Alberto Saa

Nonminimal coupling, exponential potentials and the w < - 1 regime of dark energy

Recent observations and theoretical considerations have motivated the study of models for dark energy with equation of state characterized by a parameter w=p/{rho}<-1. Such models, however, are usually believed to be inviable due to their instabilities against classical perturbations or potentially catastrophic vacuum decays. In this Brief Report, we show that a simple quintessential model with potential V({phi})=Ae{sup -{sigma}}{sup {phi}} and a gravitational coupling of the form (1+|{xi}|{phi}{sup 2})R can exhibit, for large sets of initial conditions, asymptotic de Sitter behavior with w<-1 regimes. Nevertheless, the model is indeed stable at classical and quantum level.

Superinflation, quintessence, and nonsingular cosmologies

The dynamics of a universe dominated by a self-interacting nonminimally coupled scalar field are considered. The structure of the phase space and complete phase portraits are given. New dynamical behaviors include superinflation (

Regular rotating black holes and the weak energy condition

\dot{H}>0

New no-scalar-hair theorem for black-holes

), avoidance of big bang singularities through classical birth of the universe, and spontaneous entry into and exit from inflation. This model is promising for describing quintessence as a nonminimally coupled scalar field.

Destroying a near-extremal Kerr-Newman black hole

Abstract We revisit here a recent work on regular rotating black holes. We introduce a new mass function generalizing the commonly used Bardeen and Hayward mass functions and extend the recently proposed solutions in order to accommodate a cosmological constant Λ. We discuss some aspects of the causal structure (horizons) and the ergospheres of the new proposed solutions. We also show that, in contrast with the spherically symmetrical case, the black hole rotation will unavoidably lead to the violation of the weak energy condition for any physically reasonable choice of the mass function, reinforcing the idea that the description of the interior region of a Kerr black hole is much more challenging than in the Schwarzschild case.

Challenging the weak cosmic censorship conjecture with charged quantum particles

A new no‐hair theorem is formulated which rules out a very large class of nonminimally coupled finite scalar dressing of an asymptotically flat, static, and spherically symmetric black hole. The proof is very simple and based on a covariant method for generating solutions for nonminimally coupled scalar fields starting from the minimally coupled case. Such a method generalizes the Bekenstein method for conformal coupling and other recent ones. We also discuss the role of the finiteness assumption for the scalar field.

Quantization of a spinning particle with anomalous magnetic moment

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 Walds 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.

Field propagation in de Sitter black holes

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 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 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.

Status Report of the Schenberg Gravitational Wave Antenna

A generalization of the pseudoclassical action of a spinning particle in the presence of an anomalous magnetic moment is given. The leading considerations, to write the action, are gotten from the path-integral representation for the causal Green function of the generalized (by Pauli) Dirac equation for the particle with anomalous magnetic moment in an external electromagnetic field. The action can be written in reparameterization and supergauge-invariant form. Both operator (Dirac) and path-integral quantization are discussed. The first one leads to the Dirac-Pauli equation whereas the second one gives the corresponding propagator. One of the non-trivial points in this case is that both quantization schemes demand, for consistency, that one takes into account an operator ordering problem.A generalization of the pseudoclassical action of a spinning particle in the presence of an anomalous magnetic moment is given. The leading considerations, to write the action, are gotten from the path integral representation for the causal Greens function of the generalized (by Pauli) Dirac equation for the particle with anomalous magnetic momentum in an external electromagnetic field. The action can be written in reparametrization and supergauge invariant form. Both operator (Dirac) and path-integral (BFV) quantization are discussed. The first one leads to the Dirac-Pauli equation, whereas the second one gives the corresponding propagator. One of the nontrivial points in this case is that both quantizations schemes demand for consistency to take into account an operators ordering problem.

Quasinormal modes for the Vaidya metric

We present an exhaustive analysis of scalar, electromagnetic and gravitational perturbations in the background of Schwarzchild-de Sitter and Reissner-Nordstrom-de Sitter spacetimes. The field propagation is considered by means of a semi-analytical (WKB) approach and two numerical schemes: the characteristic and general initial value integrations. The results are compared near the extreme cosmological constant regime, where analytical results are presented. A unifying picture is established for the dynamics of different spin fields.