BCSM) is analyzed. From the consistency condition of the constraints obtained from the Dirac method, we can observe that this model presents, for certain values of the $α$ parameter, two second-class constraints, which means that this system does not possess gauge invariance. However, we know that it is possible to disclose gauge symmetries in such a system by converting the original second-class system into a first-class one. This procedure can be done through the gauge unfixing (GU) formalism by acting with a projection operator directly on the original second-class Hamiltonian, without adding any extra degrees of freedom in the phase space. One of the constraints becomes the gauge symmetry generator of the theory and the other one is disregarded. At the end, we have a first-class Hamiltonian satisfying a first-class algebra. Here, our goal is to apply a new scheme of embedding second-class constrained systems based on the GU formalism, named improved GU formalism, in the BCSM. The original second-class variables are directly converted into gauge invariant variables, called GU variables. We have verified that the Poisson brackets involving the GU variables are equal to the Dirac brackets between the original second-class variables. Finally, we have found that our improved GU variables coincide with those obtained from an improved BFT method after a particular choice for the Wess-Zumino terms.

hep-th↗

arXiv2022

Statistical approaches and the Bekenstein bound conjecture in Schwarzschild black holes

One of the challenges of today's theoretical physics is to fully understand the connection between a geometrical object like area and a thermostatistical one like entropy, since area behaves analogously like entropy. The Bekenstein bound suggests a universal constraint for the entropy of a region in a flat space. The Bekenstein-Hawking entropy of black holes satisfies the Bekenstein bound conjecture. In this paper we have shown that when we use important non-Gaussian entropies, like the ones of Barrow, Tsallis and Kaniadakis in order to describe the Schwarzschild black hole, then the Bekenstein bound conjecture seems to fail.

gr-qc↗

arXiv2022

Restoring the gauge invariance in non-Abelian second-class theories

In this paper, we propose a generalization of an improved gauge unfixing formalism in order to generate gauge symmetries in the non-Abelian valued systems. This generalization displays a proper and formal reformulation of second-class systems within the phase space itself. We then present our formalism in a manifestly gauge invariant resolution of the $SU(N)$ massive Yang-Mills and $SU(2)$ Skyrme models where gauge invariant variables are derived allowing then the achievement of Dirac brackets, gauge invariant Hamiltonians and first-class Lagrangians.

hep-th↗

arXiv2021

Statistical approaches on the apparent horizon entropy and the generalized second law of thermodynamics

In this work we have investigated the effects of three nongaussian entropies, namely, the modified Rényi entropy (MRE), the Sharma-Mittal entropy (SME) and the dual Kaniadakis entropy (DKE) in the investigation of the generalized second law (GSL) of thermodynamics violation. The GSL is an extension of the second law for black holes. Recently, it was concluded that a total entropy is the sum of the entropy enclosed by the apparent horizon plus the entropy of the horizon itself when the apparent horizon is described by the Barrow entropy. It was assumed that the universe is filled with matter and dark energy fluids. Here, the apparent horizon will be described by MRE, SME, and then by DKE proposals. Since GSL holds for usual entropy, but it is conditionally violated in the extended entropies, this implies that the parameter of these entropies should be constrained in small values in order for the GSL to be satisfied. Hence, we have established conditions where the second law of thermodynamics can or cannot be obeyed considering these three statistical concepts just as it was made in Barrow's entropy. Considering the $ΛCDM$ cosmology we can observe that for MRE, SME and DKE, the GSL of thermodynamics is not obeyed for small redshift values.

gr-qc↗

arXiv2021

Warp drive dynamic solutions considering different fluid sources

Alcubierre proposed in 1994 that the well known special relativistic limitation that particles cannot travel with velocities bigger than the light speed can be bypassed when such trips are considered globally within specific general relativistic frameworks. Although initial results indicated this scenario as being unphysical, since it would seem to require negative mass-energy density, recent theoretical analyses suggest that such an unphysical situation may not always be necessarily true. In this paper we review some solutions of the Einstein equations using the original Alcubierre warp drive metric endowed with various matter-energy sources, namely dust, perfect fluid, anisotropic fluid, and perfect fluid with a cosmological constant. A connection of some of these solutions featuring shock waves described by the Burgers equation is also shown.

