arXiv subjects

Alberto Saa

Publications and source records attributed to Alberto Saa.

At least 55 records · Page 3Linked to original sources

arXiv2011

Challenging the weak cosmic censorship conjecture with charged quantum particles

Motivated by the recent attempts to violate the weak cosmic censorship conjecture for near-extreme black-holes, we consider the possibility of overcharging a near-extreme Reissner-Nordström black hole by the quantum tunneling of charged particles. We consider the scattering of spin-0 and spin-1/2 particles by the black hole in a unified framework and obtain analytically, for the first time, the pertinent reflection and transmission coefficients without any small charge approximation. Based on these results, we propose some gedanken experiments that could lead to the violation of the weak cosmic censorship conjecture due to the (classically forbidden) absorption of small energy charged particles by the black hole. As for the case of scattering in Kerr spacetimes, our results demonstrate explicitly that scalar fields are subject to (electrical) superradiance phenomenon, while spin-1/2 fields are not. Superradiance impose some limitations on the gedanken experiments involving spin-0 fields, favoring, in this way, the mechanisms for creation of a naked singularity by the quantum tunneling of spin-1/2 charged fermions. We also discuss the implications that vacuum polarization effects and quantum statistics might have on these gedanken experiments. In particular, we show that they are not enough to prevent the absorption of incident small energy particles and, consequently, the formation of a naked singularity.

gr-qc↗

arXiv2011

Searching for curvature pion radiation from protons in strongly magnetized pulsars

By using rather conservative estimates based on the simplest polar cap model, we search the ATNF Pulsar Catalogue for strongly magnetized stars that could accelerate relativistic protons up to the curvature pion production threshold. The best candidate turns out to be the 16 ms pulsar J0537-6910, but the corresponding characteristic parameter $χ=a/m_p$ is yet too small to give origin to observable signals. We show that, for pulsars with period $P\approx 1\,$ms, a surface polar magnetic field $B \approx 10^{12}$G is required in order to induce detectable curvature pion radiation from accelerated protons in the magnetosphere. Some other emission processes are also considered.

astro-ph.HE↗

arXiv2011

Nonstationary regime for quasinormal modes of the charged Vaidya metric

We consider non-stationary spherically symmetric $n$-dimensional charged black holes with varying mass $m(v)$ and/or electric charge $q(v)$, described by generic charged Vaidya metrics with cosmological constant $Λ$ in double null coordinates, and perform a comprehensive numerical analysis of the fundamental quasinormal modes (QNM) for minimally coupled scalar fields. We show that the "instantaneous" quasinormal frequencies exhibit the same sort of non-stationary behavior reported previously for the four-dimensional uncharged case with $Λ=0$. Such property seems to be very robust, independent of the spacetime dimension and of the metric parameters, provided they be consistent with the existence of an event horizon. The study of time dependent Reissner-Nordström black holes allows us to go a step further and quantify the deviation of the stationary regime for QNM with respect to charge variations as well. We also look for signatures in the quasinormal frequencies from the creation of a Reissner-Nordström naked spacetime singularity. Even though one should expect the breakdown of our approach in the presence of naked singularities, we show that it is possible, in principle, to obtain some information about the naked singularity from the QNM frequencies, in agreement with the previous results of Ishibashi and Hosoya showing that it would be indeed possible to have regular scattering from naked singularities.

gr-qc↗

arXiv2011

Alternative numerical computation of one-sided Levy and Mittag-Leffler distributions

We consider here the recently proposed closed form formula in terms of the Meijer G-functions for the probability density functions $g_α(x)$ of one-sided Lévy stable distributions with rational index $α=l/k$, with $0<α<1$. Since one-sided Lévy and Mittag-Leffler distributions are known to be related, this formula could also be useful for calculating the probability density functions $ρ_α(x)$ of the latter. We show, however, that the formula is computationally inviable for fractions with large denominators, being unpractical even for some modest values of $l$ and $k$. We present a fast and accurate numerical scheme, based on an early integral representation due to Mikusinski, for the evaluation of $g_α(x)$ and $ρ_α(x)$, their cumulative distribution function and their derivatives for any real index $α\in (0,1)$. As an application, we explore some properties of these probability density functions. In particular, we determine the location and value of their maxima as functions of the index $α$. We show that $α\approx 0.567$ and $α\approx 0.605$ correspond, respectively, to the one-sided Lévy and Mittag-Leffler distributions with shortest maxima. We close by discussing how our results can elucidate some recently described dynamical behavior of intermittent systems.

cond-mat.stat-mech↗

arXiv2011

Destroying a near-extremal Kerr-Newman black hole

We revisit here a previous argument due to Wald showing the impossibility of turning an extremal Kerr-Newman black hole into a naked singularity by plunging test particles across the black hole event horizon. We extend Wald's analysis to the case of near-extremal black holes and show that it is indeed possible to destroy their event horizon, giving rise to naked singularities, by pushing test particles towards the black hole as, in fact, it has been demonstrated explicitly by several recent works. Our analysis allows us to go a step further and to determine the optimal values, in the sense of keeping to a minimum the backreaction effects, of the test particle electrical charge and angular momentum necessary to destroy a given near-extremal Kerr-Newman black hole. We describe briefly a possible realistic scenario for the creation of a Kerr naked singularity from some recently discovered candidates to be rapidly rotating black holes in radio galaxies.

gr-qc↗

arXiv2011

Local electromagnetic duality and gauge invariance

Bunster and Henneaux and, separately, Deser have very recently considered the possibility of gauging the usual electromagnetic duality of Maxwell equations. By using off-shell manipulations in the context of the Principle of least action, they conclude that this is not possible, at least with the conventional compensating gauge fields scheme. Such a conclusion contradicts, however, an old result of Malik and Pradhan, who showed that it is indeed possible to introduce an extra abelian gauge field directly in the vacuum Maxwell equations in order to render them covariant under local duality transformations. Since it is well known that the equations of motion can, in general, admit more symmetries than the associate Lagrangian, this would not be a paradoxal result, in principle. Here, we revisit these works and identify the source of the different conclusions. We show that the Malik-Pradhan procedure does not preserve the original Maxwell gauge invariance, while Bunster, Henneaux, and Deser sought for generalizations which are, by construction, invariant under the Maxwell gauge transformation. Further, we show that the Malik-Pradhan construction cannot be adapted or extended in order to preserve the Maxwell gauge invariance, reinforcing the conclusion that it is not possible to gauge the electromagnetic duality.

hep-th↗

arXiv2011

Massive particle production from accelerated sources in high magnetic fields

Non-electromagnetic emissions from high energy particles in extreme environments has been studied in the literature by using several variations of the semi-classical formalism. The detailed mechanisms behind such emissions are of great astrophysical interest since they can alter appreciably the associated energy loss rates. Here, we review the role played by the source proper acceleration $a$ in the particle production process. The acceleration $a$ determines the typical scale characterizing the particle production and, moreover, if the massive particle production is inertially forbidden, it will be strongly suppressed for $a$ below a certain threshold. In particular, we show that, for the case of accelerated protons in typical pulsar magnetospheres, the corresponding accelerations $a$ are far below the pion production threshold.

astro-ph.HE↗

arXiv2010

Quasinormal modes for the charged Vaidya metric

The scalar wave equation is considered in the background of a charged Vaidya metric in double null coordinates $(u,v)$ describing a non-stationary charged black hole with varying mass $m(v)$ and charge $q(v)$. The resulting time-dependent quasinormal modes are presented and analyzed. We show, in particular, that it is possible to identify some signatures in the quasinormal frequencies from the creation of a naked singularity.

gr-qc↗

arXiv2010

Relativistic Weierstrass random walks

The Weierstrass random walk is a paradigmatic Markov chain giving rise to a Lévy-type superdiffusive behavior. It is well known that Special Relativity prevents the arbitrarily high velocities necessary to establish a superdiffusive behavior in any process occurring in Minkowski spacetime, implying, in particular, that any relativistic Markov chain describing spacetime phenomena must be essentially Gaussian. Here, we introduce a simple relativistic extension of the Weierstrass random walk and show that there must exist a transition time $t_c$ delimiting two qualitative distinct dynamical regimes: the (non-relativistic) superdiffusive Lévy flights, for $ t < t_c$, and the usual (relativistic) Gaussian diffusion, for $t>t_c$. Implications of this crossover between different diffusion regimes are discussed for some explicit examples. The study of such an explicit and simple Markov chain can shed some light on several results obtained in much more involved contexts.

cond-mat.stat-mech↗

arXiv2010

Ergodic transitions in continuous-time random walks

We consider continuous-time random walk models described by arbitrary sojourn time probability density functions. We find a general expression for the distribution of time-averaged observables for such systems, generalizing some recent results presented in the literature. For the case where sojourn times are identically distributed independent random variables, our results shed some light on the recently proposed transitions between ergodic and weakly nonergodic regimes. On the other hand, for the case of non-identical trapping time densities over the lattice points, the distribution of time-averaged observables reveals that such systems are typically nonergodic, in agreement with some recent experimental evidences on the statistics of blinking quantum dots. Some explicit examples are considered in detail. Our results are independent of the lattice topology and dimensionality.

cond-mat.stat-mech↗

arXiv2009

Accretion of nonminimally coupled scalar fields into black holes

By using a quasi-stationary approach, we consider the mass evolution of Schwarzschild black holes in the presence of a nonminimally coupled cosmological scalar field. The mass evolution equation is analytically solved for generic coupling, revealing a qualitatively distinct behavior from the minimal coupling case. In particular, for black hole masses smaller than a certain critical value, the accretion of the scalar field can lead to mass decreasing even if no phantom energy is involved. The physical validity of the adopted quasi-stationary approach and some implications of our result for the evolution of primordial and astrophysical black holes are discussed. More precisely, we argue that black hole observational data could be used to place constraints on the nonminimally coupled energy content of the universe.

