Search results for " relativity"
showing 10 items of 1158 documents
Geometric aspects of charged black holes in Palatini theories
2015
Charged black holes in gravity theories in the Palatini formalism present a number of unique properties. Their innermost structure is topologically nontrivial, representing a wormhole supported by a sourceless electric flux. For certain values of their effective mass and charge curvature divergences may be absent, and their event horizon may also disappear yielding a remnant. We give an overview of the mathematical derivation of these solutions and discuss their geodesic structure and other geometric properties.
On cosmic quantum tunneling from “nothing”
2015
We extend to a general Λ-Eriedmann-Lemaitre-Robertson-Walker (ΛFLRW) a previous result by Vilenkin and others according to which a closed de Sitter universe could be created from "nothing". More specifically, our main result is that only the closed ΛFLRW universe (but not the open and flat ones) could be created from a corresponding instanton, that is, from the corresponding solution with signature +4 of the Einstein field equations. Before getting this result the suitable corresponding instantons are calculated. The result is in accordance with previous results by another authors obtained by different methods.
Loop quantum gravity and Planck-size black hole entropy
2007
The Loop Quantum Gravity (LQG) program is briefly reviewed and one of its main applications, namely the counting of black hole entropy within the framework is considered. In particular, recent results for Planck size black holes are reviewed. These results are consistent with an asymptotic linear relation (that fixes uniquely a free parameter of the theory) and a logarithmic correction with a coefficient equal to -1/2. The account is tailored as an introduction to the subject for non-experts.
Mirror, Mirror on the Wall
2014
Our man stands with his face turned towards bygone times. He must spin around to catch a glimpse of the new days waiting to rise. The tornithologist has also arrived at the very essence of his own being, the point of embarkation, from which he can set off in search of the roots of each present moment. For the Torni, being a tower involves a twin-layered perceptual dimension. The height of the tower allows views to far-off places. From the top the gaze of an observer can pick out distant landscapes. Rising into the heavens also means standing out from one’s surroundings and this makes it impossible for a tower to conceal itself. In other words, it is gazed from and gazed upon. Even though th…
Relativistic MHD simulations of stellar core collapse and magnetars
2011
We present results from simulations of magneto-rotational stellar core collapse along with Alfven oscillations in magnetars. These simulations are performed with the CoCoA/CoCoNuT code, which is able to handle ideal MHD flows in dynamical spacetimes in general relativity. Our core collapse simulations highlight the importance of genuine magnetic effects, like the magneto-rotational instability, for the dynamics of the flow. For the modelling of magnetars we use the anelastic approximation to general relativistic MHD, which allows for an effective suppression of fluid modes and an accurate description of Alfven waves. We further compute Alfven oscillation frequencies along individual magneti…
On the local existence of maximal slicings in spherically symmetric spacetimes
2010
In this talk we show that any spherically symmetric spacetime admits locally a maximal spacelike slicing. The above condition is reduced to solve a decoupled system of first order quasi-linear partial differential equations. The solution may be accomplished analytical or numerically. We provide a general procedure to construct such maximal slicings.
Geometry, pregeometry and beyond
2005
This article explores the overall geometric manner in which human beings make sense of the world around them by means of their physical theories; in particular, in what are nowadays called pregeometric pictures of Nature. In these, the pseudo-Riemannian manifold of general relativity is considered a flawed description of spacetime and it is attempted to replace it by theoretical constructs of a different character, ontologically prior to it. However, despite its claims to the contrary, pregeometry is found to surreptitiously and unavoidably fall prey to the very mode of description it endeavours to evade, as evidenced in its all-pervading geometric understanding of the world. The question r…
Dynamics for a simple graph using the U(N) framework for loop quantum gravity
2012
The implementation of the dynamics in loop quantum gravity (LQG) is still an open problem. Here, we discuss a tentative dynamics for the simplest class of graphs in LQG: Two vertices linked with an arbitrary number of edges. We find an interesting global U(N) symmetry in this model that selects the homogeneous/isotropic sector. Then, we propose a quantum Hamiltonian operator for this reduced sector. Finally, we introduce the spinor representation for LQG in order to propose a classical effective dynamics for this model.
Dynamics of oscillating magnetized relativistic tori around a Schwarzschild black hole
2007
We present a comprehensive numerical study of the dynamics of magnetized relativistic axisymmetric tori orbiting in the background spacetime of a Schwarzschild black hole. The tori are modeled as having a purely toroidal magnetic field and a constant distribution of the specific angular momentum. Following previous investigations of tori in a purely hydrodynamical context, the dynamics of these objects has been studied upon the introduction of a perturbation which, for the values of the magnetic field considered here, triggers quasi-periodic oscillations (QPOs) lasting tens of orbital periods. As in the hydrodynamical case, the spectral distribution of the eigenfrequencies shows the presenc…
Labeling spherically symmetric spacetimes with the Ricci tensor
2017
We complete the intrinsic characterization of spherically symmetric solutions partially accomplished in a previous paper [Class.Quant.Grav. (2010) 27 205024]. In this approach we consider every compatible algebraic type of the Ricci tensor, and we analyze specifically the conformally flat case for perfect fluid and Einstein-Maxwell solutions. As a direct application we obtain the {\em ideal} labeling (exclusively involving explicit concomitants of the metric tensor) of the Schwarzschild interior metric and the Vaidya solution. The Stephani universes and some significative subfamilies are also characterized.