gr-qc↗

arXiv2021

Perfect fluid warp drive solutions with the cosmological constant

The Alcubierre metric describes a spacetime geometry that allows a massive particle inside a spacetime distortion, called warp bubble, to travel with superluminal global velocities. In this work we advance solutions of the Einstein equations with the cosmological constant for the Alcubierre warp drive metric having the perfect fluid as source. We also consider the particular dust case with the cosmological constant, which generalizes our previous dust solution (arXiv:2008.06560) and led to vacuum solutions connecting the warp drive with shock waves via the Burgers equation, as well as our perfect fluid solution without the cosmological constant (arXiv:2101.11467). All energy conditions are also analyzed. The results show that the shift vector in the direction of the warp bubble motion creates a coupling in the Einstein equations that requires off-diagonal terms in the energy-momentum source. Therefore, it seems that to achieve superluminal speeds by means of the Alcubierre warp drive spacetime geometry one may require a complex configuration and distribution of energy, matter and momentum as source in order to produce a warp drive bubble. In addition, warp speeds seem to require more complex forms of matter than dust for stable solutions and that negative matter may not be a strict requirement to achieve global superluminal speeds.

gr-qc↗

arXiv2021

Low-energy Chern-Simons-Proca theory

Some time ago, the infrared limit of the Abelian Chern-Simons-Proca theory was investigated. In this letter, we show how the Chern-Simons-Proca theory can emerge as an effective low energy theory. Our result is obtained by means of a procedure that takes into account the proliferation, or dilution, of topological defects presented in the system.

hep-th↗

arXiv2021

On the nature of Rényi modified entropy and the Incomplete statistics approach in black holes thermodynamics

In this work we have investigated the effects of the two highlighted nongaussian entropies which are the modified Rényi entropy and the so-called Incomplete statistics in the analysis of the thermodynamics of black holes (BHs). We have obtained the equipartition theorems and after that we obtained the heat capacities for both approaches. Depending on the values of both $λ-$parameter and $M$, the BH mass, relative to a modified Rényi entropy and, depending on the values of the $q-$parameter and $M$, the Incomplete entropy can determine if the BH has an unstable thermal equilibrium or not for each model.

gr-qc↗

arXiv2021

Charged dust solutions for the warp drive spacetime

The Alcubierre warp drive metric is a spacetime construction where a massive particle located inside a spacetime distortion, called warp bubble, travels at velocities arbitrarily higher than the velocity of light. This theoretically constructed spacetime geometry is a consequence of general relativity where global superluminal velocities, also known as warp speeds, are possible, whereas local speeds are limited to subluminal ones as required by special relativity. In this work we analyze the solutions of the Einstein equations having charged dust energy-momentum tensor as source for warp velocities. The Einstein equations with the cosmological constant are written and all solutions having energy-momentum tensor components for electromagnetic fields generated by charged dust are presented, as well as the respective energy conditions. The results show an interplay between the energy conditions and the electromagnetic field such that in some cases the former can be satisfied by both positive and negative matter density. In other cases the dominant and null energy conditions are violated. A result connecting the electric energy density with the cosmological constant is also presented, as well as the effects of the electromagnetic field on the bubble dynamics.

gr-qc↗

arXiv2021

Fluid dynamics in the warp drive spacetime geometry

The Alcubierre warp drive metric is a spacetime geometry featuring a spacetime distortion, called warp bubble, where a massive particle inside it acquires global superluminal velocities, or warp speeds. This work presents solutions of the Einstein equations for the Alcubierre metric having fluid matter as gravity source. The energy-momentum tensor considered two fluid contents, the perfect fluid and the parametrized perfect fluid (PPF), a tentative more flexible model whose aim is to explore the possibilities of warp drive solutions with positive matter density content. Santos-Pereira et al. (2020; arXiv:2008.06560) have already showed that the Alcubierre metric having dust as source connects this geometry to the Burgers equation, which describes shock waves moving through an inviscid fluid, but led the solutions back to vacuum. The same happened for two out of four solutions subcases for the perfect fluid. Other solutions for the perfect fluid indicate the possibility of warp drive with positive matter density, but at the cost of a complex solution for the warp drive regulating function. Regarding the PPF, solutions were also obtained indicating that warp speeds could be created with positive matter density. Weak, dominant, strong and null energy conditions were calculated for all studied subcases, being satisfied for the perfect fluid and creating constraints in the PPF quantities such that positive matter density is also possible for creating a warp bubble. Summing up all results,energy-momentum tensors describing more complex forms of matter, or field, distributions generate solutions for the Einstein equations with the warp drive metric where negative matter density might not be a strict precondition for attaining warp speeds.

gr-qc↗