gr-qc↗

arXiv2009

Anisotropic singularities in modified gravity models

We show that the common singularities present in generic modified gravity models governed by actions of the type $S=\int d^4x \sqrt{-g}f(R,ϕ,X)$, with $X= -{1/2}g^{ab}\partial_aϕ\partial_bϕ$, are essentially the same anisotropic instabilities associated to the hypersurface $F(ϕ)=0$ in the case of a non-minimal coupling of the type $F(ϕ)R$, enlightening the physical origin of such singularities that typically arise in rather complex and cumbersome inhomogeneous perturbation analyses. We show, moreover, that such anisotropic instabilities typically give rise to dynamically unavoidable singularities, precluding completely the possibility of having physically viable models for which the hypersurface $\frac{\partial f}{\partial R}=0$ exists. Some examples are explicitly discussed.

gr-qc↗

arXiv2009

Relativistic invariance of Lyapunov exponents in bounded and unbounded systems

The study of chaos in relativistic systems has been hampered by the observer dependence of Lyapunov exponents (LEs) and of conditions, such as orbit boundedness, invoked in the interpretation of LEs as indicators of chaos. Here we establish a general framework that overcomes both difficulties and apply the resulting approach to address three fundamental questions: how LEs transform under Lorentz and Rindler transformations and under transformations to uniformly rotating frames. The answers to the first and third questions show that inertial and uniformly rotating observers agree on a characterization of chaos based on LEs. The second question, on the other hand, is an ill-posed problem due to the event horizons inherent to uniformly accelerated observers.

nlin.CD↗

arXiv2008

Gravitational wave recoil in Robinson-Trautman spacetimes

We consider the gravitational recoil due to non-reflection-symmetric gravitational wave emission in the context of axisymmetric Robinson-Trautman spacetimes. We show that regular initial data evolve generically into a final configuration corresponding to a Schwarzschild black-hole moving with constant speed. For the case of (reflection-)symmetric initial configurations, the mass of the remnant black-hole and the total energy radiated away are completely determined by the initial data, allowing us to obtain analytical expressions for some recent numerical results that have been appeared in the literature. Moreover, by using the Galerkin spectral method to analyze the non-linear regime of the Robinson-Trautman equations, we show that the recoil velocity can be estimated with good accuracy from some asymmetry measures (namely the first odd moments) of the initial data. The extension for the non-axisymmetric case and the implications of our results for realistic situations involving head-on collision of two black holes are also discussed.

gr-qc↗

arXiv2008

Overspinning a nearly extreme black hole and the Weak Cosmic Censorship conjecture

We revisit here the recent proposal for overspinning a nearly extreme black hole by means of a quantum tunneling process. We show that electrically neutral massless fermions evade possible back reactions effects related to superradiance, confirming the view that it would be indeed possible to form a naked singularity due to quantum effects.

gr-qc↗

arXiv2008

Lifetime and decay of unstable particles in strong gravitational fields

We consider here the decay of unstable particles in geodesic circular motion around compact objects. For the neutron, in particular, strong and weak decay are calculated by means of a semiclassical approach. Noticeable effects are expected to occur as one approaches the photonic circular orbit of realistic black-holes. We argue that, in such a limit,the quasi-thermal spectrum inherent to extremely relativistic observers in circular motion plays a role similar to the Unruh radiation for uniformly accelerated observers.

gr-qc↗

arXiv2007

Non-Gaussian features of chaotic Hamiltonian transport

Some non-Gaussian aspects of chaotic transport are investigated for a general class of two-dimensional area-preserving maps. Kurtosis, in particular, is calculated from the diffusion and the Burnett coefficients, which are obtained analytically. A characteristic time scale delimiting the onset of the Markovian regime for the master equation is established. Some explicit examples are discussed.

nlin.CD↗

arXiv2007

The number of dimensional fundamental constants

We revisit, qualify, and objectively resolve the seemingly controversial question about what is the number of dimensional fundamental constants in Nature. For this purpose, we only assume that all we can directly measure are space and time intervals, and that this is enough to evaluate any physical observable. We conclude that the number of dimensional fundamental constants is two. We emphasize that this is an objective result rather than a "philosophical opinion", and we let it clear how it could be refuted in order to prove us wrong. Our conclusion coincides with Veneziano's string-theoretical one but our arguments are not based on any particular theory. As a result, this implies that one of the three usually considered fundamental constants "G", "c" or "h" can be eliminated and we show explicitly how this can be accomplished.

physics.class-ph↗