Accretion of nonminimally coupled scalar fields into black holes (2009)
Rodrigues, Manuela G., Saa, Alberto
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...
Anisotropic singularities in modified gravity models (2009)
Figueiro, Michele Ferraz, Saa, Alberto
We show that the common singularities present in generic modified gravity models governed by actions of the type $S=\int d^4x \sqrt{-g}f(R,\phi,X)$, with $X=...
Relativistic invariance of Lyapunov exponents in bounded and unbounded systems (2009)
Motter, Adilson E., Saa, Alberto
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...
Gravitational wave recoil in Robinson-Trautman spacetimes (2008)
Macedo, Rodrigo P., Saa, Alberto
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...
Overspinning a nearly extreme black hole and the Weak Cosmic Censorship conjecture (2008)
Richartz, Mauricio, Saa, Alberto
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...
Lifetime and decay of unstable particles in strong gravitational fields (2007)
Fregolente, Douglas, Saa, Alberto
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...
Non-Gaussian features of chaotic Hamiltonian transport (2007)
Venegeroles, Roberto, Saa, Alberto
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...
The number of dimensional fundamental constants (2007)
Matsas, George E. A., Pleitez, Vicente, Saa, Alberto, Vanzella, Daniel A. T.
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...
Searching for Non-Minimally Coupled Scalar Hairs (2007)
In this work we study the asymptotically flat, static, and spherically symmetric black-hole solutions of the theory described by the action S = Z d n x p \Gammag ni 1 \Gamma ¸OE 2 j R \Gamma g ¯ @...
Quasinormal mode characterization of evaporating mini black holes (2007)
According to recent theoretical developments, it might be possible to produce mini black holes in the high energy experiments in the LHC at CERN. We propose here a model based on the $n$-dimensional...
A note on a third order curvature invariant in static spacetimes (2007)
We consider here the third order curvature invariant $I=R_{\mu\nu\rho\sigma;\delta}R^{\mu\nu\rho\sigma;\delta}$ in static spacetimes ${\cal M}=R\times\Sigma$ for which $\Sigma$ is conformally flat....
Homogeneous cosmologies and the Maupertuis-Jacobi principle (2007)
Elias, Luciana A., Saa, Alberto
A recent work showing that homogeneous and isotropic cosmologies involving scalar fields are equivalent to the geodesics of certain effective manifolds is generalized to the non-minimally coupled and...
Non-Minimally Coupled Cosmology as Geodesic Motion (2007)
Elias, Luciana A., Saa, Alberto
Recent works showing that homogeneous and isotropic cosmologies involving scalar fields correspond to geodesics of certain augmented spaces are generalized to the non-minimal coupling case. As the...
N-dimensional Vaidya metric with cosmological constant in double-null coordinates (2007)
A recently proposed approach to the construction of the Vaidya metric in double-null coordinates for generic mass functions is extended to the $n$-dimensional $(n>2)$ case and to allow the inclusion...
Quasinormal modes for the Vaidya metric (2006)
We consider here scalar and electromagnetic perturbations for the Vaidya metric in double-null coordinates. Such an approach allows one to go a step further in the analysis of quasinormal modes for...
Phase Space Solutions in Scalar-Tensor Cosmological Models (2006)
Phase Space Solutions in Scalar-Tensor Cosmological Models
Phase space solutions in scalar-tensor cosmological models (2005)
An analysis of the solutions for the field equations of generalized scalar-tensor theories of gravitation is performed through the study of the geometry of the phase space and the stability of the...
Extracellular polymeric bacterial coverages as minimal area surfaces (2005)
Surfaces formed by extracellular polymeric substances enclosing individual and some small communities of {\it Acidithiobacillus ferrooxidans} on plates of hydrophobic silicon and hydrophilic mica are...
The radiation of a uniformly accelerated charge is beyond the horizon: a simple derivation (2005)
De Almeida, Camila, Saa, Alberto
We show, by exploring some elementary consequences of the covariance of Maxwell's equations under general coordinate transformations, that, despite inertial observers can indeed detect...
