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Alberto Saa

Publications and source records attributed to Alberto Saa.

87 records · Page 5Linked to original sources
arXiv2000

Chaos around the superposition of a monopole and a thick disk

We extend recent investigations on the integrability of oblique orbits of test particles under the gravitational field corresponding to the superposition of an infinitesimally thin disk and a monopole to the more realistic case, for astrophysical purposes, of a thick disk. Exhaustive numerical analyses were performed and the robustness of the recent results is confirmed. We also found that, for smooth distributions of matter, the disk thickness can attenuate the chaotic behavior of the bounded oblique orbits. Perturbations leading to the breakdown of the reflection symmetry about the equatorial plane, nevertheless, may enhance significantly the chaotic behavior, in agreement with recent studies on oblate models.

gr-qc↗
arXiv1999

Chaos around the superposition of a black-hole and a thin disk

Motivated by the strong astronomical evidences supporting that huge black-holes might inhabit the center of many active galaxies, we have studied the integrability of oblique orbits of test particles around the exact superposition of a black-hole and a thin disk. We have considered the relativistic and the Newtonian limits. Exhaustive numerical analyses were performed, and bounded zones of chaotic behavior were found for both limits. An intrinsic relativistic gravitational effect is detected: the chaoticity of trajectories that do not cross the disk.

gr-qc↗
arXiv1999

Nonexistence theorems for traversable wormholes

Gauss-Bonnet formula is used to derive a new and simple theorem of nonexistence of vacuum static nonsingular lorentzian wormholes. We also derive simple proofs for the nonexistence of lorentzian wormhole solutions for some classes of static matter such as, for instance, real scalar fields with a generic potential obeying $ϕV'(ϕ) \ge 0$ and massless fermions fields.

gr-qc↗
arXiv1998

The gravitational Vavilov-Cherenkov effect

In this essay we show that an uncharged black-hole moving superluminally in a transparent dielectric medium violates Hawking's area theorem. The violation is overcome through the emission of radiation. Since modes cannot emerge from the black hole itself, this radiation must originate from a collective effect in the medium, in complete analogy with the Vavilov-Cherenkov effect. However, because the black-hole is uncharged, the emission mechanism must be different. We discuss the physical origin of the effect and obtain a Newtonian estimative. Then we obtain the appropriate equations in the relativistic case and show that the field which is radiated away is a combination of gravitational and electromagnetic degrees of freedom. Possible astrophysical relevance for the detection of primordial black-holes and binary systems is discussed.

gr-qc↗
arXiv1997

Bound states due to an accelerated mirror

We discuss an effect of accelerated mirrors which remained hitherto unnoticed, the formation of a field condensate near its surface for massive fields. From the view point of an observer attached to the mirror, this is effect is rather natural because a gravitational field is felt there. The novelty here is that since the effect is not observer dependent even inertial observers will detect the formation of this condensate. We further show that this localization is in agreement with Bekenstein's entropy bound.

gr-qc↗
arXiv1996

Quantum dynamics of non-relativistic particles and isometric embeddings

It is considered, in the framework of constrained systems, the quantum dynamics of non-relativistic particles moving on a d-dimensional Riemannian manifold M isometrically embedded in $R^{d+n}$. This generalizes recent investigations where M has been assumed to be a hypersurface of $R^{d+1}$. We show, contrary to recent claims, that constrained systems theory does not contribute to the elimination of the ambiguities present in the canonical and path integral formulations of the problem. These discrepancies with recent works are discussed.

gr-qc↗
arXiv1996

Propagating torsion from first principles

A propagating torsion model is derived from the requirement of compatibility between minimal action principle and minimal coupling procedure in Riemann-Cartan spacetimes. In the proposed model, the trace of the torsion tensor is derived from a scalar potential that determines the volume element of the spacetime. The equations of the model are written down for the vacuum and for various types of matter fields. Some of their properties are discussed. In particular, we show that gauge fields can interact minimally with the torsion without the breaking of gauge symmetry.

gr-qc↗
arXiv1996

Searching for non-minimally coupled scalar hairs

In this work we study the asymptotically flat, static, and spherically symmetric black-hole solutions of the theory described by the action $$S = \int d^nx\sqrt{-g} \left\{\left(1-ξϕ^2 \right)R - g^{μν}\partial_μϕ\partial_νϕ\right\},$$ with $n>3$ and arbitrary $ξ$. We demonstrate the absence of scalar hairs for $ξ<0$. For $ξ>ξ_c=\frac{n-2}{4(n-1)}$, we show that there is no scalar hair obeying $|ϕ(r)| < 1/\sqrtξ$ or $|ϕ(r)| > 1/\sqrtξ$. For $0<ξ<ξ_c$, we prove the absence of scalar hairs such that $|ϕ(r)| < 1/\sqrtξ$ or $\frac{1}ξ < ϕ^2(r) < \frac{ξ_c}{ξ(ξ_c-ξ)}$.

gr-qc↗
arXiv1996

Self-organized criticality and the lattice topology

We examine exhaustively the behavior of avalanches in critical height sandpile models based in two- and three-dimensional lattices of various topologies. We get that for two-dimensional lattices the spatial and temporal distributions characterizing bulk avalanches do not depend on the lattice topology. For the three-dimensional case, we detect a small dependence of the topology for the temporal distribution, while the spatial ones are independent. The two-dimensional lattices studied are: the plane ($R^2$), the cylinder ($S^1\times R$), and the Möbius-strip ($M$); and the three-dimensional are: $R^3$, $S^1\times R^2$, $S^1\times S^1\times R$, $M\times R$, $S^2\times R$, $K\times R$, and $RP\times R$, where $K$ and $RP$ are respectively the Klein bottle and the real projective plane.

adap-org↗
arXiv1996

New no-scalar-hair theorem for black-holes

A new no-hair theorem is formulated which rules out a very large class of non-minimally coupled finite scalar dressing of an asymptotically flat, static, and spherically symmetric black-hole. The proof is very simple and based in a covariant method for generating solutions for non-minimally coupled scalar fields starting from the minimally coupled case. Such 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.

gr-qc↗
arXiv1996

Canonical quantization of the relativistic particle in static spacetimes

We perform the canonical quantization of a relativistic spinless particle moving in a curved and static spacetime. We show that the classical theory already describes at the same time both particle and antiparticle. The analyses involves time-depending constraints and we are able to construct the two-particle Hilbert space. The requirement of a static spacetime is necessary in order to have a well defined Schrödinger equation and to avoid problems with vacuum instabilities. The severe ordering ambiguities we found are in essence the same ones of the well known non-relativistic case.

gr-qc↗
arXiv1994

Is minimal coupling procedure compatible with minimal action principle?

When space-time is assumed to be non-Riemannian the minimal coupling procedure (MCP) is not compatible, in general, with minimal action principle (MAP). This means that the equations gotten by applying MCP to the Euler-Lagrange equations of a Lagrangian $\cal L$ do not coincide with the Euler-Lagrange equations of the Lagrangian obtained by applying MCP to $\cal L$. Such compatibility can be restored if the space-time admits a connection-compatible volume element. We show how these concepts can alter qualitatively the predictions of the Einstein-Cartan theory of gravity.

gr-qc↗
arXiv1993

Einstein-Cartan theory of gravity revisited

The role of space-time torsion in general relativity is reviewed in accordance with some recent results on the subject. It is shown that, according to the connection compatibility condition, the usual Riemannian volume element is not appropriate in the presence of torsion. A new volume element is proposed and used in the Lagrangian formulation for Einstein-Cartan theory of gravity. The dynamical equations for the space-time geometry and for matter fields are obtained, and some of their new predictions and features are discussed. In particular, one has that torsion propagates and that gauge fields can interact with torsion without the breaking of gauge invariance. It is shown also that the new Einstein-Hilbert action for Einstein-Cartan theory may provide a physical interpretation for dilaton gravity in terms of the non-riemannian structure of space-time.

gr-qc↗
arXiv1993

Gauge fields on Riemann-Cartan space-times

Gauge fields are described on an Riemann-Cartan space-time by means of tensor-valued differential forms and exterior calculus. It is shown that minimal coupling procedure leads to a gauge invariant theory where gauge fields interact with torsion, and that consistency conditions for the gauge fields impose restrictions in the non-Riemannian structure of space-time. The new results differ from the well established ones obtained by using minimal coupling procedure at the action formulation. The sources of these differences are pointed out and discussed.

gr-qc↗