Volume elements and torsion (2005)
Mosna, Ricardo A., Saa, Alberto
We reexamine here the issue of consistency of minimal action formulation with the minimal coupling procedure (MCP) in spaces with torsion. In Riemann-Cartan spaces, it is known that a proper use of...
Non-minimal coupling, exponential potentials and the $w (2004)
Carvalho, Fabio C., Saa, Alberto
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
On the viability of local criteria for chaos (2004)
We consider here a recently proposed geometrical criterion for local instability based on the geodesic deviation equation. Although such a criterion can be useful in some cases, we show here that, in...
Semi-analytical approach for the Vaidya metric in double-null coordinates (2004)
Girotto, Fernando, Saa, Alberto
We reexamine here a problem considered in detail before by Waugh and Lake: the solutions of spherically symmetric Einstein's equations with a radial flow of unpolarized radiation (the Vaidya metric)...
Quantum effects and superquintessence in the new age of precision cosmology (2004)
Recent observations of Type Ia supernova at high redshifts establish that the dark energy component of the universe has (a probably constant) ratio between pressure and energy density...
We show that the inclusion of a term $C_{abcd}C^{abcd}$ in the action can remove the recently described anisotropic singularity occurring on the hypersurface $F(\phi)=0$ of scalar-tensor theories of...
Abramo, L. R., Brenig, L., Gunzig, E., Saa, Alberto
We investigate the dynamics of Einstein equations in the vicinity of the two recently described types of singularity of anisotropic and homogeneous cosmological models described by the action $$...
Stability properties and asymptotics for N non-minimally coupled scalar fields cosmology (2003)
Brenig, L., Figueiredo, A., Gunzig, E., Filho, T. M. Rocha, Saa, Alberto
We consider here the dynamics of some homogeneous and isotropic cosmological models with $N$ interacting classical scalar fields non-minimally coupled to the spacetime curvature, as an attempt to...
On the singularities of gravity in the presence of non-minimally coupled scalar fields (2002)
Abramo, L. Raul, Brenig, Leon, Gunzig, Edgard, Saa, Alberto
We investigate the robustness of some recent results obtained for homogeneous and isotropic cosmological models with conformally coupled scalar fields. For this purpose, we investigate anisotropic...
Chaos around the superposition of a monopole and a thick disk (2000)
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...
Chaos around the superposition of a black-hole and a thin disk (1999)
Saa, Alberto, Venegeroles, Roberto
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...
Nonexistence theorems for traversable wormholes (1999)
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...
Comment on ``Theorem for nonrotating singularity-free universes'' (1998)
We show that Raychaudhuri's recently proposed theorem on nonrotating universes cannot be used to rule out realistic singularity-free descriptions of the universe, as suggested by him in PRL 80, 654...
The gravitational Vavilov-Cherenkov effect (1998)
Saa, Alberto, Schiffer, Marcelo
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...
Bound states due to an accelerated mirror (1996)
Saa, Alberto, Schiffer, Marcelo
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...
Quantum dynamics of non-relativistic particles and isometric embeddings (1996)
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...
Propagating torsion from first principles (1996)
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...
Searching for non-minimally coupled scalar hairs (1996)
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-\xi\phi^2 \right)R...
Self-organized criticality and the lattice topology (1996)
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...
New no-scalar-hair theorem for black-holes (1996)
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...
Canonical quantization of the relativistic particle in static spacetimes (1996)
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...
Is minimal coupling procedure compatible with minimal action principle? (1994)
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...
Einstein-Cartan theory of gravity revisited (1993)
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...
A geometrical action for dilaton gravity (1993)
We study the gravitational interaction involving the dilaton and the anti-symmetrical $B_{\mu\nu}$ fields that arises in the low-energy limit of string theory. It is shown that such interaction can...
Gauge fields on Riemann-Cartan space-times (1993)